Data Source & Methodology

This calculator uses standard formulas for determining fuel consumption and cost. The methodologies are consistent with data and testing procedures published by government agencies responsible for vehicle economy ratings.

  • Authoritative Source: U.S. Environmental Protection Agency (EPA)
  • Reference: EPA Fuel Economy Testing Procedures (40 C.F.R. Part 600)
  • Date: Revised procedures as of 2017
  • Link: fueleconomy.gov/feg/how_tested.shtml

All calculations are based on the standard formulas for fuel consumption as recognized by this agency.

The Formulas Explained

The calculator uses two primary sets of formulas depending on your selected units.

US Customary Units (MPG)

$$ \text{MPG} = \frac{\text{Distance (miles)}}{\text{Fuel Used (gallons)}} $$
$$ \text{Fuel Needed (Gallons)} = \frac{\text{Trip Distance (miles)}}{\text{MPG}} $$

Metric Units (L/100km)

Note that L/100km is a measure of consumption (lower is better), unlike MPG (where higher is better).

$$ \text{L/100km} = \left( \frac{\text{Fuel Used (liters)}}{\text{Distance (km)}} \right) \times 100 $$
$$ \text{Fuel Needed (Liters)} = \frac{\text{Trip Distance (km)} \times \text{L/100km}}{100} $$

Cost Calculation (Universal)

$$ \text{Total Cost} = \text{Fuel Needed} \times \text{Price per Unit of Fuel} $$
$$ \text{Cost per Mile/KM} = \frac{\text{Total Cost}}{\text{Trip Distance}} $$

Glossary of Variables

  • Trip Distance: The total length of your journey, measured in miles or kilometers.
  • Fuel Used: The total amount of fuel your vehicle consumed to cover that distance, measured in gallons or liters.
  • Fuel Price: The cost for one unit of fuel (one gallon or one liter).
  • Vehicle's Efficiency: Your car's rated or known fuel economy. Entering this value overrides the "Fuel Used" field for cost planning.
  • Fuel Efficiency (MPG): Miles Per Gallon. A measure of how many miles a vehicle can travel on one gallon of fuel.
  • Fuel Efficiency (L/100km): Liters per 100 kilometers. A measure of how many liters of fuel a vehicle needs to travel 100 kilometers.

How It Works: A Step-by-Step Example

Let's plan a 350-mile road trip in a car that has a known fuel efficiency of 25 MPG. The local gas price is $3.80 per gallon.

Given Inputs:

  • Trip Distance: 350 miles
  • Vehicle's Efficiency (MPG): 25
  • Fuel Price: $3.80 per gallon

Step 1: Calculate Total Fuel Needed

Using the formula: $ \text{Fuel Needed (Gallons)} = \frac{\text{Trip Distance (miles)}}{\text{MPG}} $

$ \text{Fuel Needed} = \frac{350}{25} = 14 \text{ gallons} $

Step 2: Calculate Total Trip Cost

Using the formula: $ \text{Total Cost} = \text{Fuel Needed} \times \text{Price per Gallon} $

$ \text{Total Cost} = 14 \times \$3.80 = \$53.20 $

Step 3: Calculate Cost per Mile

Using the formula: $ \text{Cost per Mile} = \frac{\text{Total Cost}}{\text{Trip Distance}} $

$ \text{Cost per Mile} = \frac{\$53.20}{350 \text{ miles}} = \$0.152 \text{ per mile} $

Frequently Asked Questions (FAQ)

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

MPG (Miles Per Gallon) is a measure of efficiency used primarily in the US and UK. A higher number is better (more miles per gallon). L/100km (Liters per 100 kilometers) is used in most other parts of the world. It measures consumption, so a lower number is better (fewer liters to drive 100 km).

Why is my car's actual MPG different from the manufacturer's rating?

Manufacturer (EPA) ratings are achieved in controlled laboratory settings. Real-world factors like driving style (aggressive acceleration, speeding), traffic, weather (cold weather and wind reduce MPG), tire pressure, and using the A/C all contribute to lower actual fuel efficiency.

How can I improve my gas mileage?

You can improve your MPG by:

  1. Driving smoothly: Avoid hard braking and rapid acceleration.
  2. Observing speed limits: Efficiency usually drops significantly above 50 mph (80 km/h).
  3. Keeping tires properly inflated: Under-inflated tires increase rolling resistance.
  4. Removing unnecessary weight: Don't carry heavy items or roof racks unless needed.
  5. Reducing idle time: Turn off the engine if you're stopped for more than a minute.

Does using the air conditioner (A/C) affect fuel efficiency?

Yes. Using your car's A/C is the largest auxiliary load on the engine and can reduce fuel efficiency by 5% to 25%, especially on short trips. In hot weather, it's a trade-off between comfort and fuel cost.

Is it more fuel-efficient to drive with windows down or use A/C?

It depends on your speed. At lower speeds (city driving, under 40 mph), open windows create less drag than the A/C compressor, making them the more efficient choice. At highway speeds (above 50 mph), open windows create significant aerodynamic drag, and it is generally more fuel-efficient to use the A/C.

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