Calculate Weighted Fuel Economy
Determine your true Combined MPG and Annual Fuel Costs using the Harmonic Mean method
The Manufacturer's rating for city driving.
Please enter a positive number.
The Manufacturer's rating for highway driving.
Please enter a positive number.
Percentage of total distance driven in stop-and-go traffic (Standard EPA is 55%).
Current cost of gas or diesel in your area.
Total distance you expect to drive in a year.
Weighted Combined MPG
23.5 Miles Per Gallon (Harmonic Mean)
Annual Fuel Cost
$1,785.00
Gallons Consumed
510.6
Cost Per Mile
$0.15
Formula Used: Combined MPG = 1 / [ (City% / CityMPG) + (Highway% / HighwayMPG) ]. This harmonic mean correctly weights fuel consumption rates rather than just averaging the MPG numbers.
Figure 1: Comparison of Annual Fuel Costs based on driving environment.
| Driving Type | Share (%) | Miles Driven | Gallons Used | Est. Cost |
|---|
Comprehensive Guide to Calculate Weighted Fuel Economy
Understanding how to calculate weighted fuel economy is essential for accurate budgeting, fleet management, and vehicle comparison. While many drivers look at the simple "City" or "Highway" numbers on a window sticker, real-life driving is rarely exclusively one or the other. To get a true picture of your fuel efficiency, you must calculate weighted fuel economy using the specific proportion of miles you drive in different environments.
What is Weighted Fuel Economy?
Weighted fuel economy is a composite metric that represents the average efficiency of a vehicle across different driving conditions, weighted by the distance driven in each condition. Unlike a simple arithmetic average, it uses a harmonic mean to account for the physical reality of fuel consumption.
This metric is critical for:
- Commuters: Who mix highway cruising with city traffic.
- Fleet Managers: Who need to forecast fuel budgets for varying routes.
- Car Buyers: Comparing vehicles where one excels in the city (like hybrids) and another on the highway (like diesels).
A common misconception is that if you drive 50% city and 50% highway, you simply average the two MPG numbers. This is mathematically incorrect because fuel consumption is inversely proportional to MPG. You consume significantly more fuel per mile in the city, so city driving drags down the average more than highway driving pulls it up.
Calculate Weighted Fuel Economy: Formula and Explanation
To calculate weighted fuel economy correctly, we use the harmonic mean formula. This ensures we are averaging the consumption (gallons per mile), not just the rate (miles per gallon).
Formula:
Weighted MPG = 1 / [ (City_Share / City_MPG) + (Highway_Share / Highway_MPG) ]
Weighted MPG = 1 / [ (City_Share / City_MPG) + (Highway_Share / Highway_MPG) ]
Variables Table
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| City_MPG | Efficiency in stop-and-go traffic | Miles per Gallon | 15 – 60 MPG |
| Highway_MPG | Efficiency at sustained high speeds | Miles per Gallon | 25 – 70 MPG |
| City_Share | Fraction of miles driven in city | Decimal (0.0 – 1.0) | 0.55 (EPA Standard) |
Practical Examples
Example 1: The Commuter Sedan
Sarah drives a sedan rated at 20 MPG City and 40 MPG Highway. She has a long commute, so she drives 80% highway and 20% city.
- Inputs: 20 MPG (City), 40 MPG (Hwy), 20% City / 80% Hwy.
- Arithmetic Mean (Incorrect): (20 * 0.2) + (40 * 0.8) = 36 MPG.
- Weighted Harmonic Mean (Correct): 1 / [ (0.2/20) + (0.8/40) ] = 33.3 MPG.
Interpretation: Even though she drives mostly highway, the inefficiency of the city miles pulls her actual MPG down lower than a simple average would suggest.
Example 2: The Delivery Van
A delivery van gets 10 MPG City and 15 MPG Highway. The route is 90% city driving.
- Inputs: 10 MPG (City), 15 MPG (Hwy), 90% City / 10% Hwy.
