When I hade my PCM tuned..they also correctd the tire Calibration for the tires I have on (285s)..Stock tune is for (275s)

 

You are correct about the speedo and odometer being off. At60mph on the speedo I'm actually doing 65mph. This has been verified by my gps and by the little radar/speed display that the cops set up around town to remind drivers that they are speeding. Unfortunately, the loss of approx. 4mpg is correct; this also verified by gpswith a couple of tanks of gas on my latest trip to Colorado. I haven't bothered to check around town mileage. Like I said before....If I was worried about mpg I would have bought a hybrid.[:-]

 

PCM tune is next on my wish list...... right after the K&N filter.

 

http://www.net-comber.com/tirecalc.html

from 285's to 315's
at 65 mph the speedo shows 61.672
and ground clearance changed 0.886


from 265's to 315's
at 65mph the speedo shows 59.453
and ground clearance changed 1.476