Currently I got some Bridgestone Potenza RE730 225/45/17 wrapped around Type S rims. I was thinking of getting some Potenza RE040 215/45/17. Does anyone know about these tires and is the decrease from 225 to 215 a bad idea? Please give me your thoughts. Thanks

 

you want to keep the diameter about the same. check out this page http://auto.howstuffworks.com/tire2.htm for a good explanation

At 225/45/17 you have 24.972 diameter
At 215/45/17 you have 24.618 diameter

The change in diameter effects the speedometer and odometer.

 

Is that a big difference in diameter? It seems small. But I was wondering if the decrease in size is ok and if it will still fit around the Type S rim

 

Too small. 225/45/17 is already too small (but within the range of acceptable).

You need to make your margin of error around 25.68 inches diameter.