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Wheels & Tyres
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.
Both tyres rolling at the same road speed
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, so stay close to the original diameter.