Tire Diameter Guide
Diameter decides fitment, gearing, and gauge accuracy.
Tire diameter equals rim diameter plus twice the sidewall height, where sidewall is width times aspect ratio divided by 25.4. A 265/70R17 works out to 31.6 inches and a 285/70R17 to 32.7 inches. Diameter is the key fitment number because it sets clearance, speedometer error, and effective gearing; width and looks are secondary.
The diameter formula
Diameter = rim + 2 x (width x aspect / 100 / 25.4). Every term is in inches after conversion. Run it once by hand to trust it, then let the calculator do it forever.
For 285/70R17: sidewall = 285 x 0.70 / 25.4 = 7.9 inches, diameter = 17 + 15.7 = 32.7 inches.
Why diameter rules fitment
Clearance is a diameter question: the tire must clear fenders, suspension, and frame at full steering lock and full bump. A 1.1-inch diameter gain, like our example, needs about 0.55 inches of extra clearance all around.
Gearing and gauges are diameter questions too. Revs per mile set the speedometer, and effective gear ratio scales directly with diameter.
Measuring a mounted tire
A tape measure across a mounted, loaded tire reads slightly less than the calculated diameter because the contact patch flattens. Measure unloaded or trust the formula for comparison.
For worn tires, subtract roughly twice the tread depth lost: a tire worn 4/32 inch is about 0.25 inches smaller in diameter than new.
Skip the arithmetic
Get any tire's diameter on the calculator.
Tire diameter questions
How is tire diameter calculated?
Diameter = rim + 2 x (width x aspect / 100 / 25.4). For 265/70R17 that is 17 + 2 x 7.3 = 31.6 inches.
Does tire diameter affect gas mileage?
A larger diameter effectively raises gearing, dropping highway RPM, but the added weight and wider contact patch usually offset the gain. Net effect is small and driving-dependent.
How do I measure tire diameter at home?
Lay a tape across the tire's widest point while it is off the ground, or just compute it from the size. A mounted, loaded tire measures a bit small because the contact patch compresses.