Fuel efficiency units and the “inverse” problem

Fuel efficiency sounds simple until you notice that the world uses two different styles of units. In North America, you’ll often see MPG (miles per gallon). In many other places, you’ll see L/100km (liters per 100 kilometers). Those are describing the same thing but from opposite directions:

• Distance per fuel (MPG, km/L, mi/L): “How far can I go on one unit of fuel?”
• Fuel per distance (L/100km, L/100mi): “How much fuel do I burn to go a fixed distance?”

The second style is called an inverse unit because it is mathematically the reciprocal of the first. That’s why people get tripped up: if you’re used to MPG, a higher number feels better. But in L/100km, a higher number is worse because it means you burn more liters to travel the same 100 km.

The easiest reliable strategy is: convert any input into a single base unit, then convert from that base to the unit you want. This calculator uses km per liter (km/L) internally because it sits nicely between metric and imperial conversions.

After we compute the base efficiency in km/L, we can produce any output unit using a small set of constants: 1 mile = 1.609344 km, 1 US gallon = 3.785411784 L, and 1 UK (imperial) gallon = 4.54609 L.

• MPG (US) → km/L: km/L = MPG × 1.609344 ÷ 3.785411784
• MPG (UK) → km/L: km/L = MPG × 1.609344 ÷ 4.54609
• km/L → MPG (US): MPG = km/L × 3.785411784 ÷ 1.609344
• km/L → MPG (UK): MPG = km/L × 4.54609 ÷ 1.609344
• L/100km → km/L: km/L = 100 ÷ (L/100km)
• km/L → L/100km: L/100km = 100 ÷ (km/L)
• L/100mi → km/L: km/L = 160.9344 ÷ (L/100mi)
• km/L → L/100mi: L/100mi = 160.9344 ÷ (km/L)

Notice the pattern: when we talk about “per 100 km” (or “per 100 mi”), the number 100 appears because the unit is scaled for readability. People usually prefer “liters per 100 kilometers” rather than “liters per kilometer” because the latter would be a tiny decimal.

MPG is “miles per gallon”, but a US gallon and a UK (imperial) gallon are different sizes. The UK gallon is larger, so the same car will often show a higher MPG number in UK units than in US units (because the “gallon” in the denominator is larger). If you’re comparing cars, always make sure you’re using the same gallon standard.

Realistic conversion examples (with intuition)

Here are a few examples that build intuition. You can paste these values into the converter to confirm. The point is not to memorize numbers, but to get a “feel” for how the units relate.

Suppose your car gets 30 MPG (US). First, convert to km/L: km/L = 30 × 1.609344 ÷ 3.785411784 ≈ 12.75 km/L. Then convert to L/100km: L/100km = 100 ÷ 12.75 ≈ 7.84. So 30 MPG (US) ≈ 7.84 L/100km. Notice how MPG went up in “goodness” while L/100km went down in “goodness” — that’s the inverse effect.

If your dashboard shows 6.0 L/100km, convert to km/L: km/L = 100 ÷ 6.0 ≈ 16.67 km/L. Then convert to MPG (US): MPG = 16.67 × 3.785411784 ÷ 1.609344 ≈ 39.2. So 6.0 L/100km ≈ 39.2 MPG (US). Again, smaller L/100km maps to bigger MPG.

Say your car is 28 MPG (US). Convert to km/L: km/L ≈ 28 × 1.609344 ÷ 3.785411784 ≈ 11.90 km/L. Now convert to MPG (UK): MPG (UK) = 11.90 × 4.54609 ÷ 1.609344 ≈ 33.6. So 28 MPG (US) ≈ 33.6 MPG (UK). That difference is mostly the gallon size difference.

Imagine you drive 180 miles and your car gets 30 MPG (US). Fuel used in gallons ≈ 180 ÷ 30 = 6 gallons. If gas is $3.49 per gallon, cost ≈ 6 × 3.49 = $20.94. The estimator on this page performs the same idea, but it works even when your input is L/100km (where you would otherwise need to flip the math).

