Fuel Consumption Unit Converter

Posted by Robbie Dickson on

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Fuel Consumption Unit Converter + Reference Table & Applications

If you're working off a European spec sheet that lists fuel use as L/100km but you're used to thinking in MPG, you'll need to convert—plus pay attention to whether it's US or UK gallons. This tool covers all the bases. Enter your value, pick units, and you'll see the equivalents in real time. Below, you'll find the formulas, tables, and examples that show why fuel economy conversions are less straightforward than people expect.

What Is Fuel Consumption Conversion?

Fuel consumption conversion bridges the gap between MPG (miles per gallon) and L/100km (litres per 100 kilometres). These units are inversely related, so you can't use a simple multiplier to switch between them.

Simple Explanation

It's like speed vs. time: if you go faster, you finish the trip in less time. Fuel consumption is the same—higher MPG means lower L/100km, but you don't just divide by two if you double the MPG. Instead, there's a fixed constant you divide by your MPG value. That constant changes with US gallons (3.785L) or UK gallons (4.546L), and that unit difference is easy to miss if you're not careful.

MPG vs L/100km — Inverse Relationship MPG (US) L/100km 10 20 30 40 50 60 4 8 12 16 20 24 SUV / Truck ~15 MPG ≈ 15.7 L/100km Avg. Sedan ~30 MPG ≈ 7.8 L/100km Hybrid ~50 MPG ≈ 4.7 L/100km L/100km = 235.215 / MPG (US gallons)

Fuel Consumption Unit Converter

30.00
36.03
7.84
12.76

Quick Reference Table

MPG (US) MPG (UK) L/100km km/L
10 12.01 23.52 4.25
15 18.02 15.68 6.38
20 24.02 11.76 8.50
25 30.02 9.41 10.63
30 36.03 7.84 12.76
35 42.04 6.72 14.88
40 48.04 5.88 17.01
50 60.05 4.70 21.26
Engineering calculation notice

This calculator is intended for education, concept evaluation, and preliminary design. Results are based on the equations and assumptions described on this page, but cannot account for every real-world load case, tolerance, material property, environmental condition, installation detail, safety factor, code, or regulatory requirement. Verify all inputs, assumptions, units, and results independently before selecting components or using the result in a real application. Safety-critical, structural, medical, lifting, transportation, or regulated applications must be reviewed by a qualified engineer.

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🎥 Video — Fuel Consumption Unit Converter

Fuel Consumption Unit Converter

How to Use This Calculator

No clicks needed—just type your number and you'll see all the unit conversions live. Here's what to do:

  1. Enter your fuel consumption value in the box—decimals and whole numbers both work.
  2. Pick the right unit in the dropdown: MPG (US), MPG (UK / Imperial), L/100km, or km/L.
  3. Compare all four units in the results area. Everything updates at once as you change the input.
  4. Use the table to sanity-check your numbers or get a ballpark for typical vehicles.

Fuel Consumption Unit Formulas

Since MPG and L/100km are inversely related, you use a constant divided by your value—not a simple multiplier. Which constant you need depends on the gallon.

MPG (US) → L/100km:
L/100km = 235.215 ÷ MPGUS
MPG (UK) → L/100km:
L/100km = 282.481 ÷ MPGUK
L/100km → MPG (US):
MPGUS = 235.215 ÷ L/100km
km/L → MPG (US):
MPGUS = km/L × 2.35215

Variable Reference

Symbol Variable Unit
MPGUS Miles per US gallon mi/gal (US)
MPGUK Miles per Imperial gallon mi/gal (UK)
L/100km Litres per 100 kilometres L/100km
km/L Kilometres per litre km/L
235.215 US gallon conversion constant —
282.481 Imperial gallon conversion constant —

Simple Example

Convert 30 MPG (US) to L/100km

Input: 30 MPG (US)

Formula: L/100km = 235.215 ÷ 30

Calculation: 235.215 ÷ 30 = 7.84 L/100km

What this means: At 30 MPG (US), the car uses 7.84 litres every 100 km. If you saw the same car on a European or Canadian website, that's about 7.8 L/100km—a typical figure for a decent midsize sedan.

Engineering Applications

Why the Inverse Relationship Matters

Most mistakes happen here. MPG and L/100km don't move in lockstep. Gaining 10 MPG from 10 to 20 saves a huge amount of fuel, but the same 10 MPG jump from 30 to 40 doesn't. With L/100km, that improvement goes from 23.5 to 11.8 (saving 11.7 L), but for 30 to 40 MPG, it's 7.8 to 5.9 (just 1.9 L saved per 100 km). This effect—often called the "MPG illusion"—can skew fleet or efficiency decisions. For system sizing—be it a generator, a hydraulic power pack, or any setup where fuel determines runtime—the L/100km metric tells you up front how much fuel you'll actually save or spend.

