Question for Experts

[Ed Hall]

If I change the gear ratio to a numerically higher ratio, the ECM will detect that the RPM's are too high and set codes. Now suppose instead, I just change the tires to a smaller diameter tire, have I altered the final drive ratio just the same as if I had switched from a 2.73 FDR to a 3.06 or 3.33? Will the computer set a code for this?

I believe that the answer to this question is no. I don't see how the computer would know if I put on smaller diameter tires. The ECM compares measured speed (not actual speed but what the speedometer says) vs. rpm's and different tires would not change this ratio.

[Bentham]

Definately not the expert you requested..

but I think you're right.. Smaller tires would alter your speed one way or another, but the speedo would NOT detect it. Therefore, your guage would be wrong, but the ratio would still be the same..

-Eric

[Flyer]

Your speedometer will read an incorrect speed by a percentage that correlates with the difference in overall diameter (or circumference) of the new tires. I know this from experience (I have 4 sets of wheels and tires, 16, 17, 17, 18") all with different rubber.

[JasonA]

If I change the gear ratio to a numerically higher ratio, the ECM will detect that the RPM's are too high and set codes. Now suppose instead, I just change the tires to a smaller diameter tire, have I altered the final drive ratio just the same as if I had switched from a 2.73 FDR to a 3.06 or 3.33? Will the computer set a code for this?

I believe that the answer to this question is no. I don't see how the computer would know if I put on smaller diameter tires. The ECM compares measured speed (not actual speed but what the speedometer says) vs. rpm's and different tires would not change this ratio.

I don't think the computer will throw a code in any case -- I don't think it knows what the rpm is supposed to be for a given speed. I think this would be pretty hard to measure, given some inaccuracy in all speedometers.

Going from a 2.73 to a 3.06 would be about an 11% change, using the same size tires. Going to 11% shorter tires, using the same final drive ratio, would give you the same results. Either way, the engine will be spinning approximately 11% faster at the same ROAD speed (not INDICATED speed).

I think it depends on where in the transmission the VSS is. Is it at the very output of one of the half shafts, measured after the final drive ratio? In THIS case, assuming that the computer DOES in fact compare the measured speed to the engine rpm, given some toleranace, I think you're right -- the computer would notice a different final drive, but not notice a different tire diameter.

Traditionally (like in RWD vehicles), the VSS is in the tailshaft of the transmission, and the speed is measured BEFORE the ring and pinion gear (which is in the rear axle). In THIS case, the computer would NOT notice the difference either way, with a ratio change or a tire diameter change, because both changes occur after the speed monitoring point.

Either way, as I think you're aware, you'll have an incorrect speedometer. I'm also surprised that the computer would measure the rpm vs. vehicle speed. I think it measures the speeds of the transmission shafts before and after the torque converter, to measure slip in the torque converter, but I'm surprised to learn that it actually compares indicated road speed to engine speed.

[Randy_W]

I can't say for sure on Caddy's, but on Dodge trucks a fairly small change in tire size will have effects on shift programs and fuel map, timing, etc... Small changes won't dissrupt the system a lot, but 11% would cause major problems!

[sprucegoose]

Your speedometer will read an incorrect speed by a percentage that correlates with the difference in overall diameter (or circumference) of the new tires. I know this from experience (I have 4 sets of wheels and tires, 16, 17, 17, 18") all with different rubber.

Woah Flyer... That's quite an arsenal of tires and wheels there!!

Yup, the tire size obviously does change the accuaracy of the speedo dramatically. Here is a nice little online calculator for this. Just pretend it says Cadillac at the top, instead of Miata!!

http://www.miata.net/garage/tirecalc.html

[cpk]

Is there any issue with ABS and/or traction control? There are sensors on each wheel to detect wheel speed.

It seems reasonable that the systems wouldn't be 100% effective if they only compared relative wheel speeds - how would they detect all four wheels locking up or two wheels slipping equally? Plus, with the differential, both front wheels (and both rear wheels) are only turning at exactly the same speed when the vehicle is going in a straight line.

I would expect that the PCM is comparing the wheel sensor inputs with what is read by the final drive sensor to insure they are within an expected parameter range. Changing the tire size would throw this off. Enough to matter? I don't know.

Bbobynski's they one to know for certain...

[Bronson]

Those systems look at the relative values coming from the 4 wheels and the rate of change of the sensor inputs. As long as you put the same size tires on all four corners the systems will not know any difference. If you were to only put shorter tires on one end then it will cause problems as you have created a differential in wheel speed between the two ends of the car.

Would a difference of .5% create a problem?

[Bronson]

Reason I'm asking is that I picked up a staple in one of my rear tires 235/55/16 the staple went through right on the edge where the tread meats the sidewall and then went through the sidewall as well.

I was looking for an excuse to change to a larger size 255/50/16, wanted to buy two for the back and later change the front. The tire size calculator shows a difference of .5% would this be enough to affect the ABS or the traction control systems?

Seems that you could have that much difference between tires of the same size in tread wear.