Fuel Economy Calculator
Convert fuel economy between MPG (US), MPG (UK), L/100km, and km/L. Plus compute trip economy from distance + fuel used + fuel price.
Fuel Economy Calculator
🔁 Unit Converter — MPG ↔ L/100km ↔ km/L
📊 Trip Calculator — distance + fuel used → actual economy + cost
US average regular: ~$0.91/L
UK premium: ~£1.45/L
How to use the Fuel Economy Calculator
Convert between fuel economy units
The top section is a pure unit converter. Type any value, pick its unit, and all four standard units are computed: MPG (US) — the US convention; MPG (UK) — the UK/Commonwealth convention (1 UK gallon = 1.20 US gallons, so UK MPG is always higher than US MPG for the same vehicle); L/100km — the metric / European convention (lower is better); km/L — Asian convention (higher is better). A car with "30 MPG (US)" = 36 MPG (UK) = 7.84 L/100km = 12.75 km/L.
Compute your actual trip economy
The "Trip Calculator" section computes your real fuel economy from a logged trip. Drive a known distance, fill up to top off the tank, note the fuel volume. Enter distance + fuel used; the calculator outputs your actual fuel economy in all three standard units. Compare this against your vehicle's EPA / NEDC / WLTP rated number — most real-world economy is 5-15% worse than rated due to traffic, hills, AC use, and ambient temperature.
Add fuel price for trip cost
Type your local fuel price (per liter or gallon). The calculator outputs total trip cost AND cost per 100km — useful for trip-budgeting, comparing routes, or computing fuel reimbursement. Reference: Singapore 95-RON ~S$2.85/L (very high), Hong Kong ~HK$28/L, UK premium ~£1.45/L, US regular ~$0.91/L equiv, Malaysia RON95 RM2.05/L (subsidised, lowest in ASEAN). Mix-and-match — feed Singapore prices into a US miles input or vice versa; the calculator handles unit conversion.
Track over time to spot maintenance issues
Log fuel economy every fill-up over a few months. Sudden 15%+ drops indicate problems: dirty air filter, low tire pressure, dragging brake, faulty O₂ sensor, clogged catalytic converter, or stuck thermostat. A gradual 5-10% decline over 50,000+ km is normal wear (engine deposits, slightly worn tires). For EVs, "fuel economy" becomes kWh/100km — works the same way using the L/100km input as a proxy if you convert kWh to "fuel equivalent" first.
Fuel economy — why your rated MPG is almost always optimistic
Every new car comes with an "official" fuel economy number — but the methodology differs dramatically by region. The US uses the EPA-estimated rating (based on city + highway combined cycles tested on a dynamometer). Europe and ASEAN use NEDC (older, very optimistic — being phased out) or WLTP (newer, more realistic). Japan uses the JC08 cycle. All are tested in laboratory conditions with controlled temperature, no AC, professional drivers driving very smoothly. Real-world economy is typically 5-15% worse than EPA, 10-25% worse than NEDC, and roughly matches WLTP. That's why your dashboard MPG indicator showing "32 MPG" when the sticker said "35 MPG" doesn't mean the car is broken — it means you're driving in the real world.
Why the four units exist and how to read them
The choice between MPG and L/100km isn't just metric vs imperial — they're structurally different scales. MPG (miles per gallon) increases with efficiency — "30 MPG" is better than "20 MPG". But the relationship is non-linear: improving from 20 to 30 MPG saves a lot more fuel than improving from 40 to 50 MPG. This is the "MPG illusion" — a 10 MPG improvement is meaningful only against a baseline. L/100km (liters per 100 km) inverts the metric — lower is better, and the relationship IS linear. Going from 10 to 8 L/100km saves exactly the same fuel per distance as going from 6 to 4. This is why most engineers and government policy work uses L/100km — the math is honest. Asian markets use km/L (kilometers per liter), which is just L/100km inverted — higher is better, non-linear like MPG.
L/100km is the only fuel economy unit where the math is honest. Going from 10 to 8 L/100km saves the same fuel as going from 6 to 4. MPG looks better but distorts the comparison.
