Gear Ratio & Speed Calculator
Find the perfect axle ratio for your operation. Calculate road speed at any RPM, find the fuel-efficient sweet spot, and see how different gear ratios compare.
Drivetrain Setup
Direct drive = 1.00, Overdrive = 0.73-0.86. Common: 0.78 (Eaton 13-speed OD).
Engine Settings
RPM Guide
1,200-1,400: Fuel efficiency sweet spot
1,400-1,600: Balanced power and economy
1,600+: Max power, higher fuel consumption
Road Speed at 1350 RPM
67.3 mph
in top gear (0.78)
RPM at 65 mph
1304
in top gear
Fuel Sweet Spot Range
60-70
mph (1,200-1,400 RPM)
Efficiency Rating
Optimal
1350 RPM in top gear
Speed at Each Gear (1350 RPM)
| Gear | Ratio | Speed | RPM @ 65 |
|---|---|---|---|
| 1st | 14.94 | 3.5 mph | 24975 |
| 2nd | 11.73 | 4.5 mph | 19609 |
| 3rd | 9.04 | 5.8 mph | 15112 |
| 4th | 7.09 | 7.4 mph | 11852 |
| 5th | 5.54 | 9.5 mph | 9261 |
| 6th | 4.35 | 12.1 mph | 7272 |
| 7th | 3.41 | 15.4 mph | 5700 |
| 8th | 2.68 | 19.6 mph | 4480 |
| 9th | 2.10 | 25.0 mph | 3511 |
| 10th | 1.65 | 31.8 mph | 2758 |
| 11th | 1.29 | 40.7 mph | 2156 |
| 12th | 1.00 | 52.5 mph | 1672 |
| 13th (OD) (top) | 0.78 | 67.3 mph | 1304 |
Gearing Recommendation
Good balanced gearing. Decent fuel economy with enough power reserve for grades. Works well for mixed OTR and regional.
Axle Ratio Comparison at 65 mph (top gear)
2.47 1046 RPMHighway
2.93 1240 RPMHighway
3.08 (yours)1304 RPMBalanced
3.36 1422 RPMBalanced
3.73 1579 RPMRegional
4.10 1736 RPMVocational
Frequently Asked Questions
The best axle ratio depends on your operation. For long-haul OTR on flat terrain, a lower ratio like 2.47-2.93 gives better fuel economy. For regional or hilly routes, 3.08-3.36 provides a good balance of power and economy. For heavy haul or vocational work, 3.55-4.10 delivers more pulling power. Most OTR trucks today spec 2.64-3.08 with an overdrive transmission.
Modern diesel engines are most fuel-efficient between 1,200-1,400 RPM at cruise speed. The engine produces peak torque in this range while burning less fuel. Running at 1,600+ RPM wastes fuel — every 100 RPM above the sweet spot costs roughly 0.1-0.2 MPG. Use the calculator to find which axle ratio puts you in this sweet spot at your preferred cruise speed.
Road speed (mph) = (RPM x 60) / (axle ratio x transmission gear ratio x tire revolutions per mile). For example, at 1,350 RPM with a 3.08 axle ratio, 0.78 overdrive, and 501 rev/mile tires: (1,350 x 60) / (3.08 x 0.78 x 501) = 67.3 mph. This calculator does the math automatically for all your gears.
Direct drive (1.00 ratio) means the transmission output spins at the same speed as the engine. Overdrive (typically 0.73-0.86 ratio) means the output spins faster than the engine, effectively lowering RPM at any given road speed. Overdrive transmissions pair well with numerically higher axle ratios (3.36-3.73), while direct drive pairs with lower ratios (2.47-2.93). The result is similar road speed at similar RPM — but through different mechanical paths.
Larger diameter tires have fewer revolutions per mile, which acts like a lower axle ratio — giving higher road speed at the same RPM. For example, switching from 295/75R22.5 (501 rev/mi) to 285/75R24.5 (479 rev/mi) effectively lowers your RPM at cruise speed by about 4%. This can improve fuel economy but reduces pulling power. Always check that your speedometer is recalibrated after changing tire sizes.