There's not a more sickening feeling than pulling out of the tire shop $1,200 lighter only to feel the sharp stab of your big, new rubber rip into your sheetmetal. A close second is hitting the twisties off-road and feeling your knobbies manhandling your suspension and frame as your axles articulate and you turn the wheel.
If only there was some way to check what size tire you could really fit under your vehicle before you buy. Well, if you've got access to such high-tech tools as a tape measure, a straight edge, and a dirt berm there is. Before you go and pony up for that new set of wheels and tires, take an hour to measure just how, where, and why your new tires may make contact. Just about every tire manufacturer lists tire dimensions on its Web sites. You can look up your dream tire's installed height and width on the company's recommended width rim. Then you can play with rim backspacing to put the tire exactly where it needs to be inside the wheelwell. Here are some things to check out.
A close look at this photo...
A close look at this photo will show where the tires rubbed on the inner fender with the suspension at full compression and on the spring with the tires turned full lock. In this case, clearance was gained by adding a set of 11/4-inch wheelspacers between the stock 15x6 rims and the axle. Instead of wheel spacers, we could have run a different wheel with less backspacing.
Like the rear, use a straight...
Like the rear, use a straight edge or small framing square to determine where the front tires may make contact with the body, as well as with steering and suspension components. It helps to mark the centerline of your tire, then divide the width of your dream tires to see how far out they will stick in each direction. Flex the suspension on an obstacle to its maximum capability, then check your measurements around the entire wheel opening with the wheels turned straight and full lock in both directions.
One of the biggest hang ups...
One of the biggest hang ups in stepping up the tire size is suspension and steering contact. Here, we see that going from 31s to 35s with the stock backspacing would place the edge of the new tires right into the spring hard. To compensate for this much tire with this narrow axle, we'd need to decrease the rim backspacing by at least 1.5 inches to move the edge of the tire away. However, doing so would then place the outside of the tire in contact with the fender. The solution is more lift, some sheetmetal cutting, or a slightly smaller tire.
Even if your tire will fit...
Even if your tire will fit under your wheelwells it may make contact as the axle articulates. When activated, the inside top of the tire is forced closer to the body. The taller the tire, the closer it will come to the inner fender. Determine how much taller the tires you want to run are than what you have, then with the suspension flexed, use a straight edge and tape measure to find where they may hit. Usually, wheel backspacing can be played with to space the tires farther away from the body. However, moving the tires too far away from the inner fender may cause contact with the outer fender opening. This is where more lift or the saber saw comes into play.