Quote:
Originally Posted by subaru604
Okay so the 17x7 I understand, thanks for explaining.
Offset, I'm confused. I'll research on that.
But the 225/40/18, I thought that the tire width was 7 and the height was 17? Isn't the size of the wheel 17x7? Why is it 18?

i think that was just an an example.
on a 17 x 7 inch rim, you could run a 215/45/17 tire. 215 is the width of the tire in milimeters, 45 in the percentage of how the tall the side walls are compared to width, so the tire in this case is 45 percent of the 215mm width. and 17 is the diameter of the rim.
so on a 18 x 7 inch rim, you could run a 215/35/18. all the numbers are the same but 18 and 35. 18 is the diameter of the rim. but why 35? because if you think about it. you want to keep the diameter of the tire, not rim, close to stock. so you would want a skinnier tire on a larger rim. and vice versa. the smaller the diameter the rim, the taller the side wall of the tire, the taller the rim, the skinnier the side walls.