I just went from P245/70R17's to LT285/70R17's. I 'm sure there will be some difference in the speedometer reading, but is it enough to warrant doing anything (if anything can be done) about it? I have a 05 Ram 1500 QC 4X4, so there's no Superchipsprogrammer available. Can this be done at a dealership (if needed)? Also, should I reset the PCM for any reason?

 

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

This little tool will help you calculate the overall diameter change, as well as the % speedometer change. You're approximately 7.2% slow. The actual amount will vary +/- 0.5% or so depending on the actual # of revolutions per mile on the tire, which depends on things like how much the sidewall flexes as it rolls and varies from tire to tire in a given size. That should get you close enough though.

 

Some difference amounts to about 4.3 mph faster then what your speedo says......and its definitely enough to warrant doing something. You should be able to go to the dealership and have them re-calibrate your speedo. I dont know what, if anything, this will cost you.

 

Thanks, fellas. I had a feeling I was driving faster than the speedo said. Now I know I was 7.2% faster!

I will call the dealer tomorrow.

 

Hi,
What did you find out at the dealership regarding re calibrating your speedo?

Thanks!