DIY check tire size setting?

[Teligence]

Is there a way to check what tire size is programmed into the PCM? I bought mine used and the tires aren't original size, so I wanted so see how much of a variance there is to determine mileage/speed accuracy. It may be that it's already been adjusted to my current tire size, but I'd like to verify...

 

[usaf2006]

Depends on what it came with from the factory . Your local dealer may tell you for free, but I doubt it.

 

[crash68]

verify your speed against a GPS, the speedometer should read within about 1 mph. Probably best to check it on the freeway at 70 mph

 

[justin13703]

It won't be off much. With my 35s, at 70 mph the speedo is only off by 3-4mph. Best bet is to check it with a gps. There is no way to know for sure what is programmed into the computer unless you program it yourself with a tuner.

 

[HemiLonestar]

DRB for absolute verification of what the pinion factor is. If you're in MD I can check it for you.