18 inch tire question....

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JAWS

18 inch tire question....

Post by JAWS »

I am getting ready to install the m parallel wheels that are 18 by 8 front and 18 by 9.5 rear with correct bore 72.56 with no rings.. offsets are 13 front and 25 rear. perfect fit but my question is on tires , I don't want to go too low profile , trying to stay 40 or above...my question is will a 225 45 18 front and a 245 45 or 40 18 work in rear with no issues?? Looking at highest sidewall possible and not too wide to avoid the car wondering with 18's....will these tires fit?
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Post by Da_Hose »

I'm running 275/35-18 in the rear and 245/35-18 in the front. Came with 265/35 in rear. That rattled my fillings loose. Changing to a wider tire, increased sidewall height and took that nasty edge off the ride.

Wish I could run a 40 profile up front to make the nose run a little softer too, but looks like there isn't enough room. I think the tire would possible rub the underside of the strut lower spring perch when at speed. EEK!

I don't have rubbing issues, but YMMV.

Jose
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JAWS

Post by JAWS »

so do you think with a 40 or 45 sidewall front and rear even with a spacer would still rub? How does yours ride with 35 sidewall? rough? I am not wanting to go that wide up front or rear , just increase sidewall number....like a 225/45/18 front and 245/40/18 rear
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Post by Da_Hose »

The rear is no problem. Tons of room with the right offset.

The problem with the front is that an 18 just has too big a diameter. Spacers won't make a difference. Distance from hub center to strut spring base doesn't change and my fear is that a taller sidewall could expand at speed and rub.

I found the 35 profile harsh in the rear with a 265 tread. Increasing the rear width to 275 added that little bit of sidewall that I needed. Even with the spherical control arm bushings I made, the 275/35 combination works well.

My M6 came to me with 18's, but I think 17 is really the biggest you would want to put on a 6'er.

Jose
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JAWS

I got what you are saying now....

Post by JAWS »

So if im ok in the rear with tons of room , then a 245 45 18 I would be ok rear right? Then if I run a 225 40 18 I would clear ok front? I have seen several people saying they are running 235 40 18 front with no issues.....so I am thinking of running a 225 40 18 think that would be ok? Thanks for your info!
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Post by GripGreg »

Just why do you feel the need to go 18"s? Are you making it a race car?
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JAWS

Post by JAWS »

18 inch m parallel's look outstanding on this car! Look at some picks....got a great deal on some factory 18 OEM m parallel's. They look outstanding from the pics I have seen on other e24's......just want to make sure I get total clearance with tire specs e24 way too heavy for a race car....lol in my opinion
Chris Wright

Post by Chris Wright »

Bottom line, I'd probably go with the recommended sizes (at least on the front):
235/40-18 and 275/35-18 as they actually give you the taller sidewalls then the other sizes you were talking about, which is what you are looking for. Here is why:

You have to be careful about the outside diameter of the tires in the front. If you look at the front suspension, you will see that the spring strut extends out over the top of the tire and and can rub if the tire is too big in diameter.

To determine the diameter of any given size tire*, you take the sidewall profile as a percentage, times the tread width, times 2 and add the rim diameter. This means that increasing the tread width also increases the sidewall height and decreasing it lessens the sidewall height. For example:

A 225/45-18 would be:
225mm x .45 = 101.25mm (The height of one sidewall)
101.25mm x 2 = 202.5mm (The height of both the top and bottom sidewalls)
202.5mm ÷ 25.4 = 7.97" (Conversion factor to convert mm to inches)
7.97" + 18" = 25.97" (Add the rim diameter to the height of both of the sidewalls to get the tire diameter, almost 26" in diameter, probably too tall for the front. See the tire diameter ranges below.)

A 225/40-18 would be:
225mm x .40 = 90mm
90mm x 2 = 180mm (sidewall height)
180mm ÷ 25.4 = 7.08"
7.08" + 18" = 25.08"

A 235/40-18 would be:
235mm x .40 = 94mm
90mm x 2 = 189mm (sidewall height)
188mm ÷ 25.4 = 7.40"
7.40" + 18" = 25.4"

As you can see, the 235 actually has a slightly taller sidewall then the 225 for the same profile.

