Tire RulerHow to read a tire size
A metric tire size like 265/70R17 has three numbers:
- 265: section width in millimetres (10.43 in).
- 70: aspect ratio. The sidewall is 70% as tall as the tire is wide: 265 × 0.70 = 185.5 mm (7.30 in).
- R17: radial construction, for a 17-inch wheel.
Overall diameter = wheel + two sidewalls = 17 + 2 × 7.30 = 31.6 in. Flotation sizes such as 33x12.50R17 already give the diameter (33 in) and width (12.5 in) in inches.
What the comparison tells you
- Diameter difference: the main number. Over ±3% usually means speedometer recalibration and possibly a lift or leveling kit.
- Ground clearance: half the diameter change, because the axle sits in the middle of the tire.
- Width: wider tires can rub on the suspension, frame or fender liner, especially at full lock.
- Revolutions per mile: fewer revolutions means a taller effective gear: less pulling power and lower RPM. Use the gear ratio calculator to regear.
Frequently asked questions
How do I convert a metric tire size to inches?
Width in inches = the first number ÷ 25.4. Sidewall = width × the second number ÷ 100. Overall diameter = wheel size + 2 × sidewall. For 265/70R17: 265 × 0.70 = 185.5 mm = 7.30 in, so 17 + 2 × 7.30 = 31.6 in tall.
What does the R mean in 275/60R20?
R means radial construction, which almost every modern tire uses. The number after it is the wheel (rim) diameter in inches.
Do LT and P tires of the same size have the same dimensions?
Yes. LT265/70R17 and P265/70R17 have the same nominal size; LT tires have a stronger construction and a load range letter. Real dimensions can still vary a little by brand and model.
How much bigger can I go without problems?
Tire shops commonly use ±3% of the original diameter as a limit. Beyond that, plan for speedometer recalibration and check clearance at full steering lock and suspension compression.
Why does my real tire measure a little different?
The calculator gives the theoretical size from the sidewall code. Actual tires vary by brand, tread depth, rim width and inflation, typically by a few tenths of an inch.