Calculate true fuel economy for modified vehicles. Get accurate MPG readings after changing gears, tires, or both. Essential for trucks, lifted vehicles, and performance builds.
Changed gears or tires? Your speedometer and odometer are now incorrect. This calculator determines your actual fuel economy by correcting for drivetrain modifications.
How to Use:
Simple fuel economy calculator for stock vehicles or tracking fuel efficiency over time. Just enter miles driven and gallons used.
Formula: MPG = Miles Driven / Gallons Used
When you change gear ratios or tire sizes, your vehicle's computer and mechanical systems are no longer calibrated correctly. The odometer measures distance based on wheel rotations, which changes when tire diameter changes. Similarly, gear ratio changes affect how many times your wheels rotate per mile traveled.
Lower Numerical Ratios (2.73, 3.08, 3.23): These "taller" gears reduce engine RPM at highway speeds, which generally improves fuel economy. Your engine works less to maintain speed. However, acceleration suffers and you may need to downshift more often.
Higher Numerical Ratios (3.73, 4.10, 4.56): These "shorter" gears increase engine RPM at all speeds, typically reducing fuel economy. They provide better acceleration and towing capacity but your engine works harder on the highway.
Larger Tires: Bigger diameter tires act like taller gearing. Each revolution covers more ground, effectively raising your gear ratio. This can improve highway fuel economy but may hurt city MPG due to increased rotational mass. Your speedometer will read slower than actual speed, and your odometer will record fewer miles than you actually travel.
Smaller Tires: Smaller diameter tires act like shorter gearing. They increase engine RPM at all speeds and typically reduce fuel economy. Your speedometer reads faster than actual speed, and your odometer overcounts miles traveled.
Example 1 - Lifted Truck: You install 35" tires to replace stock 31" tires on your truck. Your odometer now reads 12.9% fewer miles than you actually drive (35/31 = 1.129). If your odometer shows 20 MPG, your true MPG is actually 17.7 MPG.
Example 2 - Gear Change: You swap from 3.55 gears to 4.10 gears for better towing. Your speedometer now reads 15.5% slower than actual speed (4.10/3.55 = 1.155). If you calculate 18 MPG based on your odometer, your true MPG is actually 15.6 MPG.
Example 3 - Both Changed: You install 4.10 gears and 33" tires replacing 3.73 gears and 31" tires. The correction factor is (33/31) × (3.73/4.10) = 0.966. Your actual MPG is 3.4% lower than your odometer calculation.
The correction formula accounts for both tire diameter and gear ratio changes:
Correction Factor = (New Tire Diameter / Stock Tire Diameter) × (Stock Gear Ratio / New Gear Ratio)
Multiply your odometer-based MPG by this factor to get true fuel economy. If the factor is greater than 1.0, your actual MPG is better than the odometer suggests. If less than 1.0, your actual MPG is worse.
Gear Ratio Selection: For highway driving, choose the lowest numerical ratio your engine can pull comfortably in top gear at cruising speed. For city driving or towing, higher numerical ratios may be necessary despite the fuel economy penalty.
Tire Selection: Lighter tires with lower rolling resistance improve MPG. Avoid unnecessary oversizing. If you need larger tires for clearance, consider adjusting your gear ratio to compensate for the effective gear ratio change.
Driving Habits: Smooth acceleration, maintaining steady speeds, and anticipating stops dramatically improve fuel economy regardless of gear ratio or tire size.
Estimated MPG change for highway driving (assumes constant speed, no other changes):
| From Gear | To Gear | RPM Change @ 65 MPH | Estimated MPG Impact |
|---|---|---|---|
| 3.73 | 3.23 | -13.4% | +8% to +12% |
| 3.73 | 4.10 | +9.9% | -6% to -10% |
| 4.10 | 3.55 | -13.4% | +8% to +12% |
| 3.55 | 4.56 | +28.5% | -15% to -20% |
Actual results vary based on engine efficiency, aerodynamics, vehicle weight, and driving conditions. City MPG changes are typically smaller than highway changes.