Compare a stock and a new tire size: diameter, width, sidewall, circumference, revs per mile and speedometer error. Optionally see the effective gear ratio and engine RPM at speed. Runs in your browser.
FAQ
How is tire diameter calculated from the size?
For a metric size like 225/65R17, sidewall height = 225 mm × 65% = 146 mm (5.76 in), and diameter = rim (17 in) + 2 × sidewall, or about 28.5 in. Flotation sizes like 33x12.50R15 state the diameter (33 in) directly. These are nominal figures; actual tires vary by brand and tread depth.
How much bigger can I go without problems?
A common rule of thumb is staying within about ±3% of the stock diameter, but clearance, load rating, ABS/stability systems and the manufacturer's guidance matter more. Check fitment data for the specific vehicle.
Why does my speedometer read wrong with bigger tires?
A larger tire travels farther per revolution, so the vehicle goes faster than the speedometer shows. The table shows actual speed at each indicated speed, assuming the speedometer was accurate on the stock size.
What does effective gear ratio mean?
Larger tires act like a numerically lower (taller) axle ratio. The calculator shows the effective ratio with the new tires and the ratio you'd need to restore stock RPM.
Is this exact?
No. It uses nominal dimensions. Real tire diameter, tire growth, loaded radius and converter slip all vary, so use it for comparison and customer explanations, not for calibration.