I don't have a tuner yet, but I was wondering what adjustments there are for tweaking the MPH? I think I've heard people say there are 2 things that can be tweaked, the tread length of the tire and the gear ratio, is that correct?
Is there also an adjustment for transmission gearing or is the speedo based off of driveshaft RPMs and the tach straight off the crank RPMs?
The reason I am asking is I was thinking that if there are enough adjustments you could essentially calibrate the speedometer to work with a faceplate that had a different scale, such as topping out at 160 rather than 140.

As an example, if you have 3.55 gears and stock tires (235/55 17") and you were able to adjust the gear setting through the tuner to 4.06, when the needle hits the angle that reads 140 on the stock GT faceplate the speed would actually be about 160 (assuming you have enough power to get here), so you could replace the stock faceplate with a custom one that tops at 160 rather than 140.

so, what granularity is there in the settings, and does what I've described sound feasible?

 

My xcal2 allows me to adjust for gears (can't recall the if there are set options or if you can input any ratio) and for tire revs per mile. Tire revs are adjustable in 10 MPH increments (i.e. 740, 750...). I'd set the gears to exactly what you have and then adjust the tire revs per mile to get you as close to true (GPS) speed as possible.