So since the Potenzas suck i want better tires. I may be able to trade my tires to my boy who is gettin rid of his car. Ill get his tires for free. Only thing is theyre 215/40 17 tC i believe is 215/45. So will it work or will my speedometer be out of whack or something.

 

i think they should work...i have 215/40's as well but sittin on 18"

 

It will fit fine. The /40 means aspect ratio of the sidewall or percentage of the tread width the tire height is. In other words with a 215/45 your sidewall is 96.75mm or 3.8"and with a 215/40 your sidewall will be 86mm or 3.4".

Should not have a dramatic effect on your speedo if any at all. Not all tire manufacturers measure the same anyways so its not always exact.

 

Remember that it will also affect your gearing a bit. You will be cruising at a slightly higher RPM on the highway. This may not be a big deal, except the TC gearing

 

Yeah it's alittle smaller overall diameter, but it will work. The speedometer will definatley be off. I'm running a 225-45-17 Nitto winter tire on mine, with a 17x8 inch wheel for the winter.

 

I went to one of those online tire calculators and it said there would be a 3.4 percent change in diameter. it also said that staying safe means staying within 3 percent and that goin anymore percent can cause brake failure. Is it really that serious or should I do it. Remeber there is no goin back cause once we trade, his car with my tires is gone.