US Gallon vs. UK Gallon — A Real Source of Errors

US gallons hold 3.785 litres. UK (Imperial) gallons hold 4.546 litres—a 20% jump. So a car rated at 30 MPG UK is much more efficient than one at 30 MPG US. If you use the wrong one, your numbers are off by 20%. This comes up often with international spec sheets or when sourcing gear from both British and American suppliers. Always check which standard is used, or verify by converting to L/100km and seeing if the numbers make sense for the vehicle class.

L/100km as the International Engineering Standard

L/100km is standard outside the US—Canada, Europe, Australia, and much of Asia all use it. If you're building or integrating a system for export, or comparing imported equipment, think in L/100km. It's much more straightforward because the cost of fuel per distance scales directly: double the L/100km, double the cost. That makes it easier to compare running costs, run a cost/benefit analysis, or size energy storage for actuator-driven systems using combustion engines.

km/L — The Third Option

Japan and parts of Asia use km/L. It's a "higher is better" unit like MPG, but fully metric. Conversion is just a multiplication: km/L times 2.35215 gives US MPG. This one's linear, so no dividing by a constant. If you're vetting Japanese small engines or pumps, expect the spec in km/L.

Advanced Example

Scenario: Comparing a US-rated truck with a UK-rated van for a mobile actuator platform

You have two vehicles: a US truck rated at 18 MPG (US), and a UK van at 32 MPG (UK). Fuel is $1.60/litre. Which one costs less to run per 100km?

Step 1 — Convert the US truck:

L/100km = 235.215 ÷ 18 = 13.07 L/100km

Step 2 — Convert the UK van:

L/100km = 282.481 ÷ 32 = 8.83 L/100km

Step 3 — Calculate fuel cost per 100km:

US truck: 13.07 × $1.60 = $20.91 per 100km

UK van: 8.83 × $1.60 = $14.13 per 100km

Step 4 — Cross-check in US MPG for comparison:

UK van in US MPG = 235.215 ÷ 8.83 = 26.64 MPG (US)

Interpretation: The UK van is more fuel-efficient by a wide margin—saving nearly $7 per 100km. Stretch that over a year and it's a few thousand in savings. Forgetting which gallon is used can skew your calculations by around 17% in the wrong direction.

Frequently Asked Questions

Why can't I just multiply MPG by a fixed number to get L/100km? +

They move in opposite directions—you divide by your MPG. If efficiency doubles, fuel used per distance doesn't halve in a straight line. The relationship isn't linear, and a set multiplier will give the wrong answer everywhere but one point.

What's the difference between a US gallon and a UK gallon? +

A US gallon is 3.785 litres. A UK (Imperial) gallon is 4.546 litres—20% bigger. So "30 MPG" in the UK gets you much farther per litre than "30 MPG" in the US.

Where do the constants 235.215 and 282.481 come from? +

They're calculated from the unit conversions: 1 mile is 1.60934 km, 1 US gallon is 3.78541 L. Do (100 × 3.78541) ÷ 1.60934 and you get 235.215. For Imperial gallons (4.54609 L), it's 282.481. No rounding—these are the exact numbers from the fundamental unit definitions.

Is L/100km or MPG better for comparing vehicles? +

L/100km is more direct for cost and fuel-use comparison. If a vehicle uses 8 L/100km and another uses 12, the second one uses 50% more—easy to see. MPG is less helpful for direct comparisons, especially for less efficient vehicles, because the numbers don't scale linearly with actual consumption.

Can I use this for diesel, propane, or other fuels? +

Yes. The conversion just compares volume and distance. Doesn't matter if you're burning gas, diesel, or any liquid fuel. Just remember different fuels have different energy contents per litre, so two vehicles at 8 L/100km might still use more or less energy depending on fuel type.

How do I know if a spec sheet uses US or Imperial MPG? +

Usually the country gives it away: US, Canada, and most of Asia use US gallons; UK uses Imperial. If nothing's specified, convert to L/100km and check if the resulting efficiency makes sense for that class. If it looks unusually high or low, double-check which gallon they used.

What about electric vehicles — can I convert kWh/100km to MPG? +

You need a different calculation. kWh/100km is energy per distance, not volume. The EPA's MPGe rating (miles per gallon equivalent) treats 33.7 kWh as equal to one US gallon of gasoline, but that's a separate system from what's shown here.

Fuel consumption numbers look simple, but small mistakes—especially getting gallons or the formula direction wrong—can throw off your entire estimate. Whenever you're double-checking numbers between international standards or working up budgets for mobile equipment, use this live converter and avoid the common pitfalls.

About the Author

Robbie Dickson — Chief Engineer & Founder, FIRGELLI Automations

Robbie Dickson brings over two decades of engineering expertise to FIRGELLI Automations. With a distinguished career at Rolls-Royce, BMW, and Ford, he has deep expertise in mechanical systems, actuator technology, and precision engineering.

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