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Gear Ratio & Speedo Error Calculator
Changing tyre or wheel size? Enter your old and new tyre sizes to see the speedometer error and how your effective gearing changes.
Original tyre
/R
New tyre
/R
Speedo error
+1.2%
speedo reads slow (you go faster)
At an indicated 60
60.7
real speed (mph or km/h)
Bigger overall diameter also lowers effective gearing (relaxed revs); smaller raises it. Full tyre size calculator →
How it works
Your speedo is calibrated to the original rolling diameter. A larger overall tyre makes the speedo read low (you're going faster than shown) and relaxes gearing; a smaller tyre does the opposite.
Want to see mods in action, not just the numbers? Try the free demo editor →