Tire Size Calculator
Compare your current tire size with a new one before you buy. Enter metric sizes like 275/65R18 or flotation sizes like 35x12.50R20 to see the difference in diameter, width, sidewall height, revolutions per mile and speedometer reading.
| Spec | Current | New | Difference |
|---|---|---|---|
| Overall diameter (in) | 32.07 | 35.00 | +2.93 |
| Section width (in) | 10.83 | 12.50 | +1.67 |
| Sidewall height (in) | 7.04 | 7.50 | +0.46 |
| Wheel diameter (in) | 18 | 20 | +2 |
| Circumference (in) | 100.77 | 109.96 | +9.19 |
| Revolutions per mile | 629 | 576 | -53 |
| Ground clearance change (in) | - | - | +1.46 |
More than 3%: expect a noticeable speedometer error and check clearance (leveling or lift kit).
Actual speed when your speedometer shows 60 mph.
What the results mean
The overall diameter decides whether a tire fits in the wheel well and how much ground clearance you gain: half of the diameter difference is the extra clearance under the axle. A wider tire can rub on the suspension or fender at full steering lock, so check wheel offset too.
Frequently Asked Questions
+How do I read a tire size like 275/65R18?
275 is the tire width in millimeters, 65 is the sidewall height as a percentage of the width, R means radial construction and 18 is the wheel diameter in inches. Sizes like 35x12.50R20 are in inches: 35 inches tall, 12.5 inches wide, for a 20-inch wheel.
+How much can I change my tire size?
Staying within about 3% of the original overall diameter keeps the speedometer, ABS and transmission shift points close to factory. Larger changes are common on lifted trucks but usually need a speedometer recalibration.
+Will bigger tires fit my truck without a lift?
On many stock half-ton pickups, tires up to about 32 to 33 inches fit with little or no rubbing. Bigger tires usually need a leveling or lift kit and the right wheel offset.
+Do bigger tires change my speedometer?
Yes. Taller tires cover more distance per revolution, so the truck travels faster than the speedometer shows. The calculator shows your actual speed when the speedometer reads 60 mph.