Fuel Economy Calculator

Calculating Fuel Economy

Fuel economy measures how far a vehicle travels on a given amount of fuel. The US standard is miles per gallon (MPG), while most of the world uses liters per 100 kilometers (L/100km). These two scales are inversely related — MPG measures distance per unit of fuel (higher is better), while L/100km measures fuel per unit of distance (lower is better). To convert between them: L/100km = 235.215 / MPG.

The Formulas

MPG = Miles Driven / Gallons Used

L/100km = (Liters Used / Kilometers) × 100

Cost per Mile = Fuel Price / MPG

For the most accurate results, fill up to the same level each time (typically full), record the odometer reading, and divide the distance driven by the fuel added at the next fill-up. This tank-to-tank method eliminates guesswork about how much fuel is actually in the tank.

What Affects Fuel Economy

A vehicle's fuel economy is not fixed — it varies significantly depending on driving conditions and maintenance. Understanding these factors helps you identify why your MPG may be higher or lower than the EPA estimate on the window sticker.

• Driving habits: Aggressive acceleration and hard braking can lower MPG by 15-30% compared to smooth, steady driving. Maintaining a consistent speed and anticipating stops makes a measurable difference.
• Speed: Aerodynamic drag increases with the square of speed. Most vehicles hit their peak efficiency between 35-50 mph. Fuel economy drops roughly 1% for every mph above 50 mph on the highway.
• Tire pressure: Underinflated tires increase rolling resistance. Every 1 psi below the recommended pressure costs about 0.2% in fuel economy. Check pressure monthly when tires are cold.
• Vehicle weight: Every 100 lbs of extra weight reduces MPG by about 1-2%. Remove unnecessary cargo, roof racks, and toolboxes when not in use.
• Temperature: Cold engines run rich until they reach operating temperature. Short trips in cold weather can drop fuel economy by 12-20% compared to warm-weather highway driving.
• Maintenance: Dirty air filters, worn spark plugs, old engine oil, and dragging brakes all increase fuel consumption. A well-maintained engine runs closer to its rated efficiency.
• Ethanol content: E10 (10% ethanol) contains about 3% less energy than pure gasoline. E15 and E85 blends reduce MPG further because ethanol has roughly two-thirds the energy density of gasoline.

City vs. Highway Fuel Economy

EPA ratings list separate city and highway numbers because driving patterns affect fuel use dramatically. City driving involves frequent stops, idling at lights, and low-speed acceleration — all of which burn more fuel per mile. Highway driving at steady speed is more efficient because the engine operates in a narrow, optimal RPM range and there is no energy wasted on repeated acceleration from a stop. Most drivers see real-world numbers 5-15% below the EPA combined rating due to factors the test cycle does not fully capture, like cold starts, air conditioning load, and aggressive driving.

Tracking Fuel Economy Over Time

A single tank of fuel is not enough to judge your vehicle's true efficiency. Conditions vary from tank to tank — one fill-up may include a highway road trip while the next is all city errands. Track your MPG over at least five fill-ups and calculate a running average. A sudden drop in fuel economy (more than 10-15% below your baseline) often signals a maintenance issue such as a sticking caliper, failing oxygen sensor, or a leak in the evaporative emissions system.

Typical Fuel Economy by Vehicle Type

• 40+ MPG: Hybrids and efficient compact cars
• 30-40 MPG: Modern sedans and small crossovers
• 20-30 MPG: SUVs, minivans, and light trucks
• 15-20 MPG: Full-size trucks and V8 vehicles
• Under 15 MPG: Heavy-duty trucks and performance cars under hard driving