For the rears:
A 245/45-18 would be:
245mm x .45 = 110.25mm
110.25mm x 2 = 220.5mm
220.5mm ÷ 25.4 = 8.68"
8.68" + 18" = 26.68"

A 245/40-18 would be:
245mm x .40 = 98mm
98mm x 2 = 196mm
196mm ÷ 25.4 = 7.71"
7.71" + 18" = 25.71"

A 275/35-18 like Da_Hose recommended would be:
275mm x .35 = 96.25mm
96.25mm x 2 = 192.5mm (sidewall height)
192.5mm ÷ 25.4 = 7.57"
7.57" + 18" = 25.57"

As you can see, even though it is a 35 profile, it is actually a slightly taller sidewall than a 235/40-18 in the front.

* (If you look at the tire data sheets at TireRack, they give the diameters also for each brand)
______________

Here is some general info that might be helpful:

The size of the original rims and tires that came from the factory:


...................................... Rim Width x Diameter - "ET" or Offset (mm) - Tire Size
Stock wheels ('76 to '81) .............. 6" or 6.5" x 14" - ET22 with a 195/70 -14" tire that has a diameter of 24.74"
Stock wheels ('82 on) ................... 6" or 6.5" x 14" - ET22 with a 205/70 -14" tire that has a diameter of 25.30"
TRX (165mm x 390mm) ................. 6.5" x 15-3/8" - ET22 with a 220/55 -390 tire that has a diameter of 24.88"
TRX (195mm x 415mm) (M6) ........ 7.5" x 16-3/8" - ET19 with a 240/45 -415 tire that has a diameter of 24.84"
TRX (210mm x 415mm) (M6)† ... 8.26" x 16-3/8" - ET15 with a 240/45 -415 tire that has a diameter of 24.84" (early "///M" rim)

The typical replacement rim is

7" or 7.5" x 16" - ET20 or less with a 225/50-16 tire that has a diameter of 24.85"
7.5" or 8" x 17" - ET20 or less with a 235/45-17 tire that has a diameter of 25.32"
7.5" or 8" x 18" - ET20 or less with a 235/40-18 tire that has a diameter of 25.4"
(A front rim of 8.5" will need an ET of 13 or less for 235 or wider tires. An ET of 18 will bolt on but only a 225 or smaller tire will not rub and that may depend on the brand)

The Tire diameters range from 24.8" to 25.4" . It must be said that different manufacturers have different cross sections on their tires. Therefore, although a 235x45x17 tire shoiuld theoretically fit, they all do not. For instance the Falken FK 452 235/45/17 on 17x8 - ET 20 rub on the bottom of the spring perch.

The rears have plenty of space and can take an 8", 9" or even a 10" wide rim with the proper offset and tires of 255 or 265 unless you have the SLS accumulators, in which case over 255 and up can hit them in hard cornering. It is the front that is the most critical and an 8" wide rim is generally the largest without a spacer or a smaller ET (IS).

The larger the rim and tire the more stress that is put on the front suspension and the more likely to introduce a shimmy into the front unless the front suspension is tight.

All BMW rims with 5 lug bolts use a 5 x 120mm bolt pattern and are "hub centric", i.e. they are centered by a lip on the hub, not the bolts. They all have the same size hub hole (72.56mm) with the single exception of the larger hole in the e39 which will still fit fine, but they will need hub centering rings (not spacers), $15 from Discount tire. Most after-market rims have hub rings also.

The 3-series rims generally won't fit because the offset is too high, running around ET40/48 and the 7-series with an ET of 23 will generally need a hub centric spacer of 5-10mm depending on the width of the rim. Sometimes a narrower tire (215/205) can be fitted to the 7-Series rim without a spacer and not rub also (but I think is depends on the rim width).
Last edited by Chris Wright on Thu Apr 10, 2014 5:25 pm, edited 2 times in total.
JAWS

Thank you chris....

