Measurement Reality and Relative Gearing
After reading a sidewall, you might expect to have all the numbers you need. But every manufacturer's 31x10.50 or 245/75-16 are not the same size. Though section width of the tire (not including lettering or curb guards) is on the sidewall, tread width is not. The tread will be narrower than section width, though the difference usually decreases with profile.
Accurate tread width is often given in the tire-manufacturer's catalogs, along with data for true diameter, weight, section width, etc. The data listed are based on a specific rim width, though all acceptable rim widths should be listed.
Perhaps the most important numbers in the specifications are rpm (revolutions per mile) and static loaded radius. The rpm figure will allow you to make gearing calculations for 60 mph, when one minute equals one mile. This simplifies speedometer testing, changing to larger tires and axle ratios, and gives more accurate results than using mathematics and dimensions: C=D (C=circumference, D=tire diameter in feet), 5280/C=rpm, but this method doesn't consider that a tire is not a solid object-- it's an air cushion that changes radius as it revolves.
Static-loaded radius is the height of the tire from ground to center, at pressure, with a rated load on it. This figure takes into account the deflection of the sidewall that you see as a bulge, providing a better indication of potential ground clearance and a more precise number for mathematical computation. If you have no data or rpm figures, measuring your tire from the center of the hub to ground with the truck parked on it will enable a good estimate of tire rpm. From there, you can get an idea what will happen to your effective gear ratio when you swap to taller or shorter tires.