What actually affects real-world fuel economy
The biggest single factor is aerodynamic drag, which scales with the square of speed. Driving at 100 km/h uses ~30% more fuel than 80 km/h; 120 km/h uses ~60% more. Tire pressure under-inflated by 30% reduces economy by 10-15%. Cargo weight: every extra 100 kg cuts economy by ~3%. Air conditioning in tropical climates: 5-10% penalty (cold-climate winter heating from waste heat is "free"). Aggressive driving (jackrabbit starts, hard braking): 15-30% worse than smooth driving. Cold engine (first 10-15 km of any trip): 20-30% worse than warmed-up engine. The biggest "free" improvement is just driving smoother — anticipating stops, accelerating gently, maintaining steady speed on highway. Hypermilers consistently achieve 30-50% better economy than rated through technique alone.
The ASEAN fuel-economy + EV transition angle
Fuel economy matters more in ASEAN than most regions because retail fuel prices vary 4×+ across the region. Singapore: highest in ASEAN at ~S$2.85/L for 95-RON petrol (~US$2.05/L) — driven by zero domestic oil + Singapore's policy of pricing fuel close to European levels to discourage car ownership. Hong Kong: similar to Singapore at HK$28/L. Thailand / Philippines / Vietnam: ~$0.90-$1.20/L, mid-range. Malaysia: heavily subsidised — RON95 at RM2.05/L (~$0.45/L) is among the cheapest in the world. Indonesia: complex tier system — Pertalite RON90 at IDR 10,000/L (~$0.65/L), Pertamax RON92 at IDR 12,500/L. The high-fuel-price markets (SG, HK) are where EV economics have already flipped — at S$2.85/L petrol vs S$0.30-$0.40/kWh charging, a Tesla Model 3 costs ~S$0.05-$0.07 per km vs ~S$0.25-$0.30/km for a comparable petrol car. The EV transition is accelerating fastest in these markets for that reason. In low-fuel-price markets (MY, ID), EV payback is much longer and policy incentives matter more than fuel-cost arithmetic.
10 Things to Know About Fuel Economy
1 US gallon = 3.785 L. 1 UK / imperial gallon = 4.546 L (20% larger). UK MPG is always higher than US MPG for the same car.
L/100km is the only honest fuel economy unit — its math is linear. Going from 10 to 8 saves the same fuel as going from 6 to 4.
Real-world fuel economy is typically 5-15% worse than EPA rating, 10-25% worse than NEDC (Europe), and roughly matches WLTP (the newer European standard).
Aerodynamic drag scales with the square of speed. Driving 120 km/h vs 80 km/h uses ~60% more fuel for the same distance.
Under-inflated tires by 30% reduce fuel economy by 10-15%. Check tire pressure monthly — the single highest-ROI fuel-saving habit.
Air conditioning in tropical climates costs 5-10% in fuel economy. Open windows at low speed are better; closed windows + AC at highway speed are better.
The world's most fuel-efficient production car is the Volkswagen XL1 (2014, limited production) at 0.9 L/100km — 261 MPG-US.
Hypermilers routinely achieve 30-50% better fuel economy than EPA rating through technique alone (smooth acceleration, anticipating stops, coasting).
An EV's "fuel economy" is measured in kWh/100km. A Tesla Model 3 uses ~15 kWh/100km — equivalent to ~1.5 L/100km gasoline in energy terms.
Singapore charges ~S$2.85/L for 95-RON petrol vs Malaysia's RM2.05/L (~S$0.62) — driven by Malaysia's heavy fuel subsidies and Singapore's deliberate high-price policy.
Frequently Asked Questions
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Different gallon sizes. 1 US gallon = 3.785 L; 1 UK (imperial) gallon = 4.546 L — the UK gallon is 20% larger. So the same car gets a higher MPG number in UK terms than US terms. A car rated at "30 MPG US" = "36 MPG UK". Always check which gallon a published number refers to. Most modern UK car ads now use L/100km or km/L to avoid the confusion; older British car magazines still use MPG UK.
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The rated numbers are laboratory tests with controlled conditions — fixed temperature, no AC, professional driver, dynamometer (not real road). Real driving has traffic, hills, weather, AC use, cold starts, your specific driving style. Real economy is typically 5-15% worse than EPA (US rating, most realistic), 10-25% worse than NEDC (older European, very optimistic), and roughly matches WLTP (newer European standard). If you're 15-20% worse than rated, that's normal. If you're 30%+ worse, something might be wrong (low tire pressure, dragging brakes, dirty air filter).
