How much is my speedometer out?

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This question is asked loads of time on the forum. When you change your wheels/tyres your speedometer will show an incorrect reading. This little calculator will show you what difference it will make to your readout. REMEMBER: Your speedometer will not be accurate in the first place.

Select the tyre size you currently have, and then the size you’re interested in. Calculate each tyre size and then click on the “click to calculate the difference” button. It will show you all the rolling radii, circumferences, percentage differences and even speedometer error.

Current wheel/tyre	New wheel/tyre
145 155 165 175 185 195 205 215 225 230 235 245 255 265 275 285 295 305 315 325 335 / 30 35 40 45 50 55 60 65 70 75 80 85 R 12 13 14 15 15.5 16 16.5 17 18 19 19.5 20 21 22 23	145 155 165 175 185 195 205 215 225 235 245 255 265 275 285 295 305 315 325 335 / 30 35 40 45 50 55 60 65 70 75 80 85 R 12 13 14 15 16 16.5 17 18 19 19.5 20 21 22 23
Current RR: mm	New RR: mm
Current circumference: mm	New circumference: mm
Difference in circumference: mm or %
So when your speedometer reads 70mph, you’re actually traveling at mph