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Tyre Size & Speedometer Error Calculator
Compare your original (OE) tyre with a new size and see the exact speedometer error, rolling-diameter change, revs per mile and ride-height difference — before you fit them.
How tyre size affects your speedometer
Your speedometer doesn't measure the road directly — it counts how fast the wheels turn and multiplies by the rolling circumference of the tyres fitted when the car was calibrated. Change the tyre's overall diameter and that assumption breaks:
- Bigger overall diameter → the car travels further per revolution than the speedo expects → the speedo under-reads (you're going faster than it shows).
- Smaller overall diameter → the speedo over-reads (you're going slower than it shows).
Note that changing only the width or rim size while keeping the same overall diameter (a "plus-size" fitment) leaves the speedometer almost unchanged — it's the overall diameter that matters, which is why this calculator works it out for both tyres.
Reading a tyre size
A marking like 205/55 R16 means: 205 mm section width, sidewall height 55% of that width (≈113 mm), on a 16-inch rim. Overall diameter = rim (in mm) + two sidewalls.
The ±3% guideline
As a rule of thumb, keeping the new overall diameter within about ±3% of the original keeps speedometer error, gearing, ABS/traction-control calibration and arch clearance within sensible limits. This tool flags when you go beyond that.
Frequently asked questions
How do bigger tyres affect my speedometer?
A larger overall diameter covers more ground per revolution, so your true speed is higher than the speedometer shows (it under-reads). Smaller tyres make it over-read.
How much tyre size change is acceptable?
Keep the overall rolling diameter within roughly ±3% of the original. Beyond that you risk speedometer error, gearing changes, calibration issues and clearance problems.
Is the speedometer change legal?
In the UK/EU a speedometer must never read below true speed, and no more than 110% of true speed + 6.25 mph. Larger tyres make it under-read, which can breach this — stay close to the original diameter.