Did the math a little wrong. By adding a spacer, you are adding to the backspacing. Backspacing is the amount of distance the rim edge to the wms (wheel mounting surface). By adding a spacer on the wms location, you are essentially pushing the rim out farther, thus adding to the backspacing.
Offset is similar to backspacing, but it's just another way of stating it. Offset is related to the rim width, used as a centerline, then offset is either + or - to that centerline.
What I suggest, look for 16" rims. That way, you can get tires measured in metric sizes. They tend to be taller and skinnier in comparison to tires in the 15" rim comparable size. For example, 265/75-16 is a ~32" tall tire (~31.3"), 8.25" tread width, ~10" section width. In comparison to a 32x11.5-15, ~31" tall (usually low in the number), ~9+" tread width, and ~11.25 section width.
But do realize, anything taller will affect the performance of your vehicle. You may loose the 5th gear at highway speeds. It all depends on the rpm range of your vehicle as well as the torque curve. I would suggest to find out what ratios you have, research what other ratios are available, step up to a stock higher numerial ratio and just swap them in. For example, XL7's come with either 5.13 for auto or 4.30 manual trans. Take the 4.30s, swap in the 5.13's and be done. The Tracker/Vitara have other options for lockers and ratios than the XL7.
