Every month we get letters and emails from new 4x4 owners (or should we say owners new to 4x4s?) who want to know what size tire will fit on their truck. Plenty of charts are available to help determine what lift is needed to clear what tires, but what if you want bigger tires without a lift?
There are many reasons to make these measurements yourself. Suspension components can wear out, and maybe you already have a lift and just want new, slightly taller rubber. Taking the time with a tape can show you where to trim body or bumpers in case you'd rather not (or can't afford to) lift your 4x4.
Step By Step
Step one is getting correct measurements of your current tire width and height. Removing the tire and using a level as a straightedge makes this easier. Also, measuring wheel backspacing is important in case you plan on getting different wheels.
With the tire back on your truck, measure the distance from the tire to everything around it at ride height. This includes the front, top, and back of the wheelwell opening as well as the bumper-to-rubber distance if it's close. Do this for each wheel corner, as things may not be as square as you assume.
Next, turn the front wheels to full steering lock in each direction and measure again. Note the tire-to-suspension (leaf spring or links and mounts) distances as well as all the others previously mentioned. Notice if the tire gets closer at the edges, keeping in mind tire widths and backspacing. Also measure the distances of steering components because changing tire width may affect these distances.
Measure the distance from the axletube to the bumpstop and add the bumpstop's length, as they can crush to near-flat under extremely abusive off-roading like jumps. Though the bumpstop may not go flat, this adds in a slight safety margin.
Now the fun part. Find a deep ditch or gully and drive your truck in it at an angle (an RTI ramp works well also). This will twist up the suspension to near full bump. If you get one front tire and the opposing rear tire down in the ditch and the suspension flexed up so the up axle ends are at full compression, then it's time to set your parking brake and remeasure from the tire to any close hard parts.
When articulated, the tires lean over into the wheelwell and may get closer than a straight vertical compression. Turn the wheels and repeat the measuring, once left, then right. Then come through at an opposite angle to flex up the opposing tires and repeat with the tires steering straight, turned to the left, and turned to the right.
By writing down all these distances you can start to picture where your tires will hit first. It might be the front bumper, the lower valance, or a suspension link. Some of these you can easily trim, while trimming others might not be so wise (sheetmetal is more expendable, suspension less so). Also you'll know how much clearance your current tires have and whether wider or taller tires will cause problems.
If the closest thing that will hit is 6 inches away and you know the suspension will move up 4 inches before full compression (bumpstop bottomed out completely), you have roughly 2 inches of clearance. So double that and you've found your new tire size. You could probably add 4 inches of diameter to your tires max-but err on the short side, maybe plus 3 inches. Or get out the Sawzall and start chopping.