How it works
The calculator turns distance and fuel economy into fuel used, then multiplies by your price:
Towing mpg = normal mpg × (1 − loss %)
Fuel used = distance ÷ towing mpg
Trip cost = fuel used × price per gallon
In metric mode it uses L/100 km, which goes up when towing: towing L/100 km = normal L/100 km ÷ (1 − loss %), and fuel used = distance × L/100 km ÷ 100.
The MPG loss estimator
When you don’t know your towing economy, the estimator combines the trailer’s shape (drag) and weight (rolling and climbing resistance):
Loss % = shape base + 1.5% per 1,000 lb of trailer
Shape base: low 15%, medium 25%, tall 35% (result limited to 5–60%)
It is a planning estimate. Your own fill-up records will always be more accurate.
Worked example
A 400-mile trip in a truck that gets 20 mpg empty, pulling a 6,000 lb travel trailer, with fuel at $3.50 per gallon.
- Estimated loss: 35% + 6 × 1.5% = 44%, so towing economy ≈ 11.2 mpg
- Fuel: 400 ÷ 11.2 = 35.7 gallons
- Cost: 35.7 × $3.50 = $125, about $55 more than driving without the trailer.
Estimated fuel economy loss chart
Estimator output by trailer weight and shape. Use it as a starting point, not a promise.
| Trailer weight | Low | Medium | Tall | 20 mpg truck, tall trailer |
|---|---|---|---|---|
| 2,000 lb | 18% | 28% | 38% | 12.4 mpg |
| 4,000 lb | 21% | 31% | 41% | 11.8 mpg |
| 6,000 lb | 24% | 34% | 44% | 11.2 mpg |
| 8,000 lb | 27% | 37% | 47% | 10.6 mpg |
| 10,000 lb | 30% | 40% | 50% | 10.0 mpg |
| 12,000 lb | 33% | 43% | 53% | 9.4 mpg |
Planning fuel stops
Towing shrinks your range as much as it raises cost. If you enter your tank size, the calculator shows the range on a full tank at towing economy and the number of fuel stops, assuming you refill at a quarter tank. Big rigs can’t always use every pump, so plan stops at truck-friendly stations rather than running low.
Heavier trailers burn more fuel, so it pays to know what you are really towing: try theRV water and propane calculator and thetowing capacity calculator.
Common mistakes
- Using the window sticker mpg. Start from your real, everyday mileage.
- Ignoring speed. Going from 60 to 70 mph with a tall trailer can cost several mpg.
- Mixing US and UK gallons. A UK gallon is 4.546 L, a US gallon 3.785 L.
- Forgetting the return trip. Tick “Round trip” or double the distance.