Post by JAWS »

That cleared it up for me I get it now......so all that being said even in the rear I need to stay with 265/35/18? et25 so basically there isn't even enough room in the rear for a 40 sidewall correct? what I was thinking was 245/40/18 rear only......definitely goin 235/40/18 front et13 Thanks for all your help! Thanks again chris , I didn't know that wider gave you a higher sidewall. Now it makes sense to me guess 275/35/18 like he said is best setup for rear .... just thought with the extra room in the rear a 40 sidewall may work
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Post by Tim Bradham »

I came across these 18" wheels mearly by chance. There 18x8.5 front and 18x9.5 rear. The tires are not right, 245/40/18 F and 265/40/18 R. I had to dimple the front strut spring area a little. Getting ready to lower it with coil overs, and change to a 225 front and 245 rear tire. Just thought I'd share, I'm back working on my 6 after many other projects held me up.
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Re: 18 inch tire question....

Post by VGrill635 »

Hey guys. I found my dream rims. Style 65! They are staggered 8" front, 9.5" rear. After reading this thread, (great stuff btw) it's not mentioned what shocks/struts are being used. I am upgrading to Bilstien Sports which will lower ride height 1.5". Are do any of the above cars have sport shocks installed? :roll:
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Re: 18 inch tire question....

Post by Da_Hose »

I'm running Bilstein sport inserts, with Eibach springs. Now that I have gone with 275 tires, I am getting some rubbing during really aggressive cornering. I might get in there and relieve the sheetmetal liner, to prevent the rubbing.

Jose
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Re: 18 inch tire question....

Post by MrE »

VGrill635 wrote:Hey guys. I found my dream rims. Style 65! They are staggered 8" front, 9.5" rear. After reading this thread, (great stuff btw) it's not mentioned what shocks/struts are being used. I am upgrading to Bilstien Sports which will lower ride height 1.5". Are do any of the above cars have sport shocks installed? :roll:
Cool - I look forward to seeing pics once they're on.

I've got Style 65 replicas, which are 17" and chrome. I've really grown to like the look of these wheels - suitably aggressive & industrial - but yours are better because they're both staggered and shadowchrome. Image


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Re: 18 inch tire question....

Post by VGrill635 »

Great photos! I worked for the local BMW dealer selling new Bimmers back in 2002-2003. Of all the BMW rims out there these are my favorite. I will be mounting 235/40s on the front with 17mm spacers and 275/35s with 15mm spacers on the back.
Going with Continental non directionals to get a little more mileage out of them.
Here's trial sizing photo at the tire shop:
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Re: 18 inch tire question....

Post by francoid »

Disclaimer: I used to be the global head of sports tires at Michelin in France and created the Pilot Sport line. So I have some knowledge but also some bias...

I would never put 18" on an E24, at least not on a stock one. One would have to modify thoroughly all the suspension elements to go to 18". The car is optimally balanced with 225/50R16 or 235/45R17 all around.

There is nothing magical about increasing the diameter of the rim. It's 99% marketing fad. In some ways it can be counter productive as it increases the unsprung mass. The wheel itself does pretty much nothing. It's the tire that actually does the work, as we all know, and as such it's really the measurements of the tire that matter.

The most important measurement is sidewalk height. The sidewalk of the tire is the part that has to flex the most to provide the required performance elements (in particular handling and comfort). We did a lot of research and trials and found the following optima for the pseudo sidewalk height (nominal width * aspect ratio, so for example 235 * 45 = 106):

Light, track oriented sports cars (Ferrari 348, Porsche 911): 90 to 100 mm
GT type sports cars (E24 M6, E34 M5, Porsche 928): 100 to 110 mm
GT not tuned for sport (E24 635, E34 535): 110 to 120 mm
Heavy, comfort oriented (most E31 and E32, Jaguar, typical class S): 120 to 130 mm

These were found to be true regardless of the diameter of the rim. Recent cars have much bigger overall diameter than cars of the '90s so they tend to have bigger rim diameters but the tire height that works best for them has not changed, it's still pretty much the same rule of thumb as indicated above.

