[jeremy]

i have 15" 195/60 15 sized tires stock now and i am looking to upgrade, the question i have is: how large a wheel/ tire can i go before my speedometer becomes inaccurate, or i have gear to tire problems, if any? i am not really intending to go past 17" wheels, so i suppose i should be asking : what is the difference between 16" and 17" wheels on a stock pzev that comes equipped with 15" wheels?

 

[Egz]

Well, if you size the tire correctly, you won't have a problem.

For example, you can use a 205/50-16 tire, and be off by 0.6% too fast.
A 205/45-17 will be 0.2% too slow.

 

[jeremy]

is there any size/combination that will stay constant with the current calibration on the speedometer? if ford offers three sizes of wheels, is it a "+ or - 10mph" thing, or do they calibrate each car based on wheel size?

 

[Egz]

Those percentages will usully cause the speedo to be off by less than half of a MPH. Not noticible. Those are as close as you can get.

 

[Millertyme]

the digital read out of my speedo is 3~4 MPH slower than what my speedo reads. is that too much of a variance? but if you keep diameter of the tire the same (meaning lower profile tire on larger rim) you should be good