Calculate ring and pinion gear ratios, final drive ratios, and engine RPM at any speed. Compare differential ratios for towing, highway cruising, and performance applications.
See how different axle ratios affect RPM at 70 MPH (with 30" tires, direct drive)
| Ratio | RPM @ 70 | Torque | Best For |
|---|---|---|---|
| 2.73:1 | 2,150 RPM | Low | Highway fuel economy |
| 3.08:1 | 2,430 RPM | Moderate | Light duty / commuting |
| 3.42:1 | 2,700 RPM | Moderate | Balanced performance |
| 3.55:1 | 2,800 RPM | Good | General purpose |
| 3.73:1 | 2,940 RPM | Good | Towing / performance |
| 4.10:1 | 3,230 RPM | High | Heavy towing |
| 4.56:1 | 3,600 RPM | Very High | Off-road / racing |
| 4.88:1 | 3,850 RPM | Very High | Rock crawling |
Recommended: 3.73:1 to 4.30:1
Recommended: 2.73:1 to 3.23:1
Recommended: 3.90:1 to 4.56:1
Recommended: 4.56:1 to 5.38:1
Crawl ratio is the overall gear reduction from engine to wheels in the lowest gear. It's calculated by multiplying the first gear ratio, transfer case low ratio (if applicable), and axle ratio. A higher crawl ratio (100:1 or more) provides better control for rock crawling and steep descents because the engine provides more "engine braking" and the vehicle moves slower for each engine revolution. For example, a 4.10 axle ratio with 4:1 first gear and 2.72:1 transfer case gives a crawl ratio of 44.6:1.
Final drive ratio is the total gear reduction from engine to wheels, combining transmission ratio and differential (axle) ratio. For example, in 3rd gear (1.5:1) with a 3.73 axle ratio, the final drive is 5.6:1, meaning the engine rotates 5.6 times for each wheel rotation. A higher final drive ratio means more torque multiplication but lower top speed; a lower ratio means less torque but higher top speed and better fuel economy at highway speeds.
The formula is: Engine RPM = (Speed in MPH x Final Drive Ratio x 336.13) / Tire Diameter in inches. The constant 336.13 converts units (it equals 5280 feet/mile x 12 inches/foot / 60 minutes/hour / pi). For example, at 70 MPH with a 3.73 axle ratio, 0.75:1 overdrive, and 30-inch tires: RPM = (70 x 3.73 x 0.75 x 336.13) / 30 = 2,196 RPM.
For towing, numerically higher ratios (3.73, 4.10, 4.56) provide more torque multiplication, making it easier to accelerate with heavy loads and climb grades. A 3.73 or higher is typically recommended for regular towing over 5,000 lbs. However, higher ratios increase engine RPM at highway speeds, which may reduce fuel economy when not towing. Modern trucks often balance this with 8, 10, or even 12-speed transmissions that have very tall overdrive gears.
Larger tires effectively lower (numerically decrease) your gear ratio because the tire travels farther per revolution. The effect is proportional to the diameter change. Going from 30" to 33" tires (10% increase) makes your 3.73 ratio behave like a 3.39 ratio, resulting in lower RPM at a given speed but also less acceleration and towing power. To compensate, many off-road enthusiasts re-gear to a numerically higher ratio when installing larger tires.