The primary reason for this is that the stiffness, modes and frequencies associated with the tire change a lot with the height of the sidewall. The suspension needs to have matching characteristics. Otherwise what happens is a very non linear feeling, where the tire will be overpowered or underpowered and the car will react in a non uniform manner, for example a strong initial jerk from the tire followed by a much slower roll of the car, leading typically to having to correct the steering input long after the initial input. And of course the same will happen with the comfort aspects, the tire will work against the damping of the suspension or only one of the elements will do the work.

Also do realize that load carrying capacity is a function of how much air there is in the tire (i.e. higher sidewalk will make the tire able to carry more at same pressure, or to lower the pressure at same load, providing a better ride); and high speed capability is a combination of all of these and will be lower with an overloaded tire (i.e. if you don't take weight off your car as you stiffen it and put lower sidewalls you'll need to increase the tire pressure, hence further stiffening the tire, requiring even harder suspension, and having a tough ride).

I tested (both at Michelin and with BMW Motorsport) a range of wheel diameters on cars such as the E34 M5 and others and the technical guys and drivers always agreed on these results. The best mount we ever had on these cars was 235/45R17 all around (by the way, the other fad of mounting bigger tires in the back is another common misconception I will write about in the future if there is interest).

Of course that's when the marketing people came in and said we need bigger wheels, and also wider tires in the back... but everybody knew that when we departed from these optimal values, customers would pay more for a lesser result.

There is way more subtlety here (such as, what rim width to mount the tires on, depending on their aspect ratio; how tire measurement is done and on which rim width affects a lot of the characteristics of the tire (as an example, did you know that a 225/50-16 tire is not narrower than a 235/45R17 one by standard?). If there's interest, I'll try and write more.
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Re: 18 inch tire question....

Post by sansouci »

Would love to know more regarding different sized front and rears.....
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Re: 18 inch tire question....

Post by dwcains »

francoid wrote:Disclaimer: I used to be the global head of sports tires at Michelin in France and created the Pilot Sport line. So I have some knowledge but also some bias...
Thanks for such a detailed and informative post. This sort of expert data is always interesting and educational, and don't be afraid to share more often.
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Re: 18 inch tire question....

Post by dwcains »

sansouci wrote:Would love to know more regarding different sized front and rears.....
I bought my car with this staggered setup, but I'd much rather have them equal front/rear. The car already understeers enough, so the stagger only exacerbates that issue, although it can be somewhat controlled with tire pressures (don't have adjustable swaybars). This type of staggered setup was also used in the various Alpina and Hartge cars, and I could never understand why.

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Dean
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Re: 18 inch tire question....

Post by francoid »

If you ever wanted to stagger an E24, you may want to consider wider in front ;)

But more seriously, it's naturally balanced, does not have excess power for the rubber, and should ***not*** be staggered.

Little known fact: if you put a 20% wider tire on the back axle than on the front, how much bigger of a contact area do you get in the rear? 0%. How much will the cornering stiffness increase on the back axle? Essentially 0%.

No change whatsoever, unless you do unnatural things such as wildly lowering the tire pressure, which will then result in all likelihood in some sluggishness of the rear axle, possibly followed by violent (but very well hidden) oversteer...

Unless we are talking huge amounts of power or a naturally unsound car (Italy, I look thy way), there's really no good reason to stagger. I mean, for an engineer. For a marketer, that's very different!
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Re: 18 inch tire question....

Post by MrE »

francoid wrote:Disclaimer: I used to be the global head of sports tires at Michelin in France and created the Pilot Sport line. So I have some knowledge but also some bias...
That's really interesting and informative advice francoid, thanks for sharing. It's very welcome commentary as far as I'm concerned because of your background and the facts as you lay them out are pretty much irrefutable.

