Miles per Liter (mi/L) - Unit Information & Conversion

Quick Answer: What is Miles per Liter?

Miles per Liter (mi/L) is a "street unit" for fuel economy. It answers the simple question: "I just bought 10 liters of fuel; how many miles can I drive?"

• The Mix: Miles (Imperial) ÷ Liters (Metric).
• Why it's used: In the UK, you buy fuel in liters but drive in miles.
• Key Conversions:
  • 1 mi/L ≈ 4.55 MPG (UK)
  • 1 mi/L ≈ 3.79 MPG (US)
  • 1 mi/L ≈ 1.61 km/L
  • 10 mi/L ≈ 45 MPG (UK)

It is the easiest way to check your car's efficiency at a British petrol station without a calculator.

Miles per Liter Comparison Table

mi/L	Efficiency Rating	MPG (UK)	MPG (US)	L/100km
5	Very Poor	22.7	18.9	12.4
8	Poor	36.4	30.3	7.8
10	Average	45.5	37.9	6.2
12	Good	54.5	45.4	5.2
15	Very Good	68.2	56.8	4.1
20	Hybrid	90.9	75.7	3.1
25	Scooter	113.6	94.6	2.5

Definition

Miles per Liter (symbol: mi/L or mpl) is a unit of fuel economy expressing the distance in statute miles traveled per liter of fuel consumed.

Formula: $$ \text{Economy (mi/L)} = \frac{\text{Distance (Miles)}}{\text{Fuel (Liters)}} $$

The "Pump-to-Road" Logic:

• Input: You look at the pump. It says "40 Liters."
• Output: You look at your odometer. It says "400 Miles."
• Calculation: 400 / 40 = 10 mi/L.
• This is much easier than converting 40 liters to 8.8 gallons to calculate MPG.

History

The UK Metrication Gap

The United Kingdom is unique. It started metricating in 1965 but stopped halfway.

• 1980s: Petrol stations switched from Gallons to Liters.
• Result: Drivers lost their intuitive link between fuel bought and distance driven.
• The Fix: Drivers started calculating "miles per liter" in their heads. "If I get 10 miles from a liter, and I put in 20 liters, I can go 200 miles."

Racing

In endurance racing (like Le Mans), teams often use mixed units. Stint lengths are often discussed in laps (miles/km) while fuel loads are calculated in kilograms or liters. "Miles per liter" becomes a critical tactical number for the race engineer to determine pit stops.

Real-World Examples

The "Ten Mile Liter"

For a long time, 10 mi/L was the benchmark for a good car in the UK.

• 10 mi/L = 45.5 MPG (UK).
• This is a solid figure for a petrol hatchback.
• If you are getting less than 10 miles per liter (e.g., 8 mi/L), your car is thirsty.

The "Five Pound Trip"

Drivers often think in money.

• If fuel is £1.50/liter.
• And you get 10 mi/L.
• Then 10 miles costs £1.50.
• "It's a 30-mile trip, so it will cost me £4.50."
• This mental math relies entirely on the mi/L concept.

Common Uses

1. Quick Mental Checks

At the pump, you don't have a calculator to convert liters to gallons.

• "I put in 30 liters. I did 330 miles. That's 11 miles per liter. Good."

2. Motorcycles

Motorbikes have small tanks (e.g., 15 liters).

• Riders often know their range per liter precisely.
• "I get 15 miles per liter. I have 2 liters reserve. I can go 30 miles."

3. Old Cars in Modern World

Driving a classic 1960s Jaguar in 2024.

• Odometer: Miles.
• Pump: Liters.
• The driver lives in a "miles per liter" world by necessity.

Conversion Mistakes to Avoid

1. Confusing mi/L with km/L

Mistake: "10 mi/L is the same as 10 km/L." Reality: Miles are longer than kilometers.

• 10 mi/L = 16 km/L.
• A car getting 10 mi/L is 60% more efficient than a car getting 10 km/L.

2. The "4.5" Factor

Mistake: Converting mi/L to MPG (UK) using the wrong factor. Rule: Multiply by 4.546.

• Roughly: mi/L × 4.5 = MPG (UK).
• Example: 10 mi/L × 4.5 = 45 MPG.

3. The "3.8" Factor

Mistake: Converting mi/L to MPG (US). Rule: Multiply by 3.785.

• Roughly: mi/L × 3.8 = MPG (US).
• Example: 10 mi/L × 3.8 = 38 MPG.

Detailed Conversion Charts

mi/L to MPG (UK)

mi/L	MPG (UK)	Efficiency
5	22.7	Poor
8	36.4	Average
10	45.5	Good
12	54.5	Very Good
15	68.2	Excellent
20	90.9	Hybrid

mi/L to L/100km

mi/L	L/100km	Note
5	12.4	Thirsty
8	7.8	Average
10	6.2	Efficient
12	5.2	Diesel
15	4.1	Hybrid
20	3.1	PHEV

Physics of Fuel Consumption

Why is 10 mi/L a barrier?

To get 10 miles (16 km) from 1 liter of fuel requires an engine to be thermally efficient and the car to be aerodynamic.

• Energy in 1 Liter: ~34 Megajoules.
• Drag at 70 mph: Requires ~15-20 kW of power.
• Balancing this energy budget is what keeps most cars in the 10-12 mi/L range.

How to Improve Your mi/L

1. Shift Up: Lower RPMs save fuel.
2. Smoothness: Don't race to red lights.
3. Tires: Keep them inflated.
4. Weight: Clean out the boot (trunk).

Unit Frequently Asked Questions

Is mi/L an official unit?

No. You won't find it in a car manual. It is a "derived unit" used by people to make sense of mixed systems.

How do I convert mi/L to MPG (UK)?

Multiply by 4.54.

• 10 mi/L × 4.54 = 45.4 MPG.

How do I convert mi/L to km/L?

Multiply by 1.61.

• 10 mi/L × 1.61 = 16.1 km/L.

Why don't cars display mi/L?

Tradition. UK cars display MPG because that's what people are used to comparing, even though the calculation inside the computer likely uses liters and kilometers first!

Quick Reference Card

Measurement	Value
1 mi/L	4.55 MPG (UK)
1 mi/L	3.79 MPG (US)
1 mi/L	1.61 km/L
10 mi/L	~45 MPG (UK)
12 mi/L	~55 MPG (UK)
Formula	Miles ÷ Liters
Conversion	× 4.5 for MPG (UK)

Your Next Steps

Master other fuel efficiency conversions:

• Miles per Gallon (UK) - The standard unit
• Kilometers per Liter - The metric unit
• Liters per 100km - The European standard
• Liter - The volume unit