Data Source and Methodology

This calculator uses standard, universally accepted formulas for determining vehicle fuel economy and cost. The methodologies align with the principles used by government agencies, such as the **U.S. Environmental Protection Agency (EPA)**, for consumer-facing fuel economy reporting (e.g., in 40 CFR Part 600).

All calculations are based strictly on the user-provided inputs and the fundamental formulas detailed below.

The Formulas Explained

The calculations performed depend on your selected unit system and mode.

1. US Customary Units (MPG)

To find Fuel Economy (MPG):

$ \text{Fuel Economy (MPG)} = \frac{\text{Distance (miles)}}{\text{Fuel Used (gallons)}} $

To find Total Cost:

$ \text{Total Cost} = \text{Fuel Used (gallons)} \times \text{Price per Gallon} $

To find Cost per Mile:

$ \text{Cost per Mile} = \frac{\text{Total Cost}}{\text{Distance (miles)}} \quad \text{or} \quad \frac{\text{Price per Gallon}}{\text{MPG}} $

2. Metric Units (L/100km)

To find Fuel Economy (L/100km):

$ \text{Fuel Economy (L/100km)} = \frac{\text{Fuel Used (litres)} \times 100}{\text{Distance (km)}} $

To find Total Cost:

$ \text{Total Cost} = \text{Fuel Used (litres)} \times \text{Price per Litre} $

To find Cost per Kilometer:

$ \text{Cost per km} = \frac{\text{Total Cost}}{\text{Distance (km)}} \quad \text{or} \quad \frac{\text{Fuel Economy (L/100km)}}{100} \times \text{Price per Litre} $

Glossary of Variables

  • Distance: The total length of your trip or the period you are measuring, entered in either miles or kilometers.
  • Fuel Used: The total amount of fuel your vehicle consumed over that distance, in gallons or litres.
  • Fuel Price: The cost for one unit of fuel (one gallon or one litre).
  • Vehicle's Economy: Your vehicle's rated or known fuel efficiency, in MPG or L/100km. This is used when planning a trip.
  • Fuel Economy (Result): The calculated efficiency of your vehicle. Note that L/100km is an inverse measure (lower is better), while MPG is a direct measure (higher is better).
  • Total Trip Cost: The total monetary cost of the fuel required for the specified distance.
  • Cost per Mile/Km: A unit cost that shows how much you pay in fuel for every mile or kilometer you drive.

How It Works: A Step-by-Step Example

Let's walk through a common scenario to see how the calculator works.

Scenario: You are planning a road trip in the US.

  • Mode: You select 'Plan Trip Cost'.
  • Unit System: You select 'US (MPG)'.
  • Input 1 (Distance): You enter 450 miles.
  • Input 2 (Fuel Price): You enter $3.85 per gallon.
  • Input 3 (Vehicle's Economy): Your car's highway rating is 32 MPG.

Calculation Steps:

  1. Find Total Fuel Needed:
    $ \text{Fuel Needed} = \frac{\text{Distance}}{\text{Economy}} = \frac{450 \text{ miles}}{32 \text{ MPG}} = 14.06 \text{ gallons} $
  2. Find Total Trip Cost:
    $ \text{Total Cost} = \text{Fuel Needed} \times \text{Fuel Price} = 14.06 \text{ gal} \times \$3.85/\text{gal} = \$54.14 $
  3. Find Cost per Mile:
    $ \text{Cost per Mile} = \frac{\text{Fuel Price}}{\text{Economy}} = \frac{\$3.85/\text{gal}}{32 \text{ MPG}} = \$0.12 \text{ per mile} $

The calculator will display these results in the designated cards.

Frequently Asked Questions (FAQ)

Why is my calculated MPG different from my car's dashboard or the sticker value?

The sticker (EPA-rated) value is achieved in specific, controlled laboratory conditions. Your real-world fuel economy is affected by many factors, including:

  • Driving Habits: Aggressive acceleration and hard braking waste fuel.
  • Vehicle Condition: Low tire pressure, a dirty air filter, or an un-serviced engine can reduce efficiency.
  • Weather: Colder temperatures can decrease MPG. Driving into a strong headwind also uses more fuel.
  • Terrain: Driving uphill requires more energy than driving on flat ground.
  • Fuel Quality: The energy content in gasoline can vary slightly.
  • Vehicle Load: Carrying heavy cargo or using a roof rack increases drag and weight, lowering MPG.
Your car's built-in dashboard calculator can also be an estimate and may differ slightly from a manual "fill-to-fill" calculation.

What's the difference between MPG and L/100km?

They are two different ways to measure the same thing.

  • MPG (Miles Per Gallon): A measure of efficiency. It tells you how far you can go on a set amount of fuel. Higher is better.
  • L/100km (Litres per 100 Kilometers): A measure of consumption. It tells you how much fuel you use to travel a set distance. Lower is better.
You cannot convert them directly by just converting miles to km and gallons to litres. The formula to convert is: $ \text{L/100km} = \frac{235.21}{\text{MPG}} $

How can I accurately measure my "Fuel Used"?

The most accurate method is the "fill-to-fill" calculation:

  1. Fill your gas tank completely. Do not top off.
  2. Reset your vehicle's trip meter (Trip A or B) to zero.
  3. Drive normally until your tank is at least half empty (or more).
  4. Return to the gas station and fill the tank completely again. Note the exact amount of fuel it took to fill (e.g., 10.5 gallons).
  5. Record the distance on your trip meter (e.g., 315 miles).
  6. Use these two numbers (315 miles, 10.5 gallons) in the calculator.

Does fuel type (e.g., premium vs. regular) affect economy?

Only if your vehicle's manufacturer *requires* premium fuel. If your car is designed for regular (87 octane) fuel, using premium (91+ octane) will *not* improve your fuel economy and is a waste of money. If your car *requires* premium, using regular fuel may cause the engine's computer to adjust, potentially reducing both performance and fuel economy.

How do city vs. highway driving affect this calculation?

This calculator gives you the *average* economy for the distance you enter. Highway driving is typically much more fuel-efficient because it involves consistent speeds with no stop-and-go. City driving, with its constant acceleration and braking, uses significantly more fuel. To see the difference, you can use the "fill-to-fill" method (described above) on a tank of gas used *only* for highway driving, and then again on a tank used *only* for city driving.

Tool developed by Ugo Candido. Automotive content verified by the CalcDomain Editorial Board.
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