i have a 99 z28 and i'm looking at some C6R rims off ebay the rears are 20X10 and the fronts are 19X9.5 and like the title says what tire size and offset would i need for these.

 

The offsets should be within the guidelines mentioned in the FAQ (see the second page). Since the tire for a 20" wheel is most likely going to end up being a larger diameter than stock, you want an offset that will put the 20x10" wheel pretty much "centered" in the wheel opening, probably about 54-56mm. Any less offset, and its going to be close to the fender lip. Any more offset and its going to put the tire close to the inner fender liner. A typical size would be a 275/30-20 (26.5" diameter) or a 285/30-20 (26.7"), both larger than the stock 25.7" tires.

For the fronts, you need to work in the range of 45-55mm. The tire sizes for a range of 19's is shown in the FAQ.

 

thanks!!