Just bought some SSR integral wheels 17x7 +40 for the front, and 17x8 +45 for the rear. What tire size should I be using? Should I have two different sizes for the front and rear?

 

You should run 2 different size tires because the rims are different widths.

Such as a 225/40/17 on the front and a 255/40/17 on the rears. you want a wider tire for the wider rear rims.

 

Isn't a 255 for the rear too wide??

 

bump

 

Because you have a wider rim, you need a wider tire.

There are 2 things I don't have the answer to.
Will the tire rub?
Would the 225 be OK on the wider rim?

If a tire is on a rim that is too wide for it, the tread will not lay flat flat on the ground. All I can say is, pick a brand of tire and see if the manufacturer says it will work on the 8 inch rim.

The 225 & 255 numbers in the tire size are the width of the tire (the sidewall bulge, not the tread) stated in millimeters. 225 mm = 8.8 inches. 255 mm = 10.0 inches.