Gas is one of the biggest ongoing expenses of car ownership. Yet most drivers have only a vague idea of their actual fuel efficiency — and an even vaguer idea of how much a road trip will cost in fuel. Knowing your vehicle’s real-world gas mileage helps you budget accurately, spot mechanical problems early, and make smarter decisions about when to drive, what to drive, and how to drive.

This guide walks you through calculating fuel efficiency in both MPG and L/100km, estimating trip costs, and improving your gas mileage.

Why Knowing Your Fuel Efficiency Matters

Your car’s advertised MPG is measured under controlled laboratory conditions that rarely reflect real-world driving. Factors like traffic, terrain, climate, and driving habits can reduce your actual fuel efficiency by 10–30% compared to the sticker number.

Tracking your real-world gas mileage helps you:

Budget accurately for commuting and travel
Detect problems early — a sudden drop in MPG can indicate engine issues, low tire pressure, or brake drag
Compare vehicles when shopping for a new car
Make informed decisions about whether to drive, fly, or take public transit

How to Calculate MPG (Miles Per Gallon)

MPG is the standard fuel efficiency measure in the United States and a few other countries. The higher the number, the more efficient the vehicle.

MPG = Miles Driven ÷ Gallons of Fuel Used

Here’s the step-by-step method:

Fill your tank completely and reset your trip odometer to zero
Drive normally until you need to refuel (at least half a tank for accuracy)
Fill the tank completely again and note the number of gallons it takes
Record the miles driven from your trip odometer
Divide miles by gallons

For example, if you drove 380 miles and used 12.5 gallons:

MPG = 380 ÷ 12.5 = 30.4 MPG

For the most accurate results, average your calculations over three or more fill-ups. A single tank can be affected by unusual driving conditions.

How to Calculate L/100km (Liters per 100 Kilometers)

L/100km is the standard fuel efficiency measure in most countries outside the US. Unlike MPG, a lower number is better — it means you use less fuel to travel the same distance.

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

For example, if you drove 610 km and used 45 liters:

L/100km = (45 ÷ 610) × 100 = 7.38 L/100km

MPG to L/100km Conversion

Since these two scales are inversely related (higher MPG = lower L/100km), the conversion isn’t a simple multiplier. Use these formulas:

L/100km = 235.215 ÷ MPG

MPG = 235.215 ÷ L/100km

Common conversions:

MPG (US)	L/100km
15	15.7
20	11.8
25	9.4
30	7.8
35	6.7
40	5.9
50	4.7

Note: The 235.215 constant applies to US gallons. For imperial gallons (used in the UK), use 282.481 instead.

How to Calculate Trip Fuel Cost

Once you know your vehicle’s fuel efficiency, estimating the cost of any trip is straightforward:

Fuel Cost = (Distance ÷ Efficiency) × Fuel Price

For example, a 500-mile trip in a car that gets 28 MPG, with gas at $3.50/gallon:

Fuel Cost = (500 ÷ 28) × $3.50 = 17.86 × $3.50 = $62.50

For metric users:

Fuel Cost = (Distance ÷ 100) × L/100km × Fuel Price per Liter

A 800 km trip in a car that uses 7.5 L/100km, with fuel at $1.65/L:

Fuel Cost = (800 ÷ 100) × 7.5 × $1.65 = 8 × 7.5 × $1.65 = $99.00

What Affects Gas Mileage

Understanding these factors helps you interpret your fuel efficiency numbers and identify ways to improve them.

Driving Habits

Speed: Fuel efficiency typically drops significantly above 50–60 mph. Every 5 mph over 50 mph is roughly equivalent to paying an additional $0.20–$0.30 per gallon
Aggressive driving: Rapid acceleration and hard braking can lower gas mileage by 15–30% on the highway and 10–40% in city driving
Idling: An idling car gets 0 MPG. If you’ll be stopped for more than 60 seconds, it’s generally more efficient to turn off the engine

Vehicle Condition

Tire pressure: Every 1 psi drop below the recommended pressure reduces gas mileage by about 0.3%. Underinflated tires also wear faster and are less safe
Engine maintenance: A poorly tuned engine can reduce fuel efficiency by an average of 4%, and a serious problem like a faulty oxygen sensor can reduce it by up to 40%
Motor oil: Using the manufacturer’s recommended grade of motor oil can improve gas mileage by 1–2%

External Factors

Air conditioning: Using AC can reduce fuel efficiency by 5–25%, though at highway speeds it’s often more efficient than driving with windows open (which increases drag)
Extra weight: An additional 100 pounds in your vehicle reduces fuel efficiency by about 1%
Roof cargo: Roof-mounted cargo boxes can reduce fuel efficiency by 2–8% in city driving and 10–25% on the highway due to aerodynamic drag

Tips to Improve Fuel Efficiency

Maintain steady speeds: Use cruise control on highways to avoid unnecessary speed fluctuations
Observe the speed limit: Most vehicles achieve optimal fuel efficiency between 30–50 mph
Avoid excessive idling: Turn off your engine if you expect to wait more than a minute
Keep tires properly inflated: Check pressure monthly, including the spare
Remove excess weight: Clean out your trunk and remove roof racks when not in use
Plan your routes: Combine errands, avoid rush hour, and use GPS to find the most efficient routes
Use the right oil: Follow your owner’s manual recommendation for oil viscosity and consider synthetic oil for better efficiency
Replace air filters regularly: A clogged air filter can reduce acceleration and fuel economy on older vehicles

Good MPG by Vehicle Type

What counts as “good” gas mileage depends heavily on the type of vehicle:

Vehicle Type	Good MPG	Excellent MPG
Compact car	30+	40+
Midsize sedan	28+	38+
Full-size SUV	22+	28+
Pickup truck	20+	26+
Minivan	24+	30+
Hybrid	45+	55+

Electric vehicles are rated in MPGe (miles per gallon equivalent), which typically ranges from 90–130 MPGe, though this metric works differently from traditional MPG.

Calculate Your Gas Mileage

Use our free Gas Mileage Calculator to quickly calculate your vehicle’s fuel efficiency, estimate trip costs, and compare performance across fill-ups.