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Everyday Math Suite

Fuel Efficiency Calculator

Convert efficiency metrics between US/UK MPG and L/100km, or calculate total fuel consumption and travel costs for your trips.

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Understanding Fuel Consumption and Mileage

Fuel efficiency is a critical metric for estimating trip expenses, evaluating vehicle environmental impact, and understanding automotive costs. Different regions around the world use different standards to calculate how efficiently a car uses fuel.

Fuel Economy vs. Fuel Consumption

There are two main ways to express how efficiently a vehicle uses fuel:

  • Fuel Economy (Distance per Fuel): Measured in Miles per Gallon (MPG) or Kilometers per Liter (km/L). A higher number represents better efficiency (you can drive further on one unit of fuel).
  • Fuel Consumption (Fuel per Distance): Typically measured in Liters per 100 Kilometers (L/100km). A lower number represents better efficiency (you use less fuel to cover the same distance).

Conversion Formulas

Converting between fuel economy and fuel consumption involves inverse relationships. The standard conversions are:

  • US MPG to L/100km: L/100km = 235.215 / US_MPG
  • UK MPG to L/100km: L/100km = 282.481 / UK_MPG
  • km/L to L/100km: L/100km = 100 / km_L

Common Questions & Fuel Efficiency Insights

Why is fuel efficiency in Europe measured in L/100km instead of MPG?

Liters per 100 kilometers (L/100km) measures fuel consumption (how much fuel is used over a fixed distance), whereas Miles per Gallon (MPG) measures fuel economy (how far you can go on a fixed amount of fuel). L/100km is more intuitive when estimating trip costs: if you drive double the distance, you use exactly double the fuel.

What is the difference between a US gallon and a UK gallon?

A UK imperial gallon is larger than a US gallon. One UK gallon is exactly 4.54609 liters, while one US liquid gallon is exactly 3.785411784 liters. Because of this, a vehicle's fuel efficiency rated in UK MPG will appear higher than its rating in US MPG.

How does driving speed affect my fuel efficiency?

Most vehicles reach peak fuel efficiency at speeds between 45 and 60 mph (72 to 96 km/h). Driving faster increases aerodynamic drag exponentially, requiring significantly more fuel to maintain speed. For example, driving at 75 mph instead of 65 mph can reduce fuel economy by up to 15%.

How can I convert MPG to L/100km in my head?

A handy rule of thumb is the number 235. Divide 235 by your US MPG to get L/100km. For example, 30 MPG is roughly 235 / 30 = 7.8 L/100km. Conversely, dividing 235 by L/100km gets you MPG.

How much does underinflated tires affect fuel consumption?

Underinflated tires increase the rolling resistance between your vehicle and the road. For every 1 PSI drop in pressure of all four tires, fuel efficiency drops by roughly 0.2% to 0.3%. Keeping tires inflated to the manufacturer's specification helps maintain optimal MPG.

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