I'm pleased to hear your advice too - the last two sets of tyres I've had on my Sixer have been the PS3s and I've been amazingly impressed and happy with them since day one. Both in the wet and the dry they have been a fantastically impressive tyre, even when they get down to the tread wear indicators they still hang on to the road incredibly well under all sorts of duress.

Thank you for developing them! Image

And I'm pleased to hear the 17" spec you recommend too, as they're what I'm running.

My mate Paul has a good quality set of original TRX wheels on his Sixer and is prepared to spend the dollars on new TRX tyres, and he can't rate them highly enough either. Despite the bad rap they get in terms of performance, longevity etc (it would seem primarily from people who have mainly only experienced 20 or 30 year old TRX rubber), his experience is that these tyres on a stock setup work fantastically as that is how the engineering setup of the car was designed to perform. And he is correct - he and I have been head to head on motorsport skidpans and hammering with enthusiasm out on back country roads many times, and the TRXs never fail, never let up, always keep up, and his is definitely a more comfortable ride what with all that tyre sidewall compared to my 17" rims.

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Re: 18 inch tire question....

Post by sharkfan »

francoid wrote:Disclaimer: I used to be the global head of sports tires at Michelin in France and created the Pilot Sport line. So I have some knowledge but also some bias...
Great post and thank you for the information.

When I owned one a few years ago I recall lots of conversations regarding E34 M5's on the different specified tyre widths between the standard suspension, Nurburgring suspension and then the introduction of the 18" wheels; IIRC it was 235/45x17 on 8x17's all round on standard suspension (with an option for wider 9x17 rears with 255/40x17's); 235/45 and 255/40 and staggered 17" wheels on the Nurburgring suspension but then back to a square set-up of 240/45x18 all round (albeit on staggered wheels) for the M-Parrallel 18" wheel equipped cars from 1994 onwards - apparently the return to a square set-up helped reduce the slight tendency to understeer induced by the wider rear tyres (is that correct?).

Can I ask if you did any work for Alpina? They have always leaned toward larger diameter wheels but, as you quite rightly state "The suspension needs to have matching characteristics", and traditionally Alpina have only ever specified Michelin tyres.

When choosing wheels and tyres for my cars I have always thought sidewall/sidewalk height to be a key factor hence when I fitted a 9x18 square wheel set-up to my E34 Touring I opted for the E34 M5 245/40x18 fronts but 265/35x18 rears as a practice run on 235/40x18 square set-up was too harsh on the front and the power delivery a little too much for the rears - when calculated the differences appear small but the effect on the road is easily noticeable and IMH(amateur)opinion, worth it.

After many years of tyre purchasing I am not ashamed to admit that I am now unwilling to deviate from Michelin on my Alpina cars as the suspension works just perfectly with those tyres; my E39 Touring and E34 Touring are on the correct Michelin Pilot Sport tyres but a supply issue a couple of years ago left me selecting Pilot Primacy's for the huge 20" rims on my E38 B12 - although I am very pleased with them as it is still the most comfortable car in my small Fleet.
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Re: 18 inch tire question....

Post by francoid »

I only have time for a quick note on the TRX. Unfortunately, the engineering work for it stopped around 1990. It was a fantastic idea and provided some amazing capabilities but (as is often the case when trying to depart from an old, stale standard) the industry inertia was too much. IIRC Michelin sold the rights, the design and the molds to another company somewhere around 1994.
What that means is that some of the design elements are very outdated. That's the case in particular for the tread pattern (subject to irregular wear that generates vibrations and noise). That tread pattern (with blocks instead of ribs) was used briefly in the MXX tire by Michelin, but soon upgraded with the MXX2, MXX3 and the Pilot Sport line.
The other domain where the technology is quite outdated is the rubber compound used for the tread. While it's excellent on dry, it's not up to modern par on wet and cold, and it wears relatively fast.
I would only ever use TRX tires on my car for a show in order to be perfectly period conform. I would never drive on them. Pilot Sport are the way to go in my opinion.
I'll write more on Alpina when I have a chance. Alpina has had an exclusive partnership with Alpina for as long as I know (that used to be the case with BMW Motorsport as well unless big business stepped in) and the cars are definitely better on Michelin.
francoid

Re: 18 inch tire question....

