Quick Answer
- 265/70R17 (31.61") to 285/75R17 (33.83") is 7.0% taller. A 3.73 axle then acts like 3.48.
- The axle ratio that restores the original gearing is 3.73 × 33.83 ÷ 31.61 = 3.99.
- At 65 mph in a 1.00 top gear, engine RPM drops from 2,578 to 2,409. Effective ratio = axle ratio × old diameter ÷ new diameter.
Enter your original and new tire sizes (or diameters in inches) and your axle ratio.
Overall diameter used: 31.61 in
Overall diameter used: 33.83 in
Use 1.00 for a direct top gear, or the overdrive ratio from your transmission specifications.
Effective Gear Ratio
3.48
Original Ratio
3.73
Effective Ratio
3.48
Ratio Change
-6.6%
Ratio to Restore Original
3.99
RPM at 65 mph, original
2,578
RPM at 65 mph, new tire
2,409
What changes
A taller tire lowers the effective ratio from 3.73 to 3.48. Your true speed is 7.0% higher than the speedometer shows, and engine RPM at the same speed is 6.6% lower.
An axle ratio of about 3.99 would give the same effective gearing as the original tires. Manufacturers do not publish re-gear advice for other tire sizes, so ask a dealer which ratios exist for your axle.
Get a plain-English explanation of your results and what they mean for your vehicle.
Gear Ratio Chart by Tire Diameter Increase
Both charts work for any tires: find how much taller the new tire is (new diameter ÷ old diameter), then read across for your axle ratio. Calculated with the formulas below.
Effective ratio after the tire change
| New tire is taller by | 3.21 axle | 3.55 axle | 3.73 axle | 4.10 axle | 4.56 axle |
|---|---|---|---|---|---|
| 2% | 3.15 | 3.48 | 3.66 | 4.02 | 4.47 |
| 3% | 3.12 | 3.45 | 3.62 | 3.98 | 4.43 |
| 5% | 3.06 | 3.38 | 3.55 | 3.90 | 4.34 |
| 7% | 3.00 | 3.32 | 3.49 | 3.83 | 4.26 |
| 10% | 2.92 | 3.23 | 3.39 | 3.73 | 4.15 |
| 12% | 2.87 | 3.17 | 3.33 | 3.66 | 4.07 |
| 15% | 2.79 | 3.09 | 3.24 | 3.57 | 3.97 |
Axle ratio that restores the original gearing
| New tire is taller by | 3.21 axle | 3.55 axle | 3.73 axle | 4.10 axle | 4.56 axle |
|---|---|---|---|---|---|
| 2% | 3.27 | 3.62 | 3.80 | 4.18 | 4.65 |
| 3% | 3.31 | 3.66 | 3.84 | 4.22 | 4.70 |
| 5% | 3.37 | 3.73 | 3.92 | 4.30 | 4.79 |
| 7% | 3.43 | 3.80 | 3.99 | 4.39 | 4.88 |
| 10% | 3.53 | 3.91 | 4.10 | 4.51 | 5.02 |
| 12% | 3.60 | 3.98 | 4.18 | 4.59 | 5.11 |
| 15% | 3.69 | 4.08 | 4.29 | 4.71 | 5.24 |
Michelin's Truck Tire Service Manual says diameter changes of about 2% or less have a negligible effect on gear ratio, tractive effort and indicated speed, and that changes of more than 3% should be discussed with the vehicle manufacturer.
Gear Ratio Examples for Common Tire Swaps
Diameters are calculated from the size code, and the flotation size uses its stated diameter. The axle ratios are example inputs, not factory ratios for these tires.
| Original tire | New tire | Taller by | Axle ratio | Effective ratio | Restore ratio |
|---|---|---|---|---|---|
| 265/70R17 (31.61") | 285/75R17 (33.83") | 7.0% | 3.73 | 3.48 | 3.99 |
| 235/75R15 (28.88") | 33x12.50R15 (33.00") | 14.3% | 3.55 | 3.11 | 4.06 |
| 225/65R17 (28.52") | 245/65R17 (29.54") | 3.6% | 3.92 | 3.78 | 4.06 |
| 295/75R22.5 (39.92") | 285/75R24.5 (41.33") | 3.5% | 3.55 | 3.43 | 3.68 |
Actual tire diameter varies by tire model, so use your own sizes in the calculator. For truck sizes, the truck tire size calculator also takes semi truck and flotation sizes.
Engine RPM at Speed Before and After a Tire Change
RPM for a 3.73 axle with a 1.00 top gear, comparing 265/70R17 (31.61") with 285/75R17 (33.83"). Your transmission's top gear ratio changes the RPM but not the percentage difference.
| Speed | 265/70R17 | 285/75R17 | Change |
|---|---|---|---|
| 55 mph | 2,182 rpm | 2,038 rpm | -6.6% |
| 60 mph | 2,380 rpm | 2,224 rpm | -6.6% |
| 65 mph | 2,578 rpm | 2,409 rpm | -6.6% |
| 70 mph | 2,777 rpm | 2,594 rpm | -6.6% |
| 75 mph | 2,975 rpm | 2,779 rpm | -6.6% |
For your own gears and speeds, the engine RPM at highway speed calculator takes the transmission ratios and tire diameter together.
Gear Ratio Formulas
How to Use This Calculator
- Enter your original tire size (for example 265/70R17) or its overall diameter in inches. Flotation sizes such as 35x12.50R17 also work.
- Enter the new tire size or diameter.
- Enter your axle ratio, for example 3.55, 3.73 or 4.10.
- Optionally enter a speed and your top gear ratio to see engine RPM before and after the tire change.
- Read the effective ratio, the percentage change, the ratio that restores the original gearing, and the RPM at your speed.
About Gear Ratio & Tire Size Changes
How Tire Size and Gears Work Together
The axle ratio is the number of driveshaft turns for each turn of the wheel. A taller tire covers more ground per turn, so the same axle ratio gives a lower effective ratio: fewer engine revolutions per mile and less torque multiplication at the wheels. A shorter tire does the opposite. To see how big the tire change is in the first place, you can compare the stock and new tire sizes, and the speedometer error calculator shows how far the speedometer is off with the new size.
When Re‑Gearing Makes Sense
Manufacturers do not publish re-gear advice for other tire sizes. Michelin's Truck Tire Service Manual says diameter changes of about 2% or less have a negligible effect on gear ratio, tractive effort and indicated speed, and that changes of more than 3% should be discussed with the vehicle manufacturer. This calculator shows how far your effective ratio has moved and the ratio that would restore the original gearing, so you can ask a dealer which ratios are available for your axle.
Engine RPM and Effective Ratio
A higher numerical axle ratio raises engine RPM at a given speed, and a lower one reduces it. Engine RPM changes by the same percentage as the effective ratio, so the RPM tiles in the calculator move with the tire change. We have no manufacturer source for how a ratio affects fuel use, so this page does not state one.
Frequently Asked Questions
Disclaimer
This tool uses standard tire‑diameter and gear‑ratio formulas for planning and education. Actual performance depends on engine output, transmission ratios, converter or clutch behaviour, vehicle weight, aerodynamics, and driving conditions. Always consult a qualified drivetrain specialist before selecting gears for towing, off‑road, racing, or other demanding uses.