- Calculation: 1 / [ (0.9/10) + (0.1/15) ] = 1 / [0.09 + 0.0066] = 10.35 MPG.
Interpretation: The result is very close to the City MPG because the vast majority of miles—and fuel burned—occur in the city.
How to Use This Weighted Fuel Economy Calculator
Follow these steps to get the most accurate financial data:
- Enter MPG Ratings: Input the official EPA estimates or your own observed MPG for city and highway.
- Adjust Driving Share: Slide the City Percentage to match your habits. If you drive mostly on interstates, lower this number. If you drive for Uber/Lyft in downtown areas, raise it.
- Input Financials: Enter the current fuel price and your annual mileage to see the cost impact.
- Analyze Results: Look at the "Annual Fuel Cost" and "Cost Per Mile". These are the numbers that matter for your budget, not just the raw MPG.
Key Factors That Affect Results
When you calculate weighted fuel economy, several external factors influence the real-world outcome:
- Traffic Patterns: "City" driving isn't just low speed; it involves idling. Heavy idling reduces MPG to zero, drastically affecting the weighted average.
- Driving Aggression: Rapid acceleration burns significantly more fuel, effectively lowering your City MPG variable regardless of the vehicle's rating.
- Terrain: Hilly environments require more energy. If your highway driving involves steep grades, your Highway MPG input should be lowered.
- Vehicle Load: Carrying heavy cargo or towing reduces efficiency. This increases the cost per mile significantly.
- Cold Weather: Engines are less efficient until warmed up. Short city trips in winter will result in a much lower City MPG than rated.
- Fuel Price Volatility: While MPG stays constant, the financial result (Annual Cost) fluctuates. A 20% rise in gas prices increases your annual burden directly.
Frequently Asked Questions (FAQ)
Why is the weighted MPG lower than the simple average?
MPG is a reciprocal unit (miles per gallon). You spend more time and burn more fuel covering a mile at low efficiency (city) than at high efficiency (highway). The harmonic mean accounts for this disproportionate fuel consumption.
What is the EPA standard split for combined MPG?
The EPA typically uses a split of 55% City driving and 45% Highway driving to calculate the "Combined MPG" seen on window stickers.
Does this calculator work for Electric Vehicles (EVs)?
Yes, but you must use MPGe (Miles Per Gallon equivalent) for the inputs. The math for weighting efficiency based on driving mix remains the same.
How do I know my City vs Highway percentage?
Track your odometer for a week. Note how many miles were driven on freeways versus surface streets. Divide city miles by total miles to get the percentage.
Can I use this for diesel vehicles?
Absolutely. Diesel vehicles often have a larger gap between city and highway efficiency, making it even more important to calculate weighted fuel economy accurately.
Does using AC affect weighted fuel economy?
Yes. AC usage puts a load on the engine. It typically lowers city MPG more than highway MPG because the AC runs for a longer time per mile driven in the city.
Why is Cost Per Mile important?
Cost Per Mile helps in comparing the true operating cost of different vehicles. It allows you to reimburse employees fairly or calculate the profitability of gig-driving jobs.
Is it better to prioritize City or Highway MPG when buying a car?
It depends entirely on your weighted usage. If you calculate weighted fuel economy and find your driving is 80% city, a hybrid with high City MPG is far better than a diesel with high Highway MPG.
Related Tools and Internal Resources
Explore more tools to help optimize your automotive finances:
- Fuel Cost Estimator – Estimate trip costs between specific destinations.
- Car Loan Calculator – Determine your monthly payments for a new fuel-efficient vehicle.
- Commute Savings Analyzer – Compare the cost of driving vs public transit.
- EV Savings Calculator – See how much you can save by switching to electric.
- Car Depreciation Calculator – Calculate total cost of ownership including value loss.
- Monthly Budget Planner – Integrate your fuel costs into your overall household budget.