Gear Ratio Calculator
Find the rear axle ratio that puts your engine at the RPM you want for your tire size, transmission and speed. It's the tool to use when you're choosing gears or re-gearing for bigger tires, and it can also solve for RPM, tire diameter or speed.
How to find the axle ratio you need
Axle ratio = RPM × tire diameter ÷ (MPH × transmission ratio × 336)
Pick the RPM you want the engine turning at a given speed, usually your highway cruising speed, and enter the tire diameter in inches and the ratio of the gear you'll be in. The 336 converts miles per hour into tire revolutions per minute. The engine RPM calculator explains where it comes from.
Example: gearing for 33-inch tires
You want the engine at 2,000 RPM at 70 mph in a 0.70 overdrive on 33-inch tires:
2,000 × 33 ÷ (70 × 0.70 × 336) = 4.01
Axle ratios come in fixed steps, so pick the closest one offered for your axle. With 4.10 gears the engine turns 2,046 RPM at 70 mph. If the truck still has 3.73 gears, it turns 1,861.
Re-gearing for bigger tires
To get your original RPM back after a tire swap, scale the axle ratio by the change in tire diameter:
New axle ratio = old axle ratio × new tire diameter ÷ old tire diameter
Starting from 31-inch tires, this is the ratio that keeps the same RPM on bigger tires:
| Axle ratio on 31" tires | 33" tires | 35" tires | 37" tires |
|---|---|---|---|
| 3.21 | 3.42 | 3.62 | 3.83 |
| 3.42 | 3.64 | 3.86 | 4.08 |
| 3.55 | 3.78 | 4.01 | 4.24 |
| 3.73 | 3.97 | 4.21 | 4.45 |
| 4.10 | 4.36 | 4.63 | 4.89 |
Calculated with the formula above. Round to the nearest ratio available for your axle.
Effective gear ratio
If you put on bigger tires and keep the old gears, the axle behaves like a numerically lower ratio. The effective ratio is the axle ratio times the old tire diameter divided by the new one. A truck with 3.73 gears that goes from 31.6-inch to 35-inch tires has an effective ratio of 3.73 × 31.6 ÷ 35 = 3.37, so it drives like it has 3.37 gears on the original tires.
Higher and lower ratios
A numerically higher ratio, like 4.10 instead of 3.73, multiplies torque more at the wheels and turns the engine faster at any given speed. A numerically lower ratio does the opposite: fewer RPM on the highway and less torque multiplication. Use the engine RPM calculator to check your cruising RPM with the ratio you pick, and the tire size calculator to get the exact diameter of a metric tire size.
Frequently Asked Questions
How do you calculate the gear ratio you need?
Multiply the RPM you want by the tire diameter in inches, then divide by your speed in mph times the transmission gear ratio times 336.
What gear ratio do I need for bigger tires?
Multiply your current axle ratio by the new tire diameter and divide by the old one. Going from 31-inch to 35-inch tires with 3.73 gears calls for about 4.21 to keep the same RPM.
What is effective gear ratio?
It is the ratio your axle acts like after a tire size change. Multiply the axle ratio by the old tire diameter and divide by the new one. 3.73 gears with 35-inch tires in place of 31.6-inch tires act like 3.37 gears.
Is a higher or lower gear ratio better?
It depends on how you use the vehicle. A numerically higher ratio gives more torque multiplication and more RPM at a given speed. A numerically lower ratio gives fewer RPM on the highway.
What if the exact gear ratio I need is not available?
Pick the closest ratio offered for your axle, then use the engine RPM calculator to see the cruising RPM it gives you.
Does the transmission ratio matter when picking axle gears?
Yes. Engine RPM depends on the axle ratio times the transmission ratio, so a 0.70 overdrive lets you run a numerically higher axle ratio for the same highway RPM than a 1.00 direct-drive top gear.
