Tire Size Calculator

Compare two tire sizes — overall diameter and the speedometer error a swap would introduce

Comparing two tire sizes — their overall diameter and the speedometer error a size swap would introduce — matters because tire size is encoded in a compact alphanumeric format that doesn't make actual physical diameter immediately obvious, and swapping to a different size changes the effective distance covered per wheel rotation, which directly affects speedometer accuracy.

Why tire size codes don't make diameter obvious at a glance

A tire size code combines width, aspect ratio (sidewall height as a percentage of width), and wheel diameter into a compact format — but the actual overall tire diameter (which determines how far the vehicle travels per wheel rotation) isn't directly readable from the code without calculating it, since it depends on combining all three values together, not just reading off the wheel-diameter portion. Because the speedometer is calibrated based on the original tire size's rotation-to-distance relationship, fitting a genuinely different overall-diameter tire changes how far the vehicle actually travels per wheel rotation — a larger-diameter replacement tire causes the speedometer to under-read actual speed (the vehicle is going faster than displayed), while a smaller-diameter replacement causes it to over-read.

A worked example

Two tire size codes that look similar at a glance (differing in just one or two numbers) can correspond to a meaningfully different overall diameter once properly calculated — a size swap based on assuming the codes are "close enough" without checking the actual calculated diameter difference risks introducing genuine speedometer error, not just a cosmetic sizing change.

How this connects to your other vehicle calculations

Speedometer accuracy affected by a tire size change has a direct, practical consequence for trip calculations elsewhere on this site — the Fuel Cost Calculator and Mileage Calculator both rely on accurate distance figures, and a speedometer-affecting tire swap that goes unaccounted for can introduce a systematic error into odometer-based distance and mileage tracking as well.

Common mistakes

Assuming two tire sizes with similar-looking codes are close enough in actual diameter to swap without any speedometer impact, without actually calculating the diameter difference, is a common oversight — always calculate the actual overall diameter for both the original and replacement size before assuming a swap is speedometer-neutral, since the code's alphanumeric similarity doesn't reliably predict how close the calculated diameters actually are.