18" wheels

[techweenie]

In general, you can eyeball the inner and outer clearances around the wheels/tires you have now and extrapolate to the new wheels.

From what I understand, 11" rear is pushing the limit of the wheel well clearance, with some people opting to use a 265-section tire instead of the more normal-for-11" 295 section. That can vary with tire brand/model since they do have different 'shoulder' profiles.

Splitpin's explanation of offset measurement is perfect. If you are going from a stock 10" wheel with (for instance) 45mm offset, you may end up with a better fit at a different offset number. You can change 'effective offset' by using spacers -- as long as the wheel offset number is higher than your target offset number.

For instance, many OE 11" wheels have a 65mm offset. You may need a 15 to 20mm spacer to fit the wheel correctly in the wheel well... If you have the luxury of a lift and time to experiment, I've seen people make a cardboard template of a tire, tape it to the rim and mount it up to check clearances.