sactorandy

I just put C5 tires and wheels on my 96 CE. The sizes are 255/40-17 and 285/40-18. I wanted the staggard look. Big and not quite so big.

Due to the size difference, I now have both the Service ASR and Service ABS lights on. The fix is to get tires that are very near the same diameter. If they are more than a few percent different it confuses the ASR and ABS.

I'm now going to spend another $400 for 255/45-17 tires for the front. Geeze I hope that works.

Churchkey

I have read somewhere that a change of ABS reluctor rings
on the axle hub behind the brake rotor will cure the problem.

Tire diameter & revolutions per mile can be compared by tire @ tirerack.com, use the "specs" tab.

Good luck.

WVZR-1

For the front of the OP's car the hub/bearing/reluctor ring is a sealed component so that would not be an option. Being an OB2 car I don't know if correcting the speedometer calibration for the "oversized" rears would help or not but it would likely be a less expensive approach to start with.

The OP's post is a bit ODD. He doesn't seem to ask for help but is simply stating "what he's willing to try". I've never attempted but I believe I'd study a bit more before maybe just taking a stab at it. It could be possible that the tire sizing AND ALSO the calibration of the speedometer might be required.

The OP could maybe change the reluctor wheel in the "REAR" to attempt a more reasonable differential ratio front/rear and I expect that's maybe what you meant. I don't believe the change from a 40 to 45 profile tire is a wise first move.

maddogwyatt

I'm running a set of C5 5-spokes but with 245/45/17 and 275/40/18. By my calculations the depth of sidewall on mine is the same front to rear (although overall diameter is obviously an inch different). Your rear sidewall is 12mm deeper than the front. 255/45/17's on the front will give you the same sidewall as the rear, but like mine the overall diameter will still be 1 inch bigger. I've had no trouble with mine yet, but probably have less than 1000 miles on these wheels.
I'd be interested to hear from others who are more knowledgeable than me!

sactorandy

Thanks for your replys. I have no idea of how I would determine what reluctor ring to get or where to get one but that does sound like a scientific solution. Maybe a little to technical for me. Also, none of my experts here in Sacramento have mentioned the speedometer as being a possibility so I will ask about that. It is ok with me if it is a little off.

My old tire sizes were 255/40-17 and 285/40-17, 25" and 26" diameters respectively. I re-used the 255/40-17s in the front and went to 285/40-18 in the rear. The 285/40-18 tires are 27" in diameter which changes the difference between diameters from 1'' to 2". If I get the 255/45-17 tires for the front the difference will go back to only 1".

In terms of Revs per mile (RPM) the originals were 828/799 w/a difference of 29 RPMs. The set now on the car are 828/769 w/a difference of 59 RPMs. If I change to the 255/45-17 the RPM will be 802/769 w/ the difference of 33 RPMs. 33 is very close to 29 and much closer than 59 so I think that will solve the problem. I haven't been able to find any reference to what the allowable difference is.

Is there a flaw in my thought pattern? I appreciate you input. Randy

Churchkey

Tires of the same size by different manufactures will have different revs per mile. It is dependent on the sidewall construction which equates to static load radius also referred to as "squat radius", the actual working radius of a tire bearing the vehicle weight.

The amount of air pressure also determines squat radius.

IMHO your calculations are correct, 4 revolutions per mile difference should not be an issue for ABS/ASR.

maddogwyatt

"My old tire sizes were 255/40-17 and 285/40-17, 25" and 26" diameters respectively. I re-used the 255/40-17s in the front and went to 285/40-18 in the rear. The 285/40-18 tires are 27" in diameter which changes the difference between diameters from 1'' to 2". If I get the 255/45-17 tires for the front the difference will go back to only 1 inch."

That explains why I'm ok with my C5 wheels. My diameters are 1 inch different. I'll have to bear it in mind when I buy new tyres, particularly as I'd like to go wider at the rear.

sactorandy

Sheezam!! I finally found the magic number on the C5 Forum. It is .942.

The front tires can be .942 of the rear tire diameter.

Multiply the diameter of the rear tire by .942 to get the recommended diameter of the front tire.

or

Divide the front tire diameter by .942 to get the maximum tire diameter of the rear.

Hope this helps someone decide what size tire they can get away with for the staggard look.