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L/100km has linear math — going from 10 to 8 L/100km saves exactly the same fuel per distance as going from 6 to 4. MPG is non-linear: improving from 20 to 30 MPG saves a lot more fuel than improving from 40 to 50 MPG (both are "10 MPG improvements" but the first saves 2× the fuel of the second). This is the "MPG illusion" that distorts perception of fuel-economy gains. Government policy, engineering analysis, and fleet-management math worldwide use L/100km for honest comparisons.
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Smoother driving. Hypermilers routinely achieve 30-50% better economy than EPA rating without any vehicle modifications, just through technique: anticipating stops (coasting to red lights instead of accelerating then braking), accelerating gently (10-second 0-50 km/h instead of 5-second), maintaining steady highway speed (using cruise control), driving below 100 km/h on long trips (aerodynamic drag costs grow fast above 100). Second biggest: tire pressure (15% loss from 30% under-inflation). Third: removing excess weight (5% loss per 100 kg of cargo carried unnecessarily). The dashboard "instant MPG" display is a great training tool — drive smoothly to keep it high.
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Use the "fill-up to fill-up" method. (1) At start: fill tank to a clear cut-off (auto-shutoff click), reset the trip meter or note odometer reading. (2) Drive your trip. (3) At end: refill to the same level (auto-shutoff click), note the fuel volume from the pump receipt and the new odometer reading. (4) Enter distance + fuel in the Trip Calculator. The more fill-ups you average over, the more accurate — a single tank can be misleading due to fill-up variation. For best accuracy, log 5-10 fill-ups and average them. Apps like Fuelio, Drivvo, MyMPG do this automatically.
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Heavily depends on local fuel + electricity prices. Singapore: Tesla Model 3 at ~15 kWh/100km × S$0.35/kWh = S$0.05/km; comparable petrol (Toyota Corolla) at 6 L/100km × S$2.85/L = S$0.17/km. EV costs ~30% as much per km. UK: similar 3-4× EV advantage. Malaysia / Indonesia (subsidised fuel): petrol ~$0.45-0.65/L vs electricity ~$0.05/kWh — EV is still cheaper per km, but the gap is 2× not 3-4×. US: average gasoline ~$0.91/L vs electricity ~$0.13/kWh — EV ~50% cheaper per km. Always check your local rates; subsidies and time-of-use electricity tariffs change the math.
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In tropical climates: 5-10% fuel economy loss with AC on continuously. In cold climates: heating is "free" (uses engine waste heat). The math depends on speed: at low speeds (city, under 50 km/h), AC + closed windows beats open windows + no AC because the AC load is small relative to engine output. At highway speeds (80+ km/h), open windows create aerodynamic drag that costs more than AC compressor load — closed windows + AC wins. Optimum at city speed: open windows. Optimum at highway speed: closed windows + AC.
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Stop-and-go costs huge amounts of energy. Every time you accelerate from 0 to 50 km/h, you spend ~50 kJ of energy that mostly gets thrown away as heat through brakes when you stop again. City driving with frequent stops can have 40-50% of fuel energy lost to braking. Highway driving at steady speed is mostly fighting aerodynamic drag (recoverable through smoother driving) and rolling resistance (constant). EPA city ratings are typically 20-30% lower than highway ratings for this reason. Hybrids close most of the gap by regenerating braking energy back into the battery — that's why Prius city economy often beats highway economy.
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No. All calculations run entirely in your browser via JavaScript. There's no server roundtrip — open DevTools → Network and confirm zero outbound requests. Your trip data + fuel prices stay on your device. Safe for fleet-management math, business mileage tracking, or any vehicle data that shouldn't leave your machine.
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Three fuel-economy testing standards. EPA (US Environmental Protection Agency): city + highway dynamometer cycles, adjustments applied to better match real driving. Generally realistic — 5-15% worse than real-world. NEDC (New European Driving Cycle): used in Europe + many ASEAN markets until 2017, very optimistic — typically 25-30% better than real-world economy. Phased out. WLTP (Worldwide Harmonised Light Vehicles Test Procedure): replaced NEDC, more realistic — typically matches real-world economy ±5%. Asia uses JC08 (Japan) and WLTP variants. When comparing fuel economy across regions, normalise to WLTP or measure real-world for honest comparison.
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