All Tires & Wheels tools
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Tire Size Calculator: Compare Diameter and Speedometer Error
Compare two tire sizes and see the diameter change, real speed and revolutions per mile.
About these tools
This section covers tire size, speedometer error, load and pressure. A wrong tire size changes your speedometer reading, gearing, ride height and fuel use. These tools let you compare tire sizes, check how a swap affects the speedometer, and work out sensible pressures before you buy or fit anything.
Every tool in this section comes with a short guide that explains the formula, the typical mistakes and when to ask a tire shop instead of trusting a calculator.
Common questions
Will a different tire size change my speedometer?
Yes. A taller or shorter overall diameter changes how far the car travels per wheel turn, so the speedometer reads high or low. A tire size calculator shows the percentage difference.
Is the pressure on the door sticker the same as the one on the tire?
No. The sticker on the door jamb is the vehicle maker recommendation. The number moulded on the sidewall is the tire maximum, not a target.
Can I use these results for load rating decisions?
Treat them as a guide. Always confirm load index and speed rating against your owner manual or a tire professional.