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Help with tires 255/35/20 and 305/30/20?

Mdell2

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I just replaced my pp1 wheels and tires with 255/35/20 fronts and 305/30/20 rears. So my speedo is off by 5 at 90 and 3.6 at 70 which is a pain but my bigger concern is the computer thinking that either the front or rear is slipping or something like that due to the difference in wheel spin and size ? Any expert opinions or experience with this and if so what's the answer ?
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TehEno

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From my understanding you can correct this with Forscan.
 

Fly2High

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Did the PP1 come with 255/40R19 front and 275/40R19 rear? Looking up the PP1, this is what I found to be stock sizes.

I went to https://www.tire-size-calculator.info/ to see what the differences were and it is saying that the speedometer should only be off by +1.7%. At 90, it should be reading 91.53 mph and at 70 it will read 71.19 mph. This was calculated by looking only at the rear tire since the front, as I understand it, are not involved in speed calculations.

Has this car ever been setup for a different tire? Something sounds odd by the numbers you are getting. From what I understand, it sounds like someone setup the programming for tires that were 5% smaller than stock. It is as if they setup the speedo for something like a 275/30R20 which would make the speedo read 4.4% higher than actual not to mention be outside the desired 3% max difference in tire circumference (amount you can get away not having to reprogram the speedo as I understand it). I could be wrong but this is what I read from the results on that website.

The tool suggested above sounds like it will correct the problem.
 

Grintch

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A little disagreement/out of range is usually not an issue. But don't stick with ~stock sizes up front and go 3 sizes bigger on the back if this worries you. Actually this is bad idea for other reasons as well.
 

kent0464

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I just replaced my pp1 wheels and tires with 255/35/20 fronts and 305/30/20 rears. So my speedo is off by 5 at 90 and 3.6 at 70 which is a pain but my bigger concern is the computer thinking that either the front or rear is slipping or something like that due to the difference in wheel spin and size ? Any expert opinions or experience with this and if so what's the answer ?

It’s just the speed sensors doing their job, can be corrected by tuner or forscan.
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