Speed difference calculator explained
Understand how a speedometer difference table works, what reference tire means, and how to interpret indicated mph vs estimated actual mph.
Reference tire matters
This site treats Tire 1 as the speedometer reference. The table estimates what true road speed might look like if you mounted Tire 2 while the vehicle still assumed Tire 1 rollout—useful for planning, not legal certification.
How to read the grid
Pick an indicated speed on the Tire 1 row, then read the estimated actual speed on the Tire 2 row. If Tire 2 has a longer circumference, actual speed is often higher than indicated at the same wheel RPM.
Frequently asked questions
Is the table exact?
Try the free tire size calculator
Open the interactive tire size calculator to compare diameters, see a wheel size comparison side by side, and review a speedometer difference table before you buy tires or wheels. It works like a quick tire fitment checker for geometry—not a substitute for a professional install bay.