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Tire Size Calculator

Compare two tire sizes: diameter, width, sidewall, revolutions per mile and speedometer error.

Type the size from your sidewall: metric like 285/70R17 or LT285/70R17, or flotation like 35x12.50R17. Results are calculated from the size code; real tires vary slightly by brand.

Gearing and engine RPM

Top gear is your transmission’s highest gear: 0.67 on the Wrangler and Gladiator 8-speed automatic, 0.72 on the Wrangler 6-speed manual, 0.64 on the Bronco 10-speed, 0.651 on the 2024-and-later Tacoma 8-speed. Leave it at 1.00 to compare in a 1:1 gear, like most gearing charts.

Result

Enter both sizes

Current
New
MeasureCurrentNewChange
Diameter
Width
Sidewall
Revs per mile

When your speedometer shows 60 mph, you’re actually going – mph.

Reading a tire size

The size code LT285/70R17 121/118S as molded on a tire sidewall, with its seven parts numbered 1 to 7LT285/70R17121/118S1234567
  1. LT: a light-truck size, designed for pickups.
  2. 285: section width in millimeters.
  3. 70: aspect ratio. Sidewall height is 70% of the section width.
  4. R: radial construction.
  5. 17: fits a 17-inch wheel.
  6. 121/118: load index for single and dual rear wheels. Yokohama lists 121/118 as 3,197 lb single and 2,910 lb dual.
  7. S: speed rating, 112 mph.

Flotation sizes read in inches: 35x12.50R17LT is a nominal 35-inch-tall, 12.50-inch-wide radial for a 17-inch wheel. Real tires usually measure a little shorter; Yokohama lists its Geolandar A/T4 in 35x12.50R17 at 34.7 inches.

How it’s calculated

Metric sizes like 315/70R17 give the section width in millimeters (315), the sidewall height as a percentage of that width (70%), and the wheel diameter in inches (17). Sidewall height = 315 × 0.70 ÷ 25.4 = 8.7 in, and diameter = 17 + 2 × 8.7 = 34.4 in.

Flotation sizes like 35x12.50R17 give the overall diameter (35 in) and section width (12.50 in) directly, for a 17-inch wheel. That diameter is nominal: most 35-inch tires measure a little under 35 inches.

Revolutions per mile = 63,360 ÷ (diameter × π). True speed = speedometer reading × new diameter ÷ old diameter. Real tires vary slightly by brand and model, so check the maker’s spec sheet for the exact diameter of the tire you’re buying.

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