How to Calculate Fuel Economy and Gas Costs

Fuel economy measures how far your vehicle travels on a given amount of fuel. Whether you track it in miles per gallon in the United States or liters per 100 kilometers in most other countries, understanding your fuel efficiency is the first step toward reducing one of the largest recurring expenses in your household budget.

This guide explains the key metrics, walks you through the math for calculating both fuel economy and cost per mile, and provides actionable strategies to squeeze more miles from every gallon. You will also find real-world examples, a reference table comparing vehicle types, and answers to the most common questions drivers ask about gas mileage.

What Is Fuel Economy?

Fuel economy describes the relationship between the distance a vehicle can travel and the amount of fuel it consumes. A car with high fuel economy uses less fuel to cover the same distance compared to a car with low fuel economy. This metric matters for three reasons: it directly affects your wallet through fuel costs, it determines your vehicle's carbon footprint, and it influences how far you can travel between fill-ups.

Every new vehicle sold in the United States receives an EPA fuel economy rating displayed on the window sticker at the dealership. These ratings are determined through standardized laboratory tests that simulate city and highway driving conditions. Real-world fuel economy often differs from the EPA estimate by 10 to 20 percent depending on driving habits, weather, terrain, and vehicle maintenance.

MPG vs L/100km

The two most common fuel economy metrics work inversely. Miles per gallon (MPG) is a higher-is-better metric used in the United States, while liters per 100 kilometers (L/100km) is a lower-is-better metric used in Canada, Europe, Australia, and most of the world. To convert between them, use the constant 235.215:

An important nuance is that MPG improvements are not linear in terms of fuel savings. Going from 10 MPG to 20 MPG saves far more fuel (50 gallons per 1,000 miles) than going from 30 MPG to 40 MPG (8.3 gallons per 1,000 miles). This is why L/100km is considered a more intuitive metric for comparing fuel consumption — it scales linearly with actual fuel used.

How to Calculate Fuel Economy

Calculating your actual fuel economy is straightforward with the fill-up method. Follow these steps each time you refuel:

1. Fill your tank completely and record the odometer reading or reset the trip meter to zero.
2. Drive normally until you need to refuel. Do not change your driving habits — you want an accurate baseline.
3. Fill the tank completely again and note two numbers: the gallons (or liters) added and the miles (or kilometers) driven since the last fill-up.
4. Divide distance by fuel: MPG = Miles Driven / Gallons Used. For metric, divide liters used by distance in hundreds of kilometers: L/100km = (Liters Used / Kilometers Driven) x 100.
5. Repeat over multiple fill-ups to get an average. A single tank can be skewed by traffic, weather, or a long highway trip. Three to five fill-ups give a reliable average.

Cost per Mile Formula

Knowing your fuel economy lets you calculate exactly what each mile costs in fuel. This is valuable for budgeting, comparing vehicles, and deciding whether a more fuel-efficient car would save you money over time.

Practical Examples

These fictional but realistic scenarios show how fuel economy calculations apply to different driving situations.

Rachel's efficient sedan keeps her commuting cost under $72 per month. If she drove an SUV averaging 22 MPG, her annual fuel bill would jump to $1,406 — an increase of $547 per year.

For drivers with high annual mileage, even a modest improvement in MPG translates to hundreds of dollars in savings. Marcus would recoup the hybrid premium within a few years through fuel savings alone.

Fuel Economy Reference Table

This table compares average fuel economy ratings across common vehicle categories, along with estimated annual fuel costs based on 15,000 miles per year and $3.50 per gallon.

Tips to Improve Your Gas Mileage

Small changes in driving habits and vehicle maintenance can improve fuel economy by 10 to 30 percent. Here are the most effective strategies, ranked roughly by impact:

• Drive at moderate speeds. Fuel economy drops rapidly above 50 MPH. Driving 60 instead of 75 on the highway can improve your MPG by 15 to 25 percent. Use cruise control on flat highways to maintain a steady speed.
• Avoid aggressive acceleration and braking. Rapid starts and hard stops can lower fuel economy by 15 to 30 percent in city driving and 10 to 15 percent on the highway. Accelerate gently and coast to a stop when possible.
• Keep tires properly inflated. Under-inflated tires increase rolling resistance. Checking pressure monthly and inflating to the manufacturer-recommended PSI can improve fuel economy by up to 3 percent.
• Remove excess weight. Every 100 pounds of extra weight reduces fuel economy by about 1 percent. Clear out heavy items from the trunk that you do not need for the trip.
• Use the recommended motor oil. Using the manufacturer-recommended grade of motor oil can improve fuel economy by 1 to 2 percent. Look for oil labeled "Energy Conserving" which contains friction-reducing additives.
• Replace air filters on schedule. A clean air filter ensures optimal airflow to the engine. While modern fuel-injected engines compensate for dirty filters, replacement improves overall engine performance and longevity.
• Minimize idling. Idling gets zero miles per gallon. Turn off the engine if you expect to wait more than 30 seconds. Modern engines use less fuel restarting than they consume idling for 30 seconds or more.
• Plan and combine trips. A warmed-up engine is more fuel-efficient than a cold one. Combining several short errands into one trip saves fuel compared to making multiple cold starts throughout the day.

Common Mistakes to Avoid

• Warming up the engine for minutes before driving. Modern vehicles need only 30 seconds to a minute of idling before driving. Extended warm-ups waste fuel and increase engine wear.
• Ignoring the check engine light. A malfunctioning oxygen sensor alone can reduce fuel economy by up to 40 percent. Address warning lights promptly to avoid hidden fuel waste.
• Using roof racks when not needed. An empty roof rack increases aerodynamic drag and can reduce fuel economy by up to 5 percent at highway speeds. Remove it when it is not carrying cargo.
• Believing premium fuel improves mileage in regular-fuel cars. Unless your engine requires premium, higher octane fuel provides zero benefit to fuel economy or performance.
• Not tracking fuel economy over time. A sudden drop in MPG can indicate a mechanical problem. Without tracking, you may drive for months with a hidden issue costing you hundreds of dollars in wasted fuel.

Last updated: February 23, 2026