I'm orderly new tires for my 02 GT and can't decide between sizes: 255/40 or 255/45 on a stock 17" GT rim. Any suggestions or past experiences would be helpful.
i'd go for the 255/45's. they're a little taller so they'll fill up the fender gap a little better and the ride should be a little softer compared to stock.
255/40's i think are smaller than stock (height) so it'll probably look weird. 275/40's are closer to the stock size. i have the tire size formula around somewhere
255/45/17 or 255/40/17 the only difference is the middle number, the aspect ratio. It means the sidewall "height" is 45% or 40% (depending which you go with) of the width (255) of the tire. I would go with the 45 series as it will (as said above) fill out the wheelbase better.
The more sidewall you have the more flex you deal with while cornering. If ride quality and drag launches are more important, go with the 45s. If you want a little sharper steering response and a bit more road feel and harshness, go with the 40s. Keep in mind that the larger sidewall ratio and wider tire will make a larger tire overall than the stock. Overall tire height will go from 25.68" stock to 26.04" with the 45s, and shrink to 25.03 with the 40s. This will affect your speedometer (only very slightly) and gearing--making your car faster with the 40s (technically; all other variables must be identical). Again, this choice is really up to you....but I would go with the 40s.