Fuel Economy Comparison Calculator (City vs Highway MPG)
Blend city and highway MPG into a combined figure using the EPA fuel-weighted method, and estimate annual fuel cost from your mileage and gas price. Runs in your browser.
45% highway · EPA uses 55/45.
Combined MPG is the fuel-weighted (harmonic) blend of city and highway: 1 ÷ (city% ÷ cityMPG + highway% ÷ highwayMPG) — not a simple average, because MPG is miles per gallon. The EPA uses a 55% city / 45% highway split. Annual fuel cost = annual miles ÷ combined MPG × gas price (418 gallons/year here). Educational; everything runs in your browser.
About this tool
Cars are rated with separate city and highway fuel-economy numbers because they burn fuel very differently in each: stop-and-go city driving, with idling and constant acceleration, is less efficient than steady highway cruising (the reverse of how engines behave is true for hybrids, which often beat their highway figure in the city). This calculator blends the two into a realistic combined MPG for your driving mix and turns it into an annual fuel cost. The important subtlety it gets right is that you cannot simply average city and highway MPG, because miles per gallon is a rate with gallons in the denominator — averaging would overstate efficiency. Instead the combined figure is the fuel-weighted, or harmonic, blend: 1 ÷ (city-fraction ÷ city MPG + highway-fraction ÷ highway MPG). This is the same principle the EPA uses for its combined rating, which assumes a 55% city / 45% highway split; you can adjust the split to match how you actually drive. For example, 25 city and 35 highway MPG at the EPA's 55/45 split gives a combined figure of about 28.7 MPG, not the naive 30. From there the tool computes annual fuel cost (annual miles ÷ combined MPG × gas price) and, for comparison, what it would cost if you drove all city or all highway — which shows how much your driving mix matters. Use it to compare two vehicles fairly on your own commute, to estimate a year's gas budget, or to see the payoff of a more efficient car. Note these are rated/estimated figures; real-world economy varies with weather, terrain, load, tire pressure, and driving style, often running somewhat below the sticker numbers. Educational; everything runs in your browser; nothing is uploaded.
How to use it
- Enter the car's city and highway MPG (from the window sticker or fueleconomy.gov).
- Set your city/highway driving split (EPA default is 55/45).
- Enter your annual mileage and local gas price.
- Read the combined MPG and annual fuel cost, plus all-city and all-highway comparisons.
Frequently asked questions
- How do you combine city and highway MPG?
- Use the fuel-weighted (harmonic) blend, not a simple average: combined = 1 ÷ (city% ÷ city MPG + highway% ÷ highway MPG). The EPA uses a 55% city / 45% highway split. For 25 city / 35 highway that gives about 28.7 MPG.
- Why can't I just average the two numbers?
- Because MPG is miles per gallon — a rate with gallons on the bottom. Averaging the rates overweights the high number. The correct method weights by fuel used, which is the harmonic mean.
- How is annual fuel cost calculated?
- Annual miles ÷ combined MPG = gallons per year, then × gas price = annual fuel cost. The tool also shows the all-city and all-highway costs so you can see the impact of your driving mix.
- Why is city MPG usually lower than highway?
- City driving involves idling, frequent stops, and acceleration, which waste fuel, while highway driving is steady and efficient. Hybrids are an exception — regenerative braking often makes their city figure higher.
- Will I really get the rated MPG?
- Often a bit less. EPA figures are standardized estimates; real-world economy depends on weather, terrain, cargo, tire pressure, speed, and driving style. Use your own observed MPG if you have it.
- Is anything uploaded?
- No. All calculations run entirely in your browser.