Tire size & fitment calculators that explain the math
Most tire calculators spit out a number with no context. These show the work — diameter, sidewall, speedometer error, and whether a size will actually fit your wheels — so you can decide with confidence.
Compare two tire sizes
The fastest way to see what changes when you go up or down a size.
Speedometer impact
With the new tire, here's your true speed when the dash reads:
Odometer reads low by 3.5% — every indicated 100 miles is really 103.5.
Shop 285/70R17 →All calculators
Rim & Tire Fitment
Which tire widths fit your wheel? See the recommended and acceptable range, visualized.
Tire Size Comparison
Side-by-side dimensions plus speedometer and odometer impact for any two sizes.
mm → Inches Converter
Turn metric sizes like 265/70R17 into inches, with every dimension both ways.
How to read a tire size
A tire size like 265/70R17 packs three measurements into one
string. The 265 is the section width in millimetres —
how wide the tire is at its widest point. The 70 is the
aspect ratio: the sidewall height as a percentage of that width, so a 70
means the sidewall is 70% of 265mm, or about 185mm tall. The
R means radial construction, and 17 is
the wheel diameter in inches.
That mixed-units quirk — millimetres for width, a percentage for the sidewall, inches for the wheel — is exactly why tire math trips people up. The full guide walks through it with worked examples, and the calculators above do the conversions for you.