Not sure which forum this should be under but this one seemed the closest. My '02 has 3.31 gears and originally came with a 29" tall tire (235-70-16). The dealer I purchased it from installed a 31.6" tall tire (265-75-16) on it before I bought it but didn't re-flash computer to adjust the speedometer.
According to an online gear ratio/MPH calculator my speedometer reads 60 when I'm actually doing 65 (which a NYS trooper so nicely confirmed ). So I figure the online calculator is pretty accurate.
I have 31.8" tires (265-70-17) and 3.73 gears waiting in the wings. According to the gear ratio/MPH calculator with the gear and tire change the speedometer should read 60 when I'm actually doing 58.
Does this sound right? I think it does but I'm no expert and could be missing something. Also, if this is correct and it will only be off <2 MPH, is it worth buying a programmer or having the computer re-flashed? Thanks.

 

since both the axle ratio and tire size you are/ will be running is a stock config, the local ford dealer can recalibrate your comp for like $80. this will get your shift points back in line, as well as your odometer, and yes, it is worth doing.

 

If I can find a dealership that will do it for only $80 I'll probably do it eventually. Is the math right though? Will the speedometer, odometer, and shift points be pretty close to stock when I add the gear change to the tire change (end result being 3.73s and 31.8" tires)? Thanks.

 

Anybody?

 

I would get an entirely new tune, a custom VMP tune, it will accomodate for your bigger tires, better shifts, and release all the hidden power within the engine that Ford hides, and calibrate your CAI & and exhaust better.