Donaldson R F Tire Size Calculator
Compare two tire sizes (e.g. 225/45R17 vs 235/40R18) and see the difference.
Original tire
New tire
Circumference, sidewall and speedometer
Tolerance check
The 3% starting value is a widely quoted rule of thumb, not a legal limit or a figure I could confirm from a tyre maker or regulator (see the sources below). Passing this check does not mean a size is approved or will fit.
Load index and speed symbol
Sizes with a similar diameter
These are geometric matches only. Some may not exist as a tyre, and none has been checked for fit, rim width, availability or approval. A replacement must never have a lower load index or speed symbol than the original.
How the numbers are worked out
The overall diameter is the width × aspect ratio ÷ 100 for each of the two sidewalls, plus the rim diameter in inches × 25.4. The circumference is π × diameter, and revolutions per kilometre are 1,000,000 ÷ circumference in mm. A tyre with a larger diameter travels further per turn, so the speedometer and odometer read lower than the true speed and distance; a smaller diameter makes them read higher. The speed shown is your indicated speed × new diameter ÷ original diameter. Real speedometers also have their own calibration, so treat the figure as an estimate.
Load index and speed symbol. The two characters after the size, such as 91V, say how much load a tyre can carry and the top speed it is rated for. Manufacturers and the British Tyre Manufacturers' Association advise that a replacement tyre's load index and speed symbol must be equal to or higher than the original, and that the rolling circumference should stay comparable. This tool never accepts a lower value as suitable: if the new tyre is lower on either, it says so. Enter the marking as printed on the tyre, in the vehicle handbook or on the door-jamb sticker. Only load indices 70–120 and the speed symbols L to Y in the manufacturer's table are supported.
Before you buy. Different tyre and wheel sizes can rub on the body or suspension, need a different rim width or offset, and can affect ABS, traction control, pressure-monitoring systems and insurance. The manufacturer-approved sizes for your vehicle are the safest guide. Other tools you may want: Speed, Distance & Time, Fuel Economy Converter and Braking Distance Calculator.
Source / Last verified (26 September 2026): load-index (70–120) and speed-symbol (L–Y) tables — Michelin UK, tyre load rating and speed rating; replacement rules (load index and speed symbol equal or higher, rolling circumference comparable, UK law on load index, check with the vehicle or tyre maker) — British Tyre Manufacturers' Association, Replacing Car Tyres (reviewed 24 June 2014); speed rating equal or higher and rolling circumference "maintained as closely as possible" — Continental, how to up-size tyres; approved sizes and never a lower load index or speed rating — Michelin UK, can you change your tyre size. The ±3% tolerance has no numeric source among these: they say only "comparable" or "as closely as possible", so the 3% is an adjustable rule of thumb from tyre retailers, not a confirmed manufacturer or legal figure. The diameter formula is standard tyre geometry.