If you like making things shareable: try changing only the fuel price and re-screenshot the card. It becomes a quick conversation starter: “How much would this same trip cost in your area?”

What this calculator does behind the scenes

Under the hood, the logic is intentionally simple (and that’s a feature). Complex tools often hide assumptions. A converter should be transparent: you should be able to understand the steps and sanity-check the results. Here’s the exact mental model:

• Parse the input: We accept decimals, trim spaces, and reject negative or zero values.
• Normalize to km/L: Every unit has a direct formula to reach km/L.
• Convert from km/L: We compute the target unit plus a quick “multi-unit view”.
• Optional trip cost: If distance and fuel price are provided, we estimate liters used and cost.
• Save & share: You can store recent conversions locally and share a text summary.

The biggest source of human error is mixing “distance per fuel” with “fuel per distance” without flipping. Example: someone might try to treat 8 L/100km like it behaves the same direction as 30 MPG, which leads to backwards comparisons. This page always converts to km/L first, so the inversion is handled automatically and consistently.

For a fun, visual output, the meter normalizes your efficiency to MPG (US) and then maps it to a 0–100 style bar. This isn’t a “grade” and it isn’t comparing you to a database of vehicles — it is just a visualization so that if you change the number slightly, you can see the effect. (If your input is L/100km, the meter still goes up when efficiency improves.)

We convert your efficiency into km/L. Then: fuel used (liters) = distance (km) ÷ (km/L). After that, we convert the fuel price into “per liter” (if you entered per gallon), and multiply: cost = liters × price per liter. Because everything flows through liters, it works for US and UK gallons correctly.

One more practical note: this estimator assumes your displayed efficiency remains constant for the trip. Real trips vary — but for quick budgeting or comparing scenarios (“What if gas goes up by 40 cents?”), this estimate is surprisingly useful.

Frequently asked questions

• Why does my MPG number look “better” in the UK?

  Because the UK uses a larger gallon (imperial gallon). Same car, same driving, different gallon size — so the miles-per-gallon number changes. Always compare using the same standard.

• What does L/100km actually mean?

  It’s how many liters your vehicle burns to go 100 kilometers. If it says 7.0 L/100km, it means for every 100 km you travel, you’ll use about 7 liters (under similar conditions).

• Is higher L/100km better or worse?

  Worse. L/100km is fuel-per-distance. Higher means more fuel burned for the same distance. Lower L/100km is better efficiency.

• My car shows “km/L”. How do I compare with friends who use MPG?

  Convert km/L to MPG (US) using this tool. If you want a quick rough idea, km/L multiplied by ~2.35 gives a close MPG (US) estimate, but the calculator is more precise.

• Why do I get different numbers from city vs highway driving?

  Fuel economy depends heavily on speed, acceleration, stops, wind, terrain, payload, tire pressure, and even temperature. Converters don’t change that — they simply translate the units.

• Can I use this for motorcycles, trucks, hybrids, or EVs?

  For any gas/diesel vehicle that reports MPG, km/L, or L/100km, yes. For EVs, efficiency is typically expressed in kWh/100km or miles/kWh — that’s a different converter (but the “inverse unit” idea is similar).

• What’s the difference between L/100km and L/100mi?

  Same concept, different distance standard. L/100mi uses 100 miles (≈160.9344 km) instead of 100 km. It’s less common, but can be useful for US road-trip comparisons when you prefer liters.

• How accurate is the trip cost estimator?

  It’s as accurate as the efficiency number you feed it. If your MPG is a reliable average for your driving, the cost estimate will be close. If conditions change (mountains, heavy traffic), the estimate will drift.

• Does this page store my data?

  No server storage. If you click “Save”, your recent conversions are stored locally in your browser’s localStorage on this device only.

• What’s a quick sanity check to know the conversion isn’t crazy?

  If you convert from MPG to L/100km: higher MPG should produce lower L/100km. If that relationship flips, something went wrong (or the units were swapped).