Tyre Size Comparator

How this calculator works

A tyre size like 245/45 R17 tells you three things: the tyre is 245 mm wide, the sidewall height is 45% of that width, and it fits a 17-inch rim. From those three numbers the overall diameter is fixed: rim diameter plus two sidewalls. Everything on this page (circumference, revolutions per mile, speedometer error) follows from that single figure.

Why speedometer error happens

Your car doesn't measure road speed directly. It counts wheel rotations and multiplies by the rolling circumference it was calibrated for. Fit a larger-diameter tyre and each rotation covers more ground than the car assumes, so the speedo under-reads: it shows 70 mph while you're actually travelling faster. A smaller diameter does the opposite. UK regulations require a speedo never to under-read true speed, which is why manufacturers calibrate them to read slightly high from the factory. A modest diameter increase often just cancels that built-in margin, but a large one can make your displayed speed genuinely lower than your real speed.

The 3% rule of thumb

The widely used guideline is to keep the new overall diameter within about ±3% of the original. Inside that window, speedo error stays small, gearing feels unchanged, and ABS, traction control and (on automatics) shift behaviour are unaffected. Beyond it you risk arch rubbing at full lock or full compression, inaccurate odometer mileage, and on some cars warning lights from systems that compare individual wheel speeds. If you're changing diameter deliberately, for gearing or ride height, that's a valid choice, but do it knowingly.

Upsizing wheels without changing diameter

The classic reason for using this tool: fitting bigger rims while keeping overall diameter the same. Each extra inch of rim needs roughly 25 mm less total sidewall, which usually means dropping the profile by 5 to 10 points and sometimes adjusting width. For example, 245/45 R17 → 245/40 R18 keeps the diameter almost identical, while 245/45 R17 → 245/45 R18 adds over 25 mm of diameter and pushes past the 3% window.

What this tool doesn't check

Diameter is only half the fitment question. Wheel offset (ET), width and spoke clearance over brake calipers are separate issues this calculator can't see, and load and speed ratings must still meet or exceed your car's requirements. Always confirm the full fitment for your specific vehicle.