I can't speak for the 31.24" setting that you had - but as stated, that is a tall tire - something you would find on 4wd. This setting may have been right (as determined by the methods you mentioned) or it may have been wrong - either way, this is now irrelevant since you no longer have this tire.

But, I think your current setting of 28.75" is probably too low and overstating your mileage. Was 28.75" the stock setting for your truck? My '02 came with 255/70/16 stock and the diameter setting was 29.42 - that's a huge difference than what you are stating.

In order to truly detemine the correct setting to be used, you must measure the circumference of the tire in order to calculate the diameter value to be used (do not use the manufacturer value, it may be accurate, but it is not precise)

Two options in order to measure circumference:
1) Remove the wheel from the truck and use a cloth tape measure around the center of the tire (with the tire at the proper air pressure)
2) or, the easier method (physically). With the tire at the proper air pressure, mark the tire and ground where they are in contact. Roll the truck, so that the tire makes 1 complete revolution. Mark the ground using the mark previously made on the tire. Take a precise measurement of the two marks on the ground - this is your circumference.

Divide the circumference by Pi (3.141592654) in order to determine proper tire diameter to use for programming.

 

The heights I mentioned earlier were rough quesses, with me going outside with a tape measure. They were not the actually measurements used.

 

sorry to burst your bubble but using the tech2 to change the tire size in ABS<which I'm assuming has been done> does NOT change it in the pcm which is used for your speedo/odometer... it only changes the EBTCM calculation to determine wheel slip/lockup for the antilock brakes...to reprogram the tire size requires a reflash of the PCM, unless you never changed it from stock sizes in the first place then ignore my post haha

 

Really, so the Tech2 does not change the speedo? Supposedly that is what the guy did. I don't know anything about what a Tech2 does so maybe all of you are right the speedo may be off, or maybe it was off when I had the 20s, I dunno. I wonder if the Dealership did the same thing, when the 20's were installed.

Either way I am still getting better MPG. 11 gallons compared to 8.5 on the exactly same 190 mile drive on the same day.

 

I'm not doubting the better mileage at all ...with less weight<rolling mass> your bound to get better mileage. But yeah the only way the speedo can be changed is with a complete PCM reflash, if the dealer didnt charge you very much if anything at all they probably only changed the ABS module..being that we <dealer> have to get a special PCM program # ,called a VCI #, from GM and they charge us like 100$ for that alone if only the tire size is supported.. I work in a dealer and 90% of the techs/managers think that it changes the speedo when they do that so dont feel bad.

 

thanks for the input. I am guessing the dealer lied to me the two times I brought the truck for a speedo check. I bought my truck new, but they added the aftermarket 20's to get someone to buy the truck. I am thinking they never changed the speedo.

After reading some of the stuff yall said I went for a drive and recorded my RPM and Speed then I looked at one of the online RPM/Gear calculators and RPM/Speedo was pretty close to what it is now.

~65MPH @ ~1750 RPM.

 

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