Speed Odometer Re-Calibration

[Jays635]

I am running a 3.90 gear set/final drive ratio on my 1985 635csi where I once had a 3.46 gear set. What is the procedure for recalibrating the speedometer to read correctly or do you have a to change a physical part? I know that our speed cluster is electronic

regards,

Jay

[Brucey]

on E28 based cars no recalibration is required- the speedo reads a pulser on the diff crownwheel, so ratio changes don't alter the speedo but wheel diameter changes do.

Earlier E12 chassis cars (pre-82) have a mechanical speedo drive IIRC and here you can 'change gears' for the odometer certainly and maybe the speedo itself as well.

cheers

[Jays635]

on E28 based cars no recalibration is required- the speedo reads a pulser on the diff crownwheel, so ratio changes don't alter the speedo but wheel diameter changes do.

Earlier E12 chassis cars (pre-82) have a mechanical speedo drive IIRC and here you can 'change gears' for the odometer certainly and maybe the speedo itself as well.

cheers

So it must be the 17 inch wheels and tires that is throwing it off. I run 235/45 tires on 17 inch wheels

[The Sixer Kid]

Hey Jay, here are the outer tire diameters:

200/60-390 - 24.80" (Stock TRX Wheel/Tire)

235/45 - 25.33" (What you a running)

That's a 2.13% increase in diameter

[Jays635]

Hey Jay, here are the outer tire diameters:

200/60-390 - 24.80" (Stock TRX Wheel/Tire)

235/45 - 25.33" (What you a running)

That's a 2.13% increase in diameter

Thanks George this puts things in perspective

[TN_M6]

BMW speedos are usually running indicated speed 5% higher or more than actual. I put 235/45/17 on my 6 and it is still reading slightly high according to my GPS. The odometers are usually dead on and I would expect now your odometer reads a little low compared to actual miles.

I don't know if it's possible to change up anything in the odometer gears to correct. The pulse wheel inside the diff would have to be custom fabricated and you could not make a change as small as 2% or so since there are only about a dozen teeth on that wheel. Just adding one tooth might be 10% increase.

[Brucey]

BMW speedos are usually running indicated speed 5% higher or more than actual. I put 235/45/17 on my 6 and it is still reading slightly high according to my GPS. The odometers are usually dead on and I would expect now your odometer reads a little low compared to actual miles.

I don't know if it's possible to change up anything in the odometer gears to correct. The pulse wheel inside the diff would have to be custom fabricated and you could not make a change as small as 2% or so since there are only about a dozen teeth on that wheel. Just adding one tooth might be 10% increase.

I agree with all the above- FWIW the toothed wheel in the diff has eight or sixteen teeth IIRC

cheers

[hoadie]

You can get kits that allow you to recalibrate speedos. They take the pulse from the sender unit and send a corrected pulse to the speedo. I'll post details if anyone's interested.

[panthercity]

Is this the product Ben is referring to above? Has any one used a product like this to correct their speedometer on a E24?
http://shop.12oclocklabs.com/index.php? ... duct_id=59

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