I am getting my GT 17" rims/tires this week and the lot where I bought my car told me that they had no way of re-calibrating the speedometer but I could probably take it to a performance shop, BUT that if it messes up anything with the computer system then my whole warranty is voided. Has anyone done this or does anyone know the speed difference from 16's to 17's? Thanks!

 

I believe your local Ford dealer can re-flash your car for just the tire size change... No voided anything... Not sure what they would charge...

I don't think anything will void the ENTIRE warranty... The dealer must show that the problem was caused by the aftermarket part.... So if my power windows break they can't say it's not covered because I put a CAI on... Alos with the tuners... you can put the car back to stock before you take it in...

 

You can use the tire calculator to figure out if there will be any difference with you new set up. When I bought my 17 inch Bullit's and Nitto 255/50/17 tires there was almost no change in the speedo.

http://www.discounttire.com/dtcs/home.do

 

Since you are getting complete takeoff with rim & tire you won't need to recalibrate, the difference in diameter is so minimal you won't notice a change
The stock 16's have a fatter sidewall to remain 27" overall diameter, and basically with 17's the sidewall is thinner to still remain close to the 27" diameter.