Tire size calculator

Current tire size


/ R

New tire size


/ R

255/30 R21 vs 305/25 R21 comparison results:

Old New Diff %
Overall Diameter 27.02" 27.00" -0.02" (-0.50 mm) -0.07% Smaller sign. Arrow down
Width 10.04" (255.00 mm) 12.01" (305.00 mm) +1.97" (+50.00 mm) +19.62% Bigger sign. Arrow up
Sidewall Height 3.01" (76.50 mm) 3.00" (76.25 mm) -0.01" (-0.25 mm) -0.33% Smaller sign. Arrow down
Rim Diameter 21" 21" 0" 0.00%
Ground Clearance -0.01" (-0.25 mm) Smaller sign. Arrow down
Circumference 84.90" (215.64 cm) 84.84" (215.48 cm) -0.06" (-0.16 cm) -0.07% Smaller sign. Arrow down

Speedometer difference

Original Speed (mph) New Speed with New Tires (mph)
20.020.0
30.030.0
40.040.0
50.050.0
60.060.0
70.069.9
80.079.9
90.089.9
100.099.9

Other popular comparisons with this size

Welcome to our handy wheel and tire size calculator!

Put your current tire size(width, aspect ratio, and diameter) into the first form, then your desired size into the second form and you will get the difference between the new and tire size. Also, determine the speedometer reading difference and the actual speed with a new set of tires. You can use both metric and flotation sizes for your vehicle.

Our tire calculator could help decide whether tires and wheels will fit into your car. If you need to compare tires by brand and model use our tire comparison tool.