SamCalculator

Gas Mileage Calculator

Calculate fuel economy from odometer readings. Enter your current and previous odometer readings with the fuel added to get instant MPG, km/L, L/100km, and fuel cost.

Currency:

Odometer & Fuel Data

for cost estimate

Gas Mileage & Fuel Economy Explained

Gas mileage — also called fuel economy or fuel efficiency — is the single most important number for understanding what you spend on driving. Whether you track it in miles per gallon (MPG), kilometers per liter (km/L), or liters per 100 kilometers (L/100km), a higher rating means you go further on every dollar of fuel. This complete toolkit covers five key calculations: odometer-based gas mileage tracking, MPG calculation, trip fuel cost estimation, road-trip fuel planning, and fuel economy unit conversion.

How Fuel Economy Is Calculated

MPG (Miles Per Gallon)

The standard US measure. Fill your tank, record the odometer. Next fill-up, note the new odometer and gallons added. MPG = Distance (miles) ÷ Fuel (US gallons). A 300-mile drive using 10 gallons = 30 MPG.

km/L (Kilometers Per Liter)

Used widely in Asia and developing markets. km/L = Distance (km) ÷ Fuel (liters). To convert from US MPG: km/L = MPG × 0.4251.

L/100km (Liters Per 100 km)

The European and Canadian standard. Lower is better. L/100km = (Fuel liters ÷ Distance km) × 100. Convert from US MPG: L/100km = 235.21 ÷ MPG. A car rated 8 L/100km uses 8 liters to travel 100 km.

MPG (UK) vs. MPG (US)

UK and US gallons are different sizes. A UK imperial gallon = 4.546 L vs. a US gallon = 3.785 L. This means UK MPG figures are about 20% higher: 30 US MPG ≈ 36 UK MPG for the same car.

5 Ways to Use This Calculator

1

Track Fill-Up Efficiency

Use the Gas Mileage tab after every fill-up to track whether your MPG is improving or declining — an early warning for maintenance issues.

2

Road Trip Planning

Use the Trip Fuel Calculator to estimate total fuel needed, whether you'll need to stop, and approximate fuel cost before you leave home.

3

Compare Vehicle Costs

Enter two vehicles' MPG figures in the Fuel Cost tab to calculate annual fuel savings when choosing between a compact car vs. an SUV.

4

International Travel

Use the Fuel Economy Converter when renting cars abroad. Convert the European L/100km rating to familiar US MPG so you know what to expect.

5

Budget Tracking

Enter your monthly driving distance to project monthly and annual fuel spending. Helpful for household budgeting and financial planning.

Best Practices to Improve Fuel Economy

Maintain proper tire pressure

Under-inflated tires increase rolling resistance. Check monthly — every 1 PSI drop reduces MPG by ~0.2%.

Drive at steady highway speeds

Fuel economy peaks at 45–60 mph. Every 5 mph above 60 reduces efficiency by 7–14%.

Avoid aggressive acceleration

Hard acceleration can cut city MPG by 15–30%. Smooth, gradual acceleration is far more efficient.

Reduce idle time

Idling burns ~0.5–0.7 gallons/hour with zero miles traveled. Turn off the engine if stopped for more than 60 seconds.

Remove excess weight

An extra 100 lbs reduces MPG by ~1–2%. Clear cargo that isn't needed for the trip.

Service air and fuel filters

A clogged air filter can reduce engine efficiency by 10%. Replace per manufacturer schedule.

Why Fuel Economy Matters

Financial Impact

The difference between a 20 MPG and 30 MPG vehicle is $875/year at 15,000 miles and $3.50/gallon — over $13,000 across a 15-year ownership period. Fuel economy directly affects your total cost of ownership.

Environmental Impact

Burning one US gallon of gasoline produces about 8.89 kg of CO₂. A vehicle averaging 30 MPG driven 15,000 miles/year emits approximately 2,100 kg of CO₂. Improving efficiency by just 5 MPG reduces annual emissions by hundreds of kilograms.

Vehicle Health Indicator

A sudden drop in MPG between fill-ups often signals a maintenance issue — dirty air filter, failing oxygen sensor, low tire pressure, or fuel system problems. Tracking fill-up efficiency is an early warning system.

Typical Fuel Economy by Vehicle Type

Vehicle TypeMPG (US)km/LL/100kmRating
Compact CarHonda Civic, Toyota Corolla3213.67.4B
SedanToyota Camry, Honda Accord2811.98.4B
SUVToyota RAV4, Honda CR-V229.410.7C
Pickup TruckFord F-150, Chevy Silverado187.713.1D
MinivanToyota Sienna, Honda Odyssey208.511.8D
HybridToyota Prius, Hyundai Ioniq5021.34.7A
EV Equiv.Tesla Model 3, Chevy Bolt9942.12.4A+

Values are approximate combined city/highway figures. Actual MPG varies by model year, driving conditions, and maintenance.

