How to Calculate Gas Mileage and Trip Costs - Complete Guide with Examples

Gas mileage, also known as fuel economy, measures how far a vehicle can travel using a specific amount of fuel. It's typically expressed in miles per gallon (MPG) in the United States or liters per 100 kilometers (L/100km) in metric countries. Higher MPG means better fuel efficiency, while lower L/100km indicates better efficiency.

Understanding your vehicle's gas mileage helps you estimate fuel costs for trips, compare vehicle efficiency, and make informed decisions about driving habits. Whether you're planning a road trip or just curious about your car's performance, knowing how to calculate fuel economy empowers you to save money and reduce environmental impact.

The basic gas mileage formula is straightforward: MPG = Miles Traveled ÷ Gallons Used. For metric users: L/100km = (Liters Used ÷ Kilometers Traveled) × 100.

To calculate trip cost, use: Trip Cost = Distance × (Fuel Price ÷ MPG) or Trip Cost = (Distance ÷ 100) × L/100km × Fuel Price. For example, a 300-mile trip in a 25 MPG car with gas at $3.50/gallon costs: 300 × ($3.50 ÷ 25) = $42.

Example 1: You drive 350 miles and use 14 gallons of gas. Your MPG = 350 ÷ 14 = 25 MPG. If gas costs $3.80/gallon, your cost per mile is $3.80 ÷ 25 = $0.152 per mile.

Example 2: Planning a 500-mile road trip in an SUV with 20 MPG efficiency. With gas at $4.00/gallon, you'll need 500 ÷ 20 = 25 gallons, costing 25 × $4.00 = $100 total or $0.20 per mile.

Example 3: Your hybrid gets 45 MPG on highway driving. For a 225-mile trip at $3.60/gallon: 225 ÷ 45 = 5 gallons needed, total cost = 5 × $3.60 = $18, or just $0.08 per mile.

Using tank capacity instead of actual consumption: Don't assume your tank size equals fuel used. Always track how much you actually refill between fill-ups for accurate calculations.

Ignoring fuel price variations: Gas prices vary by location and time. A trip cost calculated at $3.00/gallon could be $4.50/gallon in another state, changing your budget significantly.

Not accounting for driving conditions: Highway driving typically yields 10-20% better mileage than city driving. Your actual fuel economy may differ from EPA estimates based on traffic, terrain, and driving style.