Calculator
Tire size comparison
Compare any two sizes. Metric and flotation both work. Pick your vehicle anywhere on the site and it also tells you which lift height clears the new size.
| Measure | 275/65R18 | 35x12.50R20 | Change |
|---|---|---|---|
| Overall diameter | 32.1" | 35" | +2.9" (+9%) |
| Section width | 10.8" | 12.5" | +1.7" |
| Sidewall | 7" | 7.5" | +0.5" |
| Wheel diameter | 18" | 20" | +2" |
| Circumference | 100.8" | 110" | +9.2" |
| Revs per mile | 628 | 576 | -52 |
| Ride height | — | — | +1.4" |
| Speedo reads 60, actual | 60 mph | 65.4 mph | +5.4 mph |
Diameters are nominal, computed from the size. Real tires measure a little smaller mounted and loaded. Most people stay within 3% of the original diameter unless the speedometer gets recalibrated.
What changes when you go up a size
Diameter is the number that matters for clearance, gearing and the speedometer. Width is what matters for the wheel it mounts on and for rubbing at full lock. Sidewall is what you feel: more sidewall rides softer and protects the wheel off-road, less sidewall turns in sharper.
Revs per mile drops as the tire grows, which is why a taller tire feels like a taller final-drive gear: the engine turns fewer times for the same road speed. Big jumps on a truck that tows usually call for re-gearing, which is a separate conversation we are happy to have on a quote.
The clearance panel, when your vehicle is picked, comes from the same lift table the fit guides use. It assumes a stock-width wheel at stock offset. Wider wheels with less offset push the tire outward into the fender and can take back an inch of that room; the package builder accounts for the actual wheel.
Questions people ask
- How do I read a tire size like 275/65R18?
- 275 is the section width in millimeters, 65 is the sidewall height as a percentage of that width, R means radial and 18 is the wheel diameter in inches. Overall diameter = 18 + 2 x (275 x 0.65 / 25.4) = about 32.1 inches.
- How do I read 35x12.50R20?
- That is flotation sizing, used for off-road tires. 35 is the overall diameter in inches, 12.50 is the section width in inches and 20 is the wheel diameter. A 35x12.50R20 and a 315/70R20 are close to the same tire.
- How much bigger can I go without the speedometer being off?
- Every percent of diameter change is a percent of speedometer error. Within 3% most people live with it; past that, the speedometer, odometer and some driver aids drift enough to recalibrate. Many trucks can have the tire size set with a programmer.
- Does a bigger tire raise the truck?
- By half the diameter change. Going from 32 to 35 inches raises the whole vehicle about 1.5 inches at the axle; it does not change suspension travel or fender clearance, which is what decides whether it rubs.
- What is the biggest tire that fits my truck?
- It depends on the lift, the wheel width and offset and how much trimming you accept. Pick your vehicle on this page and the calculator shows the diameter that usually clears at stock, leveled and lifted heights from our fit guides; the package builder checks a specific wheel and tire together.