Blazer Forum - Chevy Blazer Forums - 4x4 Spare different size

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Quote:

Originally Posted by El_Beautor (Post 568954)

People run different tire sizes all the time without correcting their PCM. Ideally you should have your PCM flashed so that your truck's computer has the correct information to calculate shift points, how the ABS should act, and stuff like that. That being said, a 2% difference is quite small and I really doubt that it would have any major effect on your truck's performance. You will likely see a small reduction in your highway fuel economy.

OK thank you, that helps tremendously!

When the vehicle is in motion, the vehicle speed sensors, (usually 3 of them on the vehicle) generate pulses that are sent to the PCM, (Powertrain Control Module). The PCM calculates speed by how many pulses it receives in a given amount of time. Each different tire size generates a different amount of PPM, (Pulses Per Mile). The PCM uses this data for calculating speed, distance, shift points, ABS, (brakes) and other engine calculations. The TCCM, (Transfer Case Control Module) both 2 speed selectable and Auto trac, also uses the data. P225/75R15 tires have a smaller circumference than the 235/75R15 tires so they generate more PPM. If/when tire size is changed, the PCM needs to be flashed, (reprogrammed) according to the PPM of the tires being used. It requires a bi-directional scan tool capable of flashing. GM dealers have them for sure, (Tech2 scan tool). Other than the dealer, you'd have to check around with different shops and see who has one, and knows how to do the flash. Depending on labor rates, the charge will vary. Typical labor charge is around 1 hour. And yes, if you want your vehicle to perform correctly, it is necessary.

i dont see it effecting the ABS because the ABS simply notices differences in wheel speed regardless of how fast you are traveling. with the smaller tires the PCM will think that he is going faster than he actually is BUT, the ABS will not know the difference. once the ABS sees the difference in wheel speed, thats when it kicks in. i would say that the ABS would be the system that is the LEAST effected by a change in tire size.

i would however, have the PCM reflashed because of the difference in speed and odometer readings. you dont want to put extra miles on your car because if you end up in an accident then the insurance company is going to make out better than you are because now you may have a "high mileage" vehicle.

just my .02