Gear Ratios Explained: Axle, Transmission and Tire Size
Gear ratios decide how fast the engine turns at a given road speed and how much force reaches the pavement. This guide walks through the formula, shows worked examples and explains how to pick gears for highway cruising, towing and drag racing.
Two ratios multiply
The total ratio between the engine and the wheels is the transmission gear ratio multiplied by the axle (final drive) ratio. A transmission in fourth gear at 1.00:1 with a 3.73:1 axle gives a total ratio of 3.73. An overdrive of 0.75 with the same axle gives 2.80. The torque multiplication at the wheel is the total ratio, and the engine rpm at a given speed is proportional to it.
The rpm formula
The number 336 is a constant that converts miles per hour and inches into revolutions per minute. A worked example: a vehicle at 65 mph with a 3.73 axle, 0.75 overdrive and a 28-inch tire. 65 × 3.73 × 0.75 × 336 ÷ 28 = about 2,182 rpm. Change to a 3.08 axle and the answer drops to about 1,802 rpm. The Gear Ratio Calculator solves for any one of ratio, rpm or speed when you supply the others.
Tire diameter is part of the gearing
A taller tire acts like a numerically lower axle ratio. Going from a 27-inch tire to a 30-inch tire is an 11 percent increase in diameter, which has the same effect as changing a 4.10 axle to about 3.69. This is why lifted trucks with big tires often feel sluggish until they are re-geared. The Tire Size Calculator will tell you the new diameter.
Numerically higher or lower?
A numerically higher ratio (4.10) is often called a short gear. It multiplies more torque, accelerates harder and turns more rpm at a given speed. A numerically lower ratio (2.73) is a tall gear. It lets the engine loaf on the highway and usually saves fuel, but it gives up acceleration and pulling power. Neither is better; the right choice depends on the engine's power band and what you ask of the vehicle.
| Use | Typical direction | Reason |
|---|---|---|
| Highway commuting | Numerically lower | Lower rpm, quieter, less fuel |
| Towing or hauling | Numerically higher | More torque at the wheels, less hunting between gears |
| Drag racing | Chosen to put the engine in its power band at the finish | See below |
| Large tire swap | Numerically higher | Restore the original effective ratio |
Gearing for towing
Towing is about keeping the engine in the rpm range where it makes torque while avoiding constant downshifts. The higher the trailer weight, the more it pays to use a numerically higher axle. Combine the ratio with the transmission's gear spread and check the rpm in the top gear you intend to cruise in. If the engine is working at too low an rpm in overdrive, the transmission hunts, heat builds up and fuel economy gets worse. Also check that the vehicle's tow rating allows for the axle you have, since manufacturers publish ratings by axle code. See Towing Basics for hitch ratings.
Gearing for drag racing
For the strip, the aim is for the engine to be at or near its power peak when the car crosses the finish line, in the highest gear used. Work backward: take the trap speed you expect, the peak rpm of the engine and the tire diameter, then solve for the total ratio. The Drag Racing Gear Calculator does this and the Quarter Mile Calculator estimates the elapsed time from power and weight. Too tall a gear and the engine never reaches its peak; too short and you run out of rpm before the finish and must shift or be limited.
Speedometer correction
When you change the axle ratio or the tire size, the speedometer is wrong unless it is recalibrated. Mechanical speedometers are driven by a gear in the transmission, and the Speedometer Gear Calculator finds the tooth count needed. Electronic ones can usually be reprogrammed by a shop or with a handheld tuner.
Axle details
- Axle ratio is the ring-gear teeth divided by the pinion teeth, so 41:11 gives 3.73.
- Limited-slip or locking differentials help put power down, and any of them should be matched to the new ring and pinion when you change gears.
- Set pinion depth and backlash carefully, or the new gear will whine and wear quickly. Many builders have this done by a shop that has the right tools.
- Change both front and rear axles together on four-wheel-drive vehicles so the ratios match.
Common mistakes
- Counting the overdrive as part of the axle ratio, or forgetting to multiply by it.
- Using the nominal tire size (the 28 in the name) rather than the real rolling diameter.
- Choosing gears by dyno peaks without thinking about where the car spends its time.
- Re-gearing only one axle on a four-wheel-drive or all-wheel-drive vehicle.
Frequently asked questions
What is the formula for rpm from gear ratio?
Engine rpm equals road speed in mph times the total ratio times 336, divided by the tire diameter in inches. The total ratio is the transmission gear multiplied by the axle ratio.
What axle ratio is best for towing?
A numerically higher ratio generally suits towing because it multiplies torque and keeps the transmission from hunting. The right one depends on trailer weight, engine and transmission, so check the manufacturer's towing guide for the axle codes the tow rating applies to.
Do bigger tires change my effective gear ratio?
Yes. A taller tire covers more ground per revolution, so it acts like a numerically lower axle ratio. A tire that is 10 percent taller makes a 4.10 axle behave like roughly 3.73.
Tools used in this guide
- Gear Ratio Calculator
- Drag Racing Gear Calculator
- Speedometer Gear Calculator
- Quarter Mile Calculator
- Tire Size Calculator
More guides
- Towing Basics: Hitch Classes, Tongue Weight and Ratings
- Reading a Tire Sidewall and Choosing a New Size
- Choosing Wheel Offset and Backspacing
- Sizing an Air Compressor for Your Shop
- All GarageCalcs guides
Figures in this guide are general guidance for planning. Always follow your vehicle, tool and product manufacturers' instructions and local codes, and read our disclaimer.