Tire size calculator
Your stock tire against the one you want, in metric or flotation sizes. Height, width, speedometer error, ground clearance, and what the bigger tire and a lift do to your approach, departure and breakover angles.
Type your stock tire and the one you're thinking about.
Size
Nominal sizesSpeedometer
Until you recalibrateFix it with your truck's tire size setting or a programmer. The speedometer calculator gives you the number to enter.
Clearance and angles
Estimate, worked out from factory angles and overhangs. Bumpers, sag and tire pressure change the real numbers. Use at your own risk.
Brakes
Upgrades for
Brake torque here is the bigger rotor's longer lever times any extra piston area, against your stock front brakes. Marked kits get stopping back to stock or better on these tires. Where we don't have the piston size, the number is the rotor alone, so the real gain is higher. Links go to the maker or the best write-up we found.
A taller, heavier tire is a longer lever against the same calipers, so the truck needs more brake torque to stop in the same distance and the brakes run hotter on long descents. Numbers here are estimates for planning: they assume the stock pad friction and the same weight split front to rear.
Estimate of brake torque from rotor and piston sizes, not a brake test. Use at your own risk.
Do bigger tires hurt braking?
Yes. The brakes squeeze the same rotor, but the tire is a longer lever, so the same pedal gives less stopping force at the ground. Going from a 31.6 to a 35 inch tire takes about 11% more brake torque to stop the same, before the extra weight of the bigger tire. Higher-friction pads, bigger rotors and calipers with more piston area all win it back.
Do bigger tires hurt traction control too?
On most modern 4x4s, yes. Systems like Toyota's A-TRAC and Crawl Control find traction by braking the wheel that spins, which sends torque across the open diff to the wheel with grip. A taller tire gives that same brake less bite, so the system has to clamp harder and can run out of brake before the spinning wheel stops. Bigger brakes give it that bite back, and brakes stronger than stock should in theory make it work better than it did from the factory, as long as the system isn't already holding the wheel still. Owners who upgrade often say it feels more decisive on the trail. Lockers do the same job mechanically and don't depend on the brakes at all.
How the numbers work
How do you read a metric tire size?
In 285/70R17, 285 is the section width in millimeters, 70 is the sidewall height as a percentage of that width, and 17 is the wheel diameter in inches. The overall diameter is the wheel plus two sidewalls: 17 + 2 × (285 × 0.70 ÷ 25.4) = 32.7 inches.
How do you read a flotation size?
In 35x12.50R17, 35 is the overall diameter in inches, 12.50 is the width in inches and 17 is the wheel. These are nominal: most 35x12.50s measure between 34.3 and 34.8 inches mounted. For an exact speedometer number, measure yours and type the diameter, like 34.5.
Does a lift raise my differentials?
No. Only taller tires raise the axles, and only by half the diameter gain, because the axle sits at the center of the tire. Going from a 31.6 inch tire to a 33.8 adds about 1.1 inches under the diffs. A suspension lift raises the frame, body, bumpers and skids, which is why it helps approach, departure and breakover angles but not diff clearance.
How are the new angles worked out?
Each angle is a line tangent to the tire that touches the lowest point of the bumper or belly. From the factory angle, the tire size and the overhang we work out how high that point sits, raise it by the tire and lift gain, and solve for the new tangent line. It assumes the factory bumpers and skids. A high-clearance bumper changes the overhang and the height, so enter your own figures.
How close is it to real numbers?
We checked it against factory packages that add tires and lift. Going from a Wrangler JL Rubicon to the Xtreme Recon (35s and 1.5 inches of lift), it lands within a degree of Jeep's published angles. Where a maker doesn't publish overhangs, we estimate them from overall length, and the page says so.
How far off will my speedometer be?
By the ratio of the two diameters. Going from 31.6 to 33.8 inches is 7.0% taller, so at a true 65 mph the speedometer reads about 61. Your odometer and fuel economy read low by the same 7% until you recalibrate.
Do I need to regear?
Taller tires act like a numerically lower axle ratio. Up to about 5% most trucks feel fine. Past 7% or so, especially loaded, you'll notice it on grades and in top gear. The regear calculator shows highway RPM on each ratio and the one that gets you back to stock feel.
Use at your own risk. These calculators are here to help you plan and to inform your decisions, not to sign off on a build. Many numbers are estimates worked out from factory figures, and some factory figures are not yet confirmed. Check every number against your own truck, its manuals and a qualified shop before you buy parts, cut, weld, lift, regear or drive on it. Long Range Desert Tools is not liable for anything you do based on these calculators or their results. The full terms are in our policies.