About CarCalc

CarCalc is a set of free calculators for the numbers car owners need: what a different tire size does to your speedometer, how tall a tire really is, what a road trip costs in gas and what your car actually gets per gallon.

How the numbers are calculated

Tire figures come from the size code with the standard formulas: sidewall height from width and aspect ratio, overall diameter from the wheel size plus two sidewalls, and revolutions per mile from the circumference. Fuel results are simple arithmetic on the distance, fuel economy and price you enter. Every result updates as you type, in your browser.

A note on safety

Calculators help you plan. Before changing tire size, check your owner’s manual, the tire placard in the driver’s door and the tire’s load and speed rating, and ask a tire professional if you’re unsure.