Tire Size & Axle Ratio Calculator

1. Type the stock tire size the vehicle left the factory with, and the new tire size you are fitting or already run.
2. Enter the axle ratio, and the transmission ratio of the gear you cruise in. Leave it at 1.00 for a direct top gear.
3. Set a road speed. Read the effective ratio, the RPM on each tire, the ratio that restores stock gearing, and the speedometer error.

What the results show:

Effective gear ratio is the axle ratio your vehicle behaves like once the tire diameter changes. A taller tire covers more ground per turn, so it acts like a numerically lower ratio. A 3.73 axle on tires 7.04% taller feels like a 3.48 axle.

The axle ratio itself is the number of driveshaft turns for one wheel turn. A 3.73 ratio turns the driveshaft 3.73 times for each turn of the wheel. The ring and pinion gears do not change when you change tires, but the distance the wheel travels per turn does. That is the whole effect.

This matters most to truck and SUV owners fitting larger off-road tires, and to anyone towing. A lower effective ratio means less torque at the wheels, slower acceleration and more downshifting on grades. It is not the same as a transmission ratio change: the transmission gears stay the same, and every gear is affected equally by the tire change.

It compares the two tire diameters and scales the axle ratio, the RPM and the speedometer by that ratio.

1. Work out each tire diameter from its metric code, using 25.4 mm per inch.
2. Divide the axle ratio by the new-to-stock diameter ratio to get the effective ratio. Multiply instead to get the ratio that restores stock gearing.
3. Find wheel RPM: one mile is 63,360 inches, so 1 mph is 1,056 inches per minute. Divide by the tire circumference, then multiply by the axle and gear ratios.
4. Scale the speedometer reading by the diameter ratio, because the speedometer still counts turns of the old, smaller tire.

The 1,056 / pi part is the familiar 336 gearing constant (1,056 / pi = 336.14). Use the ratio calculator to check any ratio step by hand.

A truck on stock 265/70R17 tires with a 3.73 axle is moving to 285/75R17 tires.

Diameters: the stock tire is 17 + 2 x 265 x 0.70 / 25.4 = 31.61 in. The new tire is 17 + 2 x 285 x 0.75 / 25.4 = 33.83 in, which is 7.04% taller.

Effective ratio: 3.73 / 1.0704 = 3.48. To restore stock gearing, the axle would need 3.73 x 1.0704 = 3.99.

RPM at 65 mph in a 1.00 top gear: stock tires give 2,578 rpm; the new tires give 2,409 rpm, about 170 fewer. With a 0.69 overdrive, the figures are 1,779 and 1,662 rpm.

Speedometer: it now reads 6.58% low. At 65 mph shown you are really doing about 69.6 mph, and at 50 mph shown about 53.5 mph.

What it means: the truck will cruise at lower RPM but pull harder in each gear to reach the same speed. The speedometer needs recalibrating before the next trip.

The geometric result is a clean starting point. Several real-world factors shift what you feel on the road.

Actual Tire Height

Two tires with the same code can differ in real height, and off-road tread adds to it. A measured or published diameter beats the code for a final regear choice.

Tire Weight

Larger tires are usually heavier. The extra rotating mass slows acceleration beyond what the ratio change alone predicts.

Transmission Shift Points

Automatic transmissions shift by vehicle speed and load. With the speed signal reading low, shifts can happen at the wrong road speed until the tire size setting is corrected.

Towing and Grades

A lower effective ratio hurts most when loaded. A setup that feels fine empty can hunt between gears on a long grade with a trailer.

Regearing restores torque at the wheels; recalibrating only corrects the speedometer and odometer.

Common Size Changes on a 3.73 Axle

The restore figure is exact math. Axle gear sets come in fixed steps, so the nearest ratio offered for your axle will be a little above or below it.

Planning a Lift and Larger Tires

Run the numbers before you buy tires, so you know whether a regear belongs in the budget. The gas mileage calculator helps you track fuel use before and after the change.

Checking a Used Vehicle

A used truck on oversized tires may already feel sluggish. Enter the stock size from the door placard and the fitted size to see how far the gearing has moved.

Choosing Tires for Towing

Towing adds load, so keep the effective ratio change small. Compare two candidate sizes and pick the one closer to stock.

1. Using the Speedometer Speed for RPM

After a tire change, the speedometer reads low. Base RPM checks on true speed, from GPS, not the dash reading.

2. Forgetting the Transmission Ratio

Cruising RPM depends on the gear. An overdrive top gear of 0.69 cuts RPM by 31% compared with a direct 1.00 gear.

3. Treating the Restore Ratio as a Part Number

The calculator gives an exact ratio such as 3.99. Your axle is offered in set ratios, so choose from what is available for it.

4. Mixing Inch and Metric Sizes

A flotation size such as 35x12.50R17 already states the diameter as 35 inches. This tool reads metric codes, so convert with the inches to mm converter or compare diameters directly.

5. Ignoring the Odometer

A tire 7% taller makes the odometer under-count by about the same share. Service intervals by distance come later than they should.

The ratio math is exact for the tire codes you enter; the gain or loss you feel also depends on tire weight and real height.

Bigger tires lower the effective ratio. A 3.73 axle on tires 7% taller behaves like a 3.48, so the engine turns slower at a given speed and pulls less hard.

Multiply your axle ratio by 35 divided by your stock tire diameter. From 31.6 in stock on 3.73 gears, that is 3.73 x 35 / 31.6 = 4.13, so a ratio near that restores the stock feel.

Each turn of a taller tire covers more road, so the wheels and engine turn fewer times per mile. At 65 mph, going from 31.61 in to 33.83 in tires cuts about 170 rpm in a 1.00 gear.

It reads low by the diameter ratio. Tires 7.04% taller make the speedometer read about 6.58% low, so 65 mph shown is really about 69.6 mph.

For buses and trucks under U.S. federal motor carrier rules, the speedometer must be accurate within plus or minus 5 mph at 50 mph. Rules for private cars vary by country and state.

No. Regearing restores the effective ratio, but the speedometer still counts turns of a taller tire. Update the tire size setting as well.

Not reliably. Lower cruising RPM can help on flat highways, but heavier, taller tires add rolling resistance and weight, and the engine may downshift more.

It is 63,360 inches per mile divided by 60 minutes and by pi, which equals about 336.14. It converts mph and tire diameter into wheel RPM.

Yes. Enter the overdrive ratio, such as 0.69, as the transmission gear ratio. RPM scales directly with that ratio.

No. Your tire sizes and ratios stay on your device while the math runs. A saved result lives only in this browser, and you can clear it at any time.

• NHTSA tire label guide: section width, aspect ratio and rim diameter code (NHTSA. Automated fetches may be blocked; the page loads with status 200 in a normal browser).
• 49 CFR 571.139, FMVSS 139: tire definitions and markings for light vehicles (eCFR, NHTSA).
• 49 CFR 393.82: speedometer accuracy for buses and trucks (eCFR, FMCSA).
• NIST SP 811 Appendix B.8: factors for the inch and the mile (NIST SP 811).