Tiremath: Tire Size Calculator

Tiremath shows you what changing tire size actually does.

Type the size on your sidewall and the size you are thinking about. Both tires are drawn to scale — overlaid on the same ground line, or side by side — so you can see the difference instead of imagining it.

WHAT CHANGES
• Diameter, width, sidewall height and circumference, each with the change as a percentage
• Ride height — how much higher or lower the car sits
• Revolutions per km, or per mile
• Speedometer error at 50, 80, 100 and 120 km/h (30, 50, 60 and 70 mph in imperial)
• Load index in kg or lb, and speed rating in km/h or mph, when the size includes them
• A plain verdict: within the usual range, borderline, or outside it

SIZES THAT FIT
Tiremath lists the sizes that roll about the same distance as the one you have — the same wheel, one inch up, two inches up, one inch down — with how far each moves the diameter. Tap one to compare it.

REFERENCE TABLES
• The rim widths your tread is meant to go on
• The full load index table, in kg or lb
• The full speed rating table, L to Y, in km/h or mph
• Your last ten comparisons

TYPE IT HOWEVER YOU LIKE
225/45R17, P225/45R17, 225-45-17, 225 45 17, 225/45ZR17, LT265/70R17 and flotation sizes like 33x12.50R15 all read first time, with or without a service description like 94W. If something cannot be read, Tiremath says why — "that is only the width", "the wheel size is missing" — instead of showing nothing.

METRIC OR IMPERIAL
Millimetres and km/h, or inches and mph. Your choice is remembered, and it changes every table at once.

NOTHING LEAVES YOUR PHONE
Every figure is worked out on the device. There is no account, no sign-in and no tire database to download — the app works with no connection at all. It asks for no permissions beyond what ads need: no camera, no microphone, no location, no files.

A GUIDE, NOT A GUARANTEE
Tiremath does the arithmetic. Whether a size suits your car depends on your wheels, your arches, your brakes and your vehicle handbook. Check with a tire professional before fitting anything.