I'm looking at a set of rims that are 18" x 7.5 and come in an offset of 20, 35 or 40. They have the correct bolt pattern so will any of these fit and have proper clearance on turns? What is the original offset anyway?

 

I don't know off the top of my head, but its very easy to calculate.

Offset = (rim width in inches + 1 inch)/2 - (backspacing in inches)

**1 inch = 25.4 mm
** Rim width is measured from bead to bead, whereas backspacing is measured from the outer edge of the inboard flange, which is why you need to add the extra inch to the rim width... because the typical distance between outer edge of the flange to the bead is 1/2 inch on each side.

Example... If you have an 8 inch wide rim with 4.5 inches of backspacing:

Offset = (8 + 1)/2 - 4.5
Offset = 0

 

figure out what your stock wheel is, then use this to figure which you'd want.

http://www.rimsntires.com/rt_specs.jsp