Post by francoid »

sharkfan wrote:When I owned one a few years ago I recall lots of conversations regarding E34 M5's on the different specified tyre widths between the standard suspension, Nurburgring suspension and then the introduction of the 18" wheels
Correct. That's what I allude to above in part. There was no suspension (and steering, etc) that made 18" preferable to 17". The way it happened was, marketing pushed for 18" and staggered, and we looked for a solution that was as close as possible to what was possible on 235/45R17 square. Is it any better? Overall not, but maybe there are some extreme conditions in which it's marginally better in the hands of extremely accomplished professional drivers on a race track. As a sports GT it was in the view of the technicians not an improvement.


sharkfan wrote:IIRC it was 235/45x17 on 8x17's all round on standard suspension (with an option for wider 9x17 rears with 255/40x17's); 235/45 and 255/40 and staggered 17" wheels on the Nurburgring suspension
That sounds right. The option in my view brought nothing, and that 255/40 is rather narrow compared to its nominal size.
sharkfan wrote:but then back to a square set-up of 240/45x18 all round (albeit on staggered wheels) for the M-Parrallel 18" wheel equipped cars from 1994 onwards - apparently the return to a square set-up helped reduce the slight tendency to understeer induced by the wider rear tyres (is that correct?).
There was no good handling reason for that option in my opinion, rather it was some sort of dealer option IIRC, again aimed at $$$ and marketing, I believe. 245/40R18, right?
sharkfan wrote:When choosing wheels and tyres for my cars I have always thought sidewall/sidewalk height to be a key factor hence when I fitted a 9x18 square wheel set-up to my E34 Touring I opted for the E34 M5 245/40x18 fronts but 265/35x18 rears as a practice run on 235/40x18 square set-up was too harsh on the front and the power delivery a little too much for the rears - when calculated the differences appear small but the effect on the road is easily noticeable and IMH(amateur)opinion, worth it.
To each their own. I would never put 18" on these cars, but it's just me. :D
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Re: 18 inch tire question....

Post by GazM3 »

The pilot supersports are a great performace road tyre that cope with the wet better than anything else I have ever driven on. I currently have them on my work Holden ute and supercharged 540i. I wouldn't mind upgrading the tyres on the e24 as the continental sport contact3 are fairly average as a dry performace tyre and the fronts are nearly 10yr old.

The only problem with them is the lack of a 255/40/17. Quite a few of us run a 10" rear wheel. I run a staggered set and offset the understeer by running -2deg front camber. Besides running a 235/45 on a 10" rim would be a bit of a stretch. I guess I could get away with a 245/45 on a 10" which do come in PSS.

I am also considering making up an 18" set of BBS RS wheels so I have access to more choice of rubber but as stated having a large stagger 235 front 275 rear would make the handling biased to understeer.
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Re: 18 inch tire question....

Post by dwcains »

Aesthetics (subjective) aside, the are certainly some performance (objective) negatives to larger wheels and tires that should be mentioned. Heavier wheels and tires increase the unsprung mass of the car which will negatively impact acceleration and braking, as well as the tracking ability of the suspension, and the feel of the steering. My 635 has Michelin Pilot SX; 205/55-16 and 245/45-16, and while I've never driven it with the original spec wheels and tires, I can say the steering is overly heavy, IMO, and the tramlining is pretty bad - not to mention the tendency to understeer from the staggered setup. Now, my 635 isn't a track car, nor a car I drive very aggressively, but my 350Z is, and changing from the OEM wheels and tires to a much lighter setup made a world of difference. The stock cast 18" wheels were ~26 pounds each, but the replacements are Rays 18" forged wheels at <18 pounds each. It feels like a different car, in the most positive ways.
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'08 350Z

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