Tire Size and Axle Ratio Guide
Tire size and axle ratio work together. They decide how far the vehicle travels for each driveshaft turn. A taller tire covers more distance per revolution. It can lower engine rpm at cruising speed. It can also soften acceleration. A shorter tire does the opposite. It raises rpm and usually improves launch feel.
This calculator compares the original tire with a planned tire. It converts metric tire codes into sidewall height, total diameter, and tire revolutions per mile. It then estimates cruise rpm, true speed, speedometer change, and an equivalent axle ratio. These values help you judge a gear swap before parts are ordered.
The main tire formula is simple. Tire diameter equals rim diameter plus two sidewalls. Each sidewall equals section width times aspect ratio. The result is converted from millimeters to inches. Engine rpm is estimated with this common road speed formula. RPM equals speed times gear ratio times axle ratio times transfer ratio times 336, divided by effective tire diameter.
Use the rolling diameter loss field for loaded tires. Real tires flatten slightly on the road. A small loss, such as two or three percent, gives a closer result. Leave it at zero when you want pure published tire size math.
Enter the current tire first. Then enter the new tire. Add the current axle ratio, transmission gear ratio, and transfer ratio. Use one for direct drive or two wheel drive. Add a target road speed and engine rpm. Press calculate to see the comparison above the form.
The equivalent axle ratio is useful. It shows the gear ratio needed with the new tire to keep the old rpm feel. The suggested ratio field uses your target speed and desired rpm. It is helpful for towing, crawling, highway cruising, or correcting sluggish gearing after oversized tires.
Use the CSV export for spreadsheet records. Use the report download for a quick printable file. The example table shows how different tire and axle combinations affect cruising rpm.
Always compare the result with real tire maker data. Sizes can vary by brand. Automatic transmissions may slip. Lockup state matters. Wind, grade, load, and tire pressure also change rpm. Use these numbers as planning estimates, not final shop specifications for safety.