How to calculate the fuel cost of a road trip and your real mileage

The basic formula

With fuel economy in miles per gallon, the gallons you need are the distance divided by the mpg. A 300-mile trip in a car that gets 30 mpg needs 300 ÷ 30 = 10 gallons. If your car's economy is in liters per 100 kilometers (L/100 km), the liters are the distance in km times the figure divided by 100: 450 km at 7.5 L/100 km is 450 × 7.5 ÷ 100 = 33.75 liters.

The fuel cost is that amount times the price, and tolls and other expenses are added to the total. The whole calculation hinges on one number you may not know well: your fuel economy. That is why the rest of this guide insists on measuring it.

L/100 km, km/L and mpg: three ways to say the same thing

Many countries express fuel economy in L/100 km (lower is better), others use km/L (higher is better), and the United States uses miles per gallon, mpg (higher is better). To convert: km/L = 100 ÷ (L/100 km), and mpg = 235.21 ÷ (L/100 km), using the US gallon. It is an inverse relationship, so L/100 km × mpg always equals 235.21.

Mental shortcut: to turn mpg into L/100 km, divide 235 by the mpg (30 mpg ≈ 7.8 L/100 km). Beware of the imperial gallon, which is larger (4.546 liters): 8 L/100 km is about 35.3 UK mpg and 29.4 US mpg. If you find an mpg figure, check which gallon it uses. To convert a single value, use the L/100 km to mpg converter or the mpg to L/100 km one.

How to measure your real mileage: full tank to full tank

The factory figure or the dashboard readout is only a guide. The most reliable way is to measure it yourself with the full-tank-to-full-tank method:

1. Fill the tank until the pump clicks off the first time and note the odometer (or reset the trip meter).
2. Drive normally until you need to refuel.
3. Fill up again until the first click, ideally at the same pump and with the car in the same position. Note the amount added and the distance driven.
4. Calculate: distance ÷ fuel added for mpg, or fuel added ÷ distance × 100 for L/100 km.

Example: you added 12 gallons and drove 336 miles since the last fill-up. 336 ÷ 12 = 28 mpg, which is 235.21 ÷ 28 = 8.40 L/100 km. A single measurement can vary with traffic and how you filled the tank; if you average three or four fill-ups, the number is far more reliable. In the fuel calculator, the «Calculate my real fuel economy» section does this math with the fuel and distance you type, and «Use this fuel economy» carries it over to the trip calculation.

A trip worked out from start to finish

Take a round trip of 300 miles each way in a car that gets 30 mpg. The prices are made up for the example: fuel at $4.00 per gallon and $18 in tolls in total. Three people travel, the tank holds 15 gallons and the average speed is 60 mph.

Two practical details. The 450-mile range comes from the ideal math; it is wise not to arrive with an empty tank and to refuel earlier. And the driving time does not include stops. Add a 20-minute break every two hours and the trip takes noticeably longer than those 10 hours.

How to use the fuel calculator

1. Open the fuel calculator and choose the unit system: kilometers and liters, or miles and gallons.
2. Enter the one-way distance and check «Round trip» if you are coming back: the calculator doubles the distance for you.
3. Enter your vehicle's fuel economy and its unit (L/100 km, km/L or mpg). If you do not know it, pick a vehicle type to start with a typical value and then replace it with yours.
4. Enter the fuel price, the tolls and other expenses for the whole trip, and how many people share the cost. Tolls are added once, so enter the round-trip total.
5. Optionally add the average speed to see the travel time and the tank size to see the range.

It shows the fuel needed, the total cost, and the cost per person and per mile or kilometer. It assumes no price: you have to enter it. The calculations run in your browser and what you type is not sent anywhere.

What raises or lowers fuel consumption

No percentage is universal, but these factors always play a part:

• Speed: air resistance grows a lot as speed rises, so at high highway speeds consumption is usually clearly higher than at moderate ones.
• Under-inflated tires: they increase rolling resistance and fuel use. Check them when the car is cold.
• Air conditioning: it makes the engine work harder, especially in slow traffic.
• Weight and roof loads: more load means more fuel; an empty roof rack or cargo box also adds drag.
• Traffic: frequent starting and braking burns more than a steady pace.
• Short trips on a cold engine, hills, headwinds and the condition of the vehicle (filters, spark plugs, alignment) also count.

That is why a highway trip tends to use less fuel per mile than a day in heavy city traffic, and why you should use the figure you measured in driving similar to the trip.

Factory figures vs real mileage, and the tool's limits

The fuel economy a manufacturer publishes comes from tests under controlled conditions. In daily use, with real loads, weather, traffic and driving style, fuel use is usually higher. That is why measuring your own is better than trusting the brochure.

The fuel calculator is an estimate. It does not use maps or know your route: you type the distance. It does not know fuel or toll prices. It works with liquid fuels (liters or US gallons), so it does not apply as is to an electric car or one running on CNG. It also leaves out wear, insurance and parking: it covers fuel plus the expenses you add yourself.

Frequently asked questions