Tire Size Gear Ratio Calculator
Enter the tire size, the axle ratio and the transmission gear, and this tire size gear ratio calculator gives the engine RPM at a road speed. The example is a 265/70R17 tire on 3.73 gears in direct drive: 2,578 rpm at 65 mph.
Engine speed
- Engine speed 691.28 tire rpm × 1.00 × 3.73. Tire rpm = 65 mph × 63,360 ÷ (60 × π × 31.606").
- 2,578 rpm
- Overall ratio Transmission 1.00 × axle 3.73. The engine turns 3.73 times for each turn of the tires.
- 3.73:1
- Tire Sidewall 185.5 mm (7.3"), twice that plus the 17" wheel. 638 revolutions per mile.
- 265/70R17, 31.61" tall
- Tire speed
- 691.28 rpm
- Driveshaft speed Tire rpm times the axle ratio: the speed coming out of the transmission.
- 2,578 rpm
| Road speed (mph) | Engine speed (rpm) | Tire speed (rpm) |
|---|---|---|
| 30 | 1,190 | 319.05 |
| 45 | 1,785 | 478.58 |
| 55 | 2,182 | 584.93 |
| 65 | 2,578 | 691.28 |
| 70 | 2,777 | 744.45 |
| 75 | 2,975 | 797.63 |
| 80 | 3,174 | 850.81 |
- The tire diameter is the unloaded figure from the size. A loaded tire rolls on a slightly smaller radius, so the true engine speed runs a little higher than this.
- A locked torque converter or a manual clutch is assumed. An automatic with the converter unlocked slips, and the engine turns faster than the gearing alone gives.
How the Gear Ratio Calculator works
Engine speed at a given road speed depends on three numbers: the tire's diameter, the axle ratio and the transmission gear. The formula is rpm = mph × 63,360 ÷ (60 × π × tire diameter) × transmission ratio × axle ratio. 63,360 is the inches in a mile, and 63,360 ÷ (60 × π) = 336.14, the "336" in the rule of thumb rpm = mph × ratio × 336 ÷ tire diameter.
The tire diameter comes from the size, the same way the tire size calculator works it out. A 265/70R17 has a sidewall 70% of 265 mm tall, which is 185.5 mm or 7.30 inches. Twice that plus the 17-inch wheel is 31.61 inches. That tire turns 638 times a mile, and 691.28 times a minute at 65 mph. For a tire sized in inches, such as 33x12.50R15, enter the first number as the diameter.
With 3.73 axle gears in direct drive (a 1.00 transmission ratio), 691.28 tire rpm becomes 2,578 engine rpm. In a 0.70 overdrive the same speed is 1,805 rpm. The table under the result runs the same gearing from 30 to 80 mph.
This is the arithmetic behind re-gearing after a tire change. Swap the 31.61-inch tires for 33-inch ones and, on the same 3.73 gears, the engine drops to 2,470 rpm at 65 mph, and the true road speed runs 4.4% above what an uncorrected speedometer shows. To bring the engine speed back to 2,578 rpm the axle ratio would have to rise in proportion to the tire, to 3.73 × 33 ÷ 31.61 = 3.89. Axles come in fixed ratios, so the choice is between the nearest ones either side, and the calculator shows what each does.
Run it the other way with the speed mode. At 2,500 rpm on the same tires and gears, the truck does 63.0 mph in direct drive. Use the gear ratio of each transmission gear in turn to see the road speed at a given rpm in every gear.
The figures assume no slip. A loaded tire rolls a little shorter than its unloaded diameter, and an automatic with its torque converter unlocked lets the engine run faster than the gearing gives, so a tachometer may read slightly above these numbers.
Gear Ratio Calculator FAQ
How do I calculate RPM from tire size and gear ratio?
Work out the tire diameter, then rpm = mph × 336.14 × axle ratio × transmission ratio ÷ diameter. A 265/70R17 is 31.61 inches, so 65 mph on 3.73 gears in direct drive is about 2,578 rpm.
What axle ratio do I need for bigger tires?
To keep the same engine speed, scale the ratio by the change in diameter: new ratio = old ratio × new diameter ÷ old diameter. Going from 31.61-inch to 33-inch tires on 3.73 gears calls for about 3.89.
How does an overdrive gear change RPM?
An overdrive has a ratio below 1, so it lowers engine speed in proportion. At 65 mph on 31.61-inch tires and 3.73 gears, direct drive (1.00) is 2,578 rpm and a 0.70 overdrive is 1,805 rpm.