Big wheels combined with a relatively small increase in tire size means the sidewall height of the tire is reduced. While this does decrease sidewall deflection on the highway, it does so at the expense of a rougher ride. In the dirt, you have less margin for keeping the rim from being damaged at low air pressures, and the rim rides that much closer to offending obstacles that can destroy a bead. Larger wheels usually add more unsprung weight on your axles and work your brakes harder.
To measure backspacing on a wheel, place a straight-edge across the outside surface of the inboard bead seat (not the inboard flange edge). Then use a ruler to measure the distance from the hub mating surface on the back of the wheel (shown here) to your straight-edge. This dimension is the wheel backspacing.
Offset is a bit trickier to measure, but can be calculated from a backspacing measurement fairly easily. It is defined as the backspacing minus the centerline. The centerline is measured as one-half of the overall outside width of the wheel. If the centerline number is greater than the backspacing number, you have a negative offset. This wheel shows it has a -6mm offset dimension. Offset specs are often expressed in millimeters, so you can convert millimeters to inches by dividing by 25.4.
A good rule of thumb when choosing tires you want use off-road is to never have a wheel that is more than half the diameter of the tire (i.e. 40s on 20s, 35s on 17s, or 30s on 15s). This allows you to keep sidewall height down to a point where the sidewall is not excessively squirmy, yet retain good flex and sidewall distance to soak up bumps that severely deflect the tire carcass.
When choosing a tire, you’ll first want to determine size, whether it be the same as the stock size or something larger. Metric passenger car tires (sometimes used on light trucks) sizes are designated with a P prefix and light-truck tire sizes designated with an ‘LT’ prefix.
This tire has a designation of LT285/70R17. The 285 number is the nominal overall tire width in millimeters. R designates radial-tire construction. The 70 designates the cross-section aspect ratio of the tire, and the 17 is the tire rim size in inches. To determine the overall tire height, we simply figure twice the sidewall height (2 x 285mm x 0.70 aspect ratio) and add that number to the rim diameter to get the approximate tire diameter. In this case, the nominal tire height turns out to be about 33 inches tall.