GarageCalc Combined MPG estimates fuel economy across city and highway driving. The US EPA's window-sticker calculation uses 55% city miles and 45% highway miles. It adds the fuel each part would use, then divides the total distance by the total fuel. It is not the arithmetic average of the two MPG figures.
What do city, highway and combined MPG mean?
City and highway MPG are standardized estimates for different driving conditions. They help you compare vehicles on the same basis; neither predicts exactly what your car will return on every route. The EPA's combined number represents a specific distance mix: 55 miles of city driving and 45 miles of highway driving per 100 miles. Your mix may be different.
The EPA defines combined fuel economy as 1 ÷ [(0.55 ÷ city MPG) + (0.45 ÷ highway MPG)]. The division matters because gallons, rather than MPG values, add across trip segments. See the EPA Green Vehicle Guide glossary for the definition and formula.
Worked example: 25 city MPG and 35 highway MPG
These are illustrative ratings, not a particular vehicle's EPA label. Over 100 miles at the EPA's 55/45 split:
- 55 city miles ÷ 25 MPG = 2.20 gallons.
- 45 highway miles ÷ 35 MPG = 1.29 gallons.
- 100 miles ÷ 3.486 total gallons = 28.69 combined MPG.
A simple 55/45 average of the MPG figures gives 29.5 MPG, which is too high for this example. It would imply less fuel used than the two segments actually require. For an actual vehicle, use its published combined rating when you have it: the city and highway numbers printed on a label are rounded, so recomputing from those displayed figures may not reproduce the label exactly.
What if your driving mix is different?
Use your estimated share of miles, not the share of time spent in traffic. For any mix, add city miles ÷ city MPG and highway miles ÷ highway MPG to estimate gallons used. Then divide total miles by those gallons. Here is the same hypothetical car across several 100-mile routes:
| City miles | Highway miles | Fuel used | Trip MPG |
|---|---|---|---|
| 0% | 100% | 2.86 gal | 35.00 |
| 25% | 75% | 3.14 gal | 31.82 |
| 50% | 50% | 3.43 gal | 29.17 |
| 55% | 45% | 3.49 gal | 28.69 |
| 75% | 25% | 3.71 gal | 26.92 |
| 100% | 0% | 4.00 gal | 25.00 |
For example, a trip with 25 city miles and 75 highway miles would use about 3.14 gallons and average 31.82 MPG. The 55/45 number is a comparison standard, not a promise that every driver will get 28.69 MPG.
Which MPG should you use for a trip budget?
Start with the city, highway or combined figure that most closely fits your route. For a mostly highway trip, the highway estimate is more relevant than the combined label. If you have measured MPG on similar drives, use that instead. Speed, weather, traffic and vehicle load can change real fuel use; EPA testing is adjusted for several real-world factors but still cannot represent every trip. The EPA explains how its fuel-economy tests and adjustments work.
To check what your own car delivered, enter miles and gallons from two comparable fill-ups in our MPG calculator. Then enter that observed result, or a suitable EPA estimate, in the fuel cost calculator. A full-tank measurement covers all driving since the last fill-up, so it will not separate city from highway use unless you track those segments separately.