Core Fuel Economy Formulas

MPG (US) = Distance (miles) ÷ Fuel (US gallons)

The US standard measurement

km/L = Distance (km) ÷ Fuel (liters)

Metric equivalent used in Asia and emerging markets

L/100km = (Fuel liters ÷ Distance km) × 100

European standard — lower is more efficient

MPG (UK) = MPG (US) × 1.20095

Because UK gallon = 4.546 L vs. US 3.785 L

km/L = MPG (US) × 0.4251

Quick conversion between US MPG and metric km/L

L/100km = 235.215 ÷ MPG (US)

Convert US MPG to European L/100km

Trip Fuel Cost = (Distance ÷ MPG) × Price per gallon

Estimated cost in US dollar terms

Annual Fuel Cost = (Annual miles ÷ MPG) × Price per gallon

Total yearly spend based on driving habits

Common Fuel Economy Mistakes

Confusing US and UK gallons

Always confirm whether your vehicle's spec sheet or rental car displays US MPG or UK MPG. The 20% difference can significantly over- or under-estimate fuel needs.

Not accounting for driving style

EPA ratings are measured on standardized cycles. Aggressive city driving can reduce real-world MPG by 20–40% below the rated figure.

Cold weather efficiency loss

Cold engines run rich during warm-up, and cold air increases aerodynamic drag. Winter MPG is typically 10–25% lower than summer figures.

Short trips skewing averages

Very short trips never allow the engine to reach optimal operating temperature, dramatically lowering per-trip efficiency. Track only longer fill-up cycles for accurate readings.

Ignoring altitude effects

Higher altitudes mean less oxygen, requiring fuel system adjustments that reduce efficiency. Mountain driving typically shows 5–10% lower MPG.

About This Calculator

All formulas follow EPA, EU Directive 1999/94/EC, and ISO metric conversion standards. Unit conversion factors use internationally recognized exact values: 1 US gallon = 3.785411784 L, 1 UK gallon = 4.54609 L, 1 mile = 1.609344 km. CO₂ emission estimates use EPA-published figures of 8.89 kg per US gallon of gasoline. This tool is provided for educational and planning purposes and should be verified with your actual vehicle's manufacturer specifications.

Frequently Asked Questions

Gas mileage, or fuel economy, is the distance a vehicle travels per unit of fuel. In the US it's measured in miles per gallon (MPG); globally as kilometers per liter (km/L) or liters per 100 kilometers (L/100km).

MPG = Distance traveled (miles) ÷ Fuel used (US gallons). For example, if you drove 300 miles and used 10 gallons, your MPG is 30.

A US gallon is 3.785 liters while a UK (imperial) gallon is 4.546 liters. This means UK MPG figures are about 20% higher for the same vehicle — so 40 US MPG ≈ 48 UK MPG.

L/100km is the fuel consumption rate used in most of Europe and Canada: how many liters of fuel a vehicle needs to travel 100 kilometers. Lower is better. Convert from US MPG: L/100km = 235.21 ÷ MPG.

A good MPG depends on vehicle type. For compact cars: 30–40 MPG; sedans: 25–35 MPG; SUVs: 20–28 MPG; pickup trucks: 15–22 MPG; hybrids: 45–60 MPG. The US EPA average for all new passenger vehicles is around 26 MPG.

Key improvements include maintaining correct tire pressure, replacing air filters, using cruise control on highways, reducing idling, removing excess weight, and driving at steady speeds. Aggressive acceleration and braking can reduce MPG by 15–30%.

Highway driving is typically 20–40% more fuel-efficient than city driving because consistent speeds reduce stop-and-go fuel waste. Hybrids and EVs are an exception — they often perform better in city traffic due to regenerative braking.

Trip Fuel Cost = (Trip Distance ÷ MPG) × Fuel Price per Gallon. For metric: Trip Fuel Cost = (Distance km ÷ 100) × L/100km × Price per Liter.

Kilometers per liter (km/L) tells you how many kilometers a vehicle travels on one liter of fuel. It's the metric equivalent of US MPG. To convert: km/L = MPG × 0.4251.

Fill your tank completely. Record the odometer reading. Drive normally. When you refuel, record the new odometer reading and the gallons added. MPG = (New odometer − Old odometer) ÷ Gallons added.

MPG varies due to driving conditions (city vs. highway), weather (cold reduces efficiency by 15–24%), speed (optimal efficiency is 45–60 mph), air conditioning use, cargo weight, tire pressure, and fuel quality.

Use the Trip Fuel Calculator: enter trip distance, vehicle MPG, tank size, and current fuel level. It calculates how many refueling stops you'll need and estimates total trip fuel cost.

Use the Fuel Economy Converter tab to instantly convert between MPG (US), MPG (UK), km/L, L/100km, miles per liter, and gallons per 100 miles.

Burning one US gallon of gasoline produces approximately 8.89 kg (19.6 lb) of CO₂. Per liter, that's approximately 2.31 kg. Our calculator estimates your fill-up and annual CO₂ based on your fuel consumption.

Annual Fuel Cost = (Annual Miles ÷ MPG) × Fuel Price per Gallon. Based on the US average of 15,000 miles/year and $3.50/gallon, a 30 MPG car costs about $1,750/year in fuel vs. $2,625 for a 20 MPG vehicle — a $875 annual difference.

Related Automotive Calculators