Bike & Riding

Tyre Size Calculator: Compare Two Tyre Sizes and Speedometer Error

Free tool · Updated October 2, 2026 · By CouponTalk

Your result

Enter your measurements to see your result.

Quick answer

A tyre marked 185/65 R15 is 185 mm wide, its sidewall is 65% of that width (120 mm), and it fits a 15-inch rim. Overall diameter = rim × 25.4 + 2 × sidewall, so it is 621 mm tall. Keep a new size within about 3% of the original diameter. A 195/60 R15 is 615 mm, about 1% smaller, so at a true 60 km/h your speedometer shows about 60.6 km/h.

How to use this calculator

Read the size on your current tyre's sidewall, for example 185/65 R15.
Enter the size you want to change to.
Keep the difference within ±3% and check your car manual for approved sizes.

Tyre size chart

Common Indian car tyre sizes
Tyre size	Sidewall	Overall diameter	Distance per turn
155/80 R13	124 mm	578.2 mm	1.816 m
165/80 R14	132 mm	619.6 mm	1.947 m
175/65 R14	114 mm	583.1 mm	1.832 m
185/65 R15	120 mm	621.5 mm	1.952 m
195/55 R16	107 mm	620.9 mm	1.951 m
205/55 R16	113 mm	631.9 mm	1.985 m
215/60 R16	129 mm	664.4 mm	2.087 m
235/65 R17	153 mm	737.3 mm	2.316 m

Check your owner's manual for approved tyre sizes, and keep the same load and speed rating or higher.

Frequently asked questions

Can I fit a wider tyre on my car?

Usually one size wider is fine on the same rim, if the diameter stays within 3% and the tyre does not rub at full lock. Check your owner's manual and insurer.

Does tyre size change mileage?

Wider tyres have more rolling resistance and can cut mileage slightly. A larger diameter also makes the odometer under-read, so calculated mileage looks lower.

What does R mean in a tyre size?

R means radial construction. The number after it is the rim diameter in inches.