How to combine MPG for a trip with different driving conditions
Add fuel used on each leg before calculating combined MPG. Compare equal and unequal distances, changing prices and a metric example with worked numbers.
A trip with one slow leg and one efficient highway leg needs more care than averaging the two MPG figures. Miles per gallon is a ratio. Calculate the fuel used on each leg, add it, and then divide total distance by total fuel. That keeps the estimate tied to what you will actually buy.
Why a simple MPG average underestimates this trip
Suppose you drive 100 miles at 20 US MPG and another 100 miles at 40 US MPG. Use an invented price of $3.60 per US gallon throughout. The first leg uses five gallons; the second uses two and a half.
| Leg | Distance | Economy | Fuel | Cost at $3.60/US gal |
|---|---|---|---|---|
| A | 100 mi | 20 US MPG | 5 US gal | $18.00 |
| B | 100 mi | 40 US MPG | 2.5 US gal | $9.00 |
| Total | 200 mi | 26.666… US MPG | 7.5 US gal | $27.00 |
The arithmetic average of 20 and 40 is 30 MPG. Applying 30 MPG to the entire 200 miles predicts only 6.666… gallons, or $24. That is $3 below the correctly combined estimate. Combined economy is 200 ÷ 7.5 = 26.666… MPG.
The general calculation
Total fuel = distance A ÷ MPG A + distance B ÷ MPG B + …
Combined MPG = total miles ÷ total gallons.
The same approach handles unequal distances. For 50 miles at 20 MPG and 150 miles at 40 MPG, the fuel quantities are 2.5 and 3.75 gallons. The 200-mile total therefore uses 6.25 gallons and achieves 32 MPG. Weighting MPG directly by distance would give 35 MPG, which is still the wrong result. Add consumption first.
Keep US and Imperial gallons separate. A UK rating in Imperial MPG cannot be combined directly with a US MPG figure. Convert the units consistently first; the US versus Imperial MPG guide explains the distinction.
Run the legs in TripFuel
- Choose miles, US MPG and a price per US gallon.
- Enter the first leg with one trip and the return option off; record its fuel and cost.
- Replace the inputs with the second leg and record that result.
- Add the unrounded quantities where available. Round the final money estimate for your budget.
TripFuel applies one economy assumption to the distance currently entered. It does not provide a multi-leg route editor. Repeating a scenario or selecting a return journey assumes the same entered economy and price. Calculate the return separately if its conditions differ, rather than also selecting a return option and counting it twice.
Litres per 100 km behave differently
Consumption expressed as L/100 km is already fuel per distance. For 100 km at 10 L/100 km and 100 km at 5 L/100 km, the fuel total is 15 litres, so the combined rate is 7.5 L/100 km. For unequal distances, weight the consumption rate by kilometres: 50 km at 10 plus 150 km at 5 uses 12.5 litres over 200 km, or 6.25 L/100 km.
Changing prices and real-world uncertainty
If the same 7.5 gallons are valued at different prices, calculate each quantity at its relevant price. Five gallons at $3.60 plus 2.5 at $4.00 totals $28. This is a cost-allocation assumption, not a prediction of where a partially filled tank will need refueling. Actual cash spent at stations can differ because fuel remains in the tank.
Economy figures are inputs, not guarantees. Repeat the calculation with a less efficient scenario when traffic, weather or load is uncertain. Keep tolls and other travel expenses separate. The methodology documents conversions and scope; FuelEconomy.gov explains why driving conditions affect fuel use.