Tire Size Combination

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jps635
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Re: Tire Size Combination

Post by jps635 »

I run Goodyear Eagle F1 Assymetric 2's in 255/40/17 and believe these are still available

Tyre Calculator https://www.tacomaworld.com/tirecalc?ti ... -255-40r17
'85 635csi JPS (RA2-66)
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tschultz
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Re: Tire Size Combination

Post by tschultz »

This is a great website if the number get you confused: https://www.rimsntires.com/specspro.jsp
http://www.Drive4Corners.com
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88m6
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Re: Tire Size Combination

Post by 88m6 »

Thank you both as those tire calculators are very helpful.
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OnTheFritz
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Re: Tire Size Combination

Post by OnTheFritz »

I've got a question for the braintrust here.

At the end of the coming summer, I plan to finish restoring my RC090s and put some nice rubber on them. Currently I'm running 225/50/15 on the stock 15x7 ET20 style 5s (I know, the tires are slightly undersized, speedo is slightly off). I've been convinced for months that 235/45/17s are the best way to go after reading all the accounts of this size tire fitting great with no rubbing. However, I started to dig a little deeper into tire selection and wheel/tire size calculators and comparisons. After comparing a 235/45/17 to a 225/60/15 which I know to rub (previous owner had on the car, since replaced), the clearances are very close between these sizes (7mm shorter, 5mm wider compared larger to smaller).
So I started looking at 245/40/17s, and these, compared to the 225/60/15, offer 13mm of vertical clearance, but are 10mm closer to the lower spring perch. Now, I haven't exactly taken detailed measurements of tire clearance to the suspension, but is there enough clearance there? Has anyone successfully run these 245-sized tires on a similar setup? (17x8 ET20, ideally no spacers) I have months to decide, so I'm still well within the info gathering period.
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TN_M6
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Re: Tire Size Combination

Post by TN_M6 »

OnTheFritz wrote: Thu Mar 14, 2019 6:19 pmAfter comparing a 235/45/17 to a 225/60/15 which I know to rub
Everyone has probably at one time been prone to overthink this.

I'm going to say the 225/60/15 was simply too tall by almost 1" inch (1/2" radius) and probably rubbed on the spring perch. Stock tires were never more than 25" diameter on these cars, e24 or e28 in any size metric or std.

If you have the RC090's like I do in the 17x8 et20 then the 235/45-17 is THE best choice IMO and will be ~1/4" taller than stock while a 240 would be ~1/4" shorter than stock. I prefer a little more sidewall on the 17" wheel (already quite low profile) and I think the chassis will too making the 235/45 the tire. It should not rub unless you have a damaged or bent strut, which is possible you know.

Be prepared to update your front suspension if you get the shimmies. Even a brand new stock UCA bushing will fail pretty soon IME and you need Meyle HD, milled 750 or Moosehead UCA bushings plus all steering joints to be tight. A modern tire with the bigger wheel will exacerbate ANY issues with the front end.

Just out of curiosity are you refinishing your RC090's yourself and what do you have planned. I'm in the same boat needing to refinish the wheels and get new rubber. I'm thinking the darker silver grey (Nagaro)on the centers to match the door trim and polished barrels with new chrome bolts. Waiting for the usual Memorial Day tire deals to pick up those.
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OnTheFritz
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Re: Tire Size Combination

Post by OnTheFritz »

Thank you for the definitive answer! I'm sure it's been discussed before, but I haven't seen as clear/concise info in all my searching yet. I agree about sidewall height, I want as much as I can to get back some comfort and to make the wheels look smaller and less like huge "pimp" wheels on a classic. Now the hard part is what sticky tires to put on. I'll either be more reasonable and try General G-Max RS summers, or go nuts and waste money on RE71Rs. Thankfully I have 5 months to mull that decision over :-k

I completely understand the potential for these to spotlight deficiencies in the suspension, and I hope all my bushes will hold up well. At my last inspection a couple thousand miles back, everything still felt quite taught and good, but I know I haven't replaced any suspension parts (other than front struts) in the 4.5 years I've had the car. If any issues do arise, It's probably time for new hardware, and I've definitely had some form of upgraded bits in mind (E34 thrust arm and aluminum UCA with factory stiffer bushings).

As for the wheels, I've been doing the work mostly myself, aside from repainting the centers. I've completely stripped, sanded, and polished the rims, and am waiting to return from my study abroad to have the centers re-painted (in the correct color this time #-o ). It's a royal pain, but the results are completely worth it. I've documented my journey thus far here: https://imgur.com/a/XIxKbsZ I plan on having the centers just painted to what my current wheels (and pinstripes) are colored, which I now think will look quite good. I also plan to use black polished steel bolts and finish with the black/gold BBS cap. Still undecided on regular polished silver or polished gold for the valve stem, but, like the wheels, I have time to think
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TN_M6
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Re: Tire Size Combination

Post by TN_M6 »

Looking good. One thing I would highly recommend is that you clean and scrub (scotch bright pad) the bead seal area (maybe lightly sand) until smooth and put a fresh coat of paint there too. I've seen older aluminum wheels get so rough from corrosion and/or full of old tire bits you never get rid of the rim leaks. I've refinished the insides of several sets of wheels that looked OK on the outside but suffered from tiny rim leaks.

I did mine years ago (about 8 iirc) but only a real quick respray of the cetners. This next time I'll strip and use 2k paint on them. I had my bolts nickel plated (not very expensive) and they looked great but still began to rust a few years later on. I also found mirror polish difficult to keep up especially around the bolts. I wonder if a clear anodize like the bumpers is possible? Another option might be clear sealer like Sharkhide for example.

Since the original M6 had black bolts it's something I am considering also. Be curious to see how yours come out.

My last set of tires were Pirelli P Zero Nero All-Season. Summer tread with winter compound and I liked them in almost every way but the way they would flat spot when it got cold. These are not worn out but aged out at 8 years or more now.
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