Miles per gallon (MPG) and liters per 100 kilometers (L/100km) are both measures of fuel efficiency, but they work in opposite ways. MPG is a "more is better" number — higher MPG means the car goes further per unit of fuel. L/100km is a "less is better" number — lower means more efficient.

This inverse relationship is what makes comparing them confusing. A car that gets 30 MPG isn't twice as efficient as one that gets 15 MPG in the way the numbers suggest — the actual fuel consumed per distance doesn't scale that cleanly. Understanding both systems helps when comparing cars across US, European, and Australian specifications.

The Conversion Formulas

MPG (US) to L/100km: ` L/100km = 235.215 ÷ MPG `

L/100km to MPG (US): ` MPG = 235.215 ÷ L/100km `

The constant 235.215 comes from converting miles to kilometers and US gallons to liters simultaneously.

Examples:

• 25 MPG = 235.215 ÷ 25 = 9.4 L/100km
• 40 MPG = 235.215 ÷ 40 = 5.9 L/100km
• 6 L/100km = 235.215 ÷ 6 = 39.2 MPG
• 10 L/100km = 235.215 ÷ 10 = 23.5 MPG

For UK imperial MPG (which uses the larger imperial gallon), the constant is 282.5 instead of 235.215.

Use the volume converter to cross-check fuel volume conversions — for instance, if you're comparing a fuel price in liters vs gallons.

Quick Reference: MPG to L/100km

How to Calculate Your Actual Fuel Consumption

Manufacturer fuel economy ratings are measured under standardized test conditions that often don't match real-world driving. Calculating your actual consumption is straightforward:

Method 1: Full tank to full tank

1. Fill your tank completely and record the odometer reading (or reset the trip counter to zero). 2. Drive until you need to refuel, then fill the tank completely again. 3. Note the fuel volume added (in liters or gallons) and the distance driven.

Calculating L/100km: ` L/100km = (fuel added in liters ÷ distance in km) × 100 `

Example: You drove 450 km and added 38 liters to fill the tank. ` L/100km = (38 ÷ 450) × 100 = 8.4 L/100km `

Calculating MPG: ` MPG = miles driven ÷ gallons used `

Example: You drove 280 miles and added 9.5 gallons. ` MPG = 280 ÷ 9.5 = 29.5 MPG `

Why Your Real-World MPG Differs From the Rating

Fuel economy ratings in most countries are measured on dynamometers (rolling test machines) under controlled conditions. Real-world consumption differs because:

Driving style: Aggressive acceleration and braking uses significantly more fuel than smooth, steady driving. Hard acceleration from a stop can increase fuel consumption by 15–30%.

Speed: Aerodynamic drag increases with the square of speed. Driving at 110 km/h (68 mph) uses meaningfully more fuel than 90 km/h (56 mph) for the same distance. Most cars achieve their best fuel efficiency between 80–100 km/h.

Cold starts: Engines use more fuel when cold. Short trips never let the engine fully warm up, so city driving with many short trips shows much worse fuel consumption than highway driving.

Air conditioning: Running AC adds roughly 0.5–1 L/100km to consumption on most cars (5–10% increase). More significant in slow city traffic than on highways.

Load and cargo: Extra weight increases fuel consumption. An extra 100 kg in the car increases consumption by roughly 3–5%.

Tire pressure: Underinflated tires increase rolling resistance. Tires at 20% below recommended pressure can increase fuel consumption by 2–3%.

Comparing Fuel Costs Between Cars

Fuel consumption ratings become most useful when comparing the ongoing cost of running different vehicles. Here's how to calculate annual fuel cost:

annual fuel cost = (annual km ÷ 100) × L/100km × fuel price per liter

Example: You drive 15,000 km per year. Car A uses 7 L/100km; Car B uses 10 L/100km. Fuel costs $1.80/liter.

• Car A: (15,000 ÷ 100) × 7 × $1.80 = $1,890/year
• Car B: (15,000 ÷ 100) × 10 × $1.80 = $2,700/year
• Annual difference: $810/year in Car A's favor

Over 5 years, that's $4,050 in fuel savings — relevant context when comparing purchase prices.

In US terms with MPG: ` annual fuel cost = (annual miles ÷ MPG) × fuel price per gallon `

Electric Vehicles: MPGe and kWh/100km

Electric vehicles use their own efficiency metrics:

MPGe (miles per gallon equivalent): The US EPA's measure of EV efficiency. It converts the energy content of a gallon of gasoline (33.7 kWh) to find the equivalent miles per gallon. A car rated at 100 MPGe uses about 33.7 kWh to travel 100 miles.

kWh/100km (or kWh/mile): The direct energy consumption metric, more common in Europe. Lower is better. Most EVs achieve 14–22 kWh/100km, compared to 50–90 kWh/100km equivalent for petrol cars.

Wh/mile: The same measure in watt-hours per mile. Common in US EV community reporting. A Tesla Model 3 achieving 250 Wh/mile is equivalent to about 134 MPGe or 15.5 kWh/100km.

Comparing EV efficiency to petrol/diesel efficiency requires converting both to a common energy unit — kWh or MJ per 100km is the cleanest comparison, using the actual energy content of the fuel rather than the volume.

A Practical Note on US vs Imperial Gallons

The US gallon (3.785 L) and the UK imperial gallon (4.546 L) are not the same. A car rated at 40 MPG in the UK is not the same as one rated at 40 MPG in the US — the UK car is actually getting about 33 US MPG.

When reading fuel economy claims from different markets, always check which gallon is being used. UK, Canadian, and Australian car reviews sometimes quote different gallon standards, which makes direct MPG comparison misleading without conversion.

The volume converter handles the US/imperial gallon conversion directly — a useful sanity check when comparing international specifications.