Gear Ratio Calculator
Calculate engine RPM from gear ratio, final drive, and tire size.
Reviewed by the ToolNestr Editorial Team — July 2026
Enter any 4 of 5 values to solve for the missing one
How gear ratios work
A gear ratio is the number of teeth on the output gear divided by the number of teeth on the input gear. In a car, the transmission provides multiple gear ratios (1st, 2nd, 3rd, etc.) that multiply torque for acceleration and reduce RPM for cruising. The final drive ratio (differential) provides a fixed multiplication between the transmission output and the wheel axles.
The formula
The relationship between engine RPM, vehicle speed, gear ratio, final drive ratio, and tire diameter is given by the following formula:
RPM = (Speed × Final Drive × Gear Ratio × 336) ÷ Tire Diameter
The constant 336 comes from the unit conversions needed: (5280 ft/mile × 12 in/ft) ÷ (60 min/hr × π) ≈ 336.1. It converts miles per hour and tire diameter in inches into the equivalent revolutions per minute at the wheel, which is then multiplied through the drivetrain ratios to get engine RPM.
To solve for any of the five variables when the other four are known, rearrange the formula:
Speed = (RPM × Tire) ÷ (Final × Gear × 336) Gear = (RPM × Tire) ÷ (Speed × Final × 336) Final = (RPM × Tire) ÷ (Speed × Gear × 336) Tire = (Speed × Final × Gear × 336) ÷ RPM Worked example
Let us calculate engine RPM for a car cruising at 70 mph in 5th gear. The transmission gear ratio for 5th gear is 0.75:1 (overdrive), the final drive ratio is 3.73:1, and the tire diameter is 28 inches.
| Variable | Value | Notes |
|---|---|---|
| Speed | 70 mph | Highway cruising speed |
| Gear ratio | 0.75:1 | 5th gear overdrive |
| Final drive | 3.73:1 | Common axle ratio |
| Tire diameter | 28 in | Typical passenger car tire |
| Engine RPM | ~2,352 RPM | Calculated result |
Step-by-step calculation
Step 1: Total drivetrain ratio = Gear ratio × Final drive = 0.75 × 3.73 = 2.7975:1
Step 2: Apply formula: RPM = (70 × 3.73 × 0.75 × 336) ÷ 28
Step 3: RPM = (70 × 2.7975 × 336) ÷ 28
Step 4: RPM = 65,818.8 ÷ 28 ≈ 2,351 RPM
Common use cases
Choosing gears for racing
Racers select transmission and final drive ratios to keep the engine in the power band at key track sections. A shorter final drive (higher numeric ratio) provides quicker acceleration out of corners but reduces top speed. Use the calculator to experiment with different combinations and find the optimal gearing for your track layout.
Highway RPM estimation
Before buying a car or modifying your drivetrain, estimate what RPM the engine will turn at highway speeds. A lower cruising RPM means less engine noise, better fuel economy, and reduced wear. This is especially important for vehicles used for long-distance commuting or road trips.
Re-gearing for larger tires
When you install larger tires on a truck or SUV, the effective gear ratio becomes numerically smaller (taller), which reduces torque at the wheels and increases load on the transmission. Re-gearing with a lower (higher numeric) final drive restores performance and keeps the engine in its efficient operating range. Enter your original tire size and new tire size to find the ideal compensating final drive ratio.
Tips for optimizing gear ratios
Lower RPM = better fuel economy
Engines consume less fuel at lower RPM for the same road speed. An overdrive transmission (gear ratio below 1.0:1) combined with a numerically small final drive (e.g. 2.73:1 instead of 3.73:1) can drop highway RPM by 500-800 RPM, improving fuel economy by 10-20% on the highway. However, if the gearing is too tall, the engine may lug and require downshifting on hills.
Taller gears for highway cruising
Taller gears (lower numeric ratios) reduce engine RPM at any given speed, which lowers noise, vibration, and fuel consumption. Modern 8, 9, and 10-speed automatic transmissions use very tall top gears (0.60-0.65:1) combined with a moderate final drive to achieve both strong acceleration in lower gears and low RPM cruising in top gear. A 10-speed automatic in top gear at 70 mph may turn only 1,500-1,800 RPM in many vehicles.
Shorter gears for towing
For towing heavy loads, shorter gearing (higher numeric ratios like 4.10:1 or 4.56:1) keeps the engine higher in the RPM range where it produces maximum torque. This reduces strain on the transmission and helps maintain speed on grades. The trade-off is higher RPM at highway speeds and reduced fuel economy when not towing.
Understanding the numbers
The gear ratio is written as the number of drive shaft revolutions to one wheel revolution. For example, a 3.73:1 final drive means the drive shaft turns 3.73 times for each wheel revolution. Transmission gear ratios work the same way — a 1st gear of 3.50:1 means the engine turns 3.50 times for each transmission output shaft revolution. Multiply them together to get the total reduction: in 1st gear with a 3.73 final drive, the total ratio is 3.50 × 3.73 = 13.06:1. This massive torque multiplication is what allows a small engine to move a heavy vehicle from a standstill.
Tire diameter is measured as the overall height of the tire from the ground to the top of the tread. A larger tire effectively raises the final drive ratio because the wheel covers more ground per revolution. This is why trucks with oversized tires often feel sluggish off the line — the effective gear ratio has become taller. Re-gearing the differential compensates for this by restoring the effective ratio back to a shorter value.
When working with km/h instead of mph, simply convert the speed before applying the formula: 1 mph = 1.60934 km/h. The calculator handles this conversion automatically when you select km/h. The formula with the 336 constant is designed for mph, so km/h values are converted internally before the calculation runs.
Related tools
Frequently asked questions
What is the difference between gear ratio and final drive?
Gear ratio is the transmission gear (e.g. 3rd gear 1.35:1), final drive is the differential ratio (e.g. 3.73:1). Total ratio = gear × final.
How do I find my gear ratios?
Check your vehicle owner manual or transmission specifications online.
Does tire size affect RPM?
Yes — larger tires lower RPM at the same speed; smaller tires increase it.
What is a good highway RPM?
Usually 1500-2500 RPM at 60-70 mph for fuel efficiency, depending on the vehicle.
Sources & references
This tool uses standard formulas and reference values from:
- • SAE International standards (e.g. J1349 engine power, J1634 EV range). sae.org
- • U.S. DOE / EPA — fueleconomy.gov, official efficiency, MPGe and charging figures. fueleconomy.gov
- • Vehicle manufacturer service specifications — always defer to the OEM figures for your specific vehicle.
General estimates. Follow your manufacturer’s published specifications and a qualified mechanic for safety-critical work.