How to use it
- Enter the current and the new tire size, metric or in inches.
- Enter the axle ratio, such as 3.73. It is often on the axle tag, in the build sheet or decoded from the axle code on the door sticker.
- Enter the top gear ratio: 1.00 for a direct top gear, or the overdrive ratio, often between 0.69 and 0.85, from your transmission's specs.
- Pick the speed you cruise at most, then read the rpm before and after, the ratio that restores it and the nearest common ratios.
Why bigger tires feel sluggish
A taller tire travels farther with each turn, so at the same speed the wheels, the driveshaft and the engine all turn slower. Cruising rpm drops in proportion: a tire 7% taller turns about 6.6% slower. It acts like a taller gear: the engine sits lower in its power band, the transmission downshifts more on hills and when towing, and acceleration is weaker. The bigger tire is usually heavier too, which makes it worse.
The formula
Engine rpm = speed ÷ tire circumference × axle ratio × gear ratio. Speed in meters per minute is mph × 1,609.344 ÷ 60. Circumference in meters is π × diameter, and the diameter in millimeters is rim × 25.4 + 2 × width × aspect ÷ 100. Dividing gives wheel turns per minute; the axle ratio turns that into driveshaft rpm and the gear ratio into engine rpm.
The shortcut most gear charts use gives the same result in inches: rpm ≈ mph × axle ratio × gear ratio × 336 ÷ tire diameter in inches. It assumes the torque converter is locked up, which is normal at a steady cruise.
Worked example: 265/70R17 to 285/75R17 on 3.73 gears
265/70R17 is 802.8 mm (31.61 in) tall and 285/75R17 is 859.3 mm (33.83 in), 7.04% taller. At 65 mph, which is 1,743.5 meters per minute, the stock tire turns about 691 times a minute and the new one about 646.
| Setup | Engine rpm | Against stock |
|---|---|---|
| 265/70R17, 3.73 | 1,779 | — |
| 285/75R17, 3.73 | 1,662 | −6.6% |
| 285/75R17, 3.92 | 1,747 | −1.8% |
| 285/75R17, 4.10 | 1,827 | +2.7% |
The ratio that brings back the original rpm is 3.73 × 859.3 ÷ 802.8 ≈ 3.99. Nobody sells 3.99, so the choice is between 3.92 and 4.10. Many owners pick 4.10 because it also makes up for the heavier tire. With a direct 1.00 top gear the same swap goes from 2,578 to 2,409 rpm, and 4.10 brings it to 2,648.
When regearing is worth it
- Small changes of about 3% or less rarely need new gears.
- Going up one tire size, such as 31 in to 33 in, is usually fine on an unloaded truck with plenty of power. It is felt more with a smaller engine, a heavy build or a trailer.
- Two sizes or more, such as 31 in to 35 in, usually call for new gears, especially if you tow, drive in the mountains or crawl off-road.
- Signs you need it: the transmission hunts between gears on the highway, the top gear never stays engaged at cruise, or fuel economy drops more than the speedometer error explains.
Buying the right gears
A ring-and-pinion set is made for one axle model, such as a Dana 44 or a Ford 8.8 in, and the ratio must exist for that axle. On some axles, ratios above a certain number need a different carrier. On a 4x4, the front and rear axles must get the same ratio, or the transfer case and driveline will bind in four-wheel drive. Setting up gears takes special tools and measurements, so most owners leave it to a shop.
The speedometer is a separate fix
New gears bring the rpm back, not the speedometer. The tire size change still makes it read low, and on vehicles that measure speed at the transmission output, the new axle ratio changes the reading as well. After the job, enter the new tire size and axle ratio with a programmer or at the dealer, and check the result with GPS.
Diameters are nominal. A real tire under load is a little shorter than the size says, and the same size varies by brand, so measured rpm can differ a little from the result.
Questions
What gear ratio do I need for 35-inch tires?
Multiply your current ratio by 35 and divide by your current tire height in inches. From a 31.61 in 265/70R17 on 3.73 gears, that is about 4.13, so 4.10 is the closest common ratio. From 3.55 gears it is about 3.93, so 3.92.
Do bigger tires lower engine rpm?
Yes. Engine rpm at a given speed is inversely proportional to tire diameter: a tire 7% taller drops cruising rpm by about 6.6%, as if you had swapped in a taller axle ratio.
Should I regear for 33s?
From 31 in tires it is optional on many trucks and helps most when you tow or the engine is small. From 265/70R17 on 3.73, the ratio that restores rpm with 33x12.50R17 tires is about 3.89, so 3.92.
Does regearing fix the speedometer?
No. The speedometer still has to be told the new tire size, and on many trucks the new axle ratio as well. Use a programmer or the dealer and check with GPS.