MPG Calculator — fuel economy in mpg & L/100km

What does this calculator do?

The MPG calculator handles three different jobs in one browser-side tool, with no server upload and no tracking:

Mode 1 — calculate fuel economy. Type how far you drove and how much fuel you put in. The calculator returns the result in three units in parallel: liters per 100 km, miles per US gallon and miles per UK imperial gallon. Click “Add another fill-up” and you can log several tanks; the running average uses the correct weighted formula (sum of fuel divided by sum of distance times 100), not the wrong arithmetic average of individual tank ratios.

Mode 2 — convert units. Type one value in your familiar unit, see the other two. A US window sticker promising 35 MPG becomes 6.72 L per 100 km and 42.03 MPG (UK) — numbers that are hard to compute mentally and easy to get wrong with the wrong conversion constant.

Mode 3 — annual fuel cost. Combine economy, fuel price and annual mileage to get the yearly fuel bill. Useful for break-even math when comparing an ICE car against a hybrid or an EV against a gas car.

The unit choice (DE/US/UK) and the mode persist across browser sessions via localStorage. No cookies, no server state, no tracking pixel. Clearing browser data clears the settings — that’s the only way they leave your device.

How does the MPG formula work?

The base formula is almost embarrassingly simple:

MPG = miles driven / gallons of fuel used

Drive 300 miles, use 10 gallons, get 30 MPG. The metric inversion in liters per 100 km is:

L per 100 km = (liters used × 100) / kilometers driven

Drive 700 km, use 50 liters, get 7.14 L per 100 km. Both formulas are derived from the same physical fact — distance per unit of fuel — but they invert each other. MPG goes up when efficiency improves; L per 100 km goes down. That fact alone causes confusion when readers see “lower is better” and “higher is better” in the same article and miss the unit switch.

The conversion uses L per 100 km as the pivot because the divide-by-zero behavior is cleanest there:

From	To	Formula
L per 100 km	MPG (US)	235.2146 / x
L per 100 km	MPG (UK)	282.4809 / x
MPG (US)	L per 100 km	235.2146 / x
MPG (UK)	L per 100 km	282.4809 / x
MPG (US)	MPG (UK)	multiply by 1.20095
MPG (UK)	MPG (US)	multiply by 0.83267

The 235.21 constant is (100 × 3.785411784) / 1.609344 — liters per US gallon, scaled to 100 km, divided by km per mile. The 282.48 constant is the same calculation with the 4.54609-liter imperial gallon. Both constants are exact mathematical consequences of the gallon and mile definitions, not approximations.

Why is the multi fill-up mode different from a simple average?

Most online MPG calculators handle a single fill-up only. That gives an MPG snapshot of one tank, which is useful but not representative — a single highway road trip can show 40 MPG in a car that delivers 28 in mixed daily driving. To get the right average across multiple tanks, you have to weight by distance.

Example: Tank A covered 100 miles at 25 MPG (so 4 gallons used). Tank B covered 900 miles at 35 MPG (so 25.7 gallons used). The naive arithmetic average is (25 + 35) / 2 = 30 MPG. That’s wrong.

The correct weighted average is (100 + 900) / (4 + 25.7) = 1000 / 29.7 = 33.7 MPG. Tank B dominates because it covered nine times the distance.

This calculator uses the weighted formula automatically when you add more than one fill-up. The output shows the running average plus the total miles and total fuel summed across all entries, so you can sanity-check the inputs at a glance. The same logic applies in any unit — sum the fuel in liters, sum the distance in kilometers, divide.

How accurate is the trip computer?

Trip computers in modern cars estimate fuel economy by integrating injector pulse durations over time and dividing by the stored tank-shape geometry. Both factors are model parameters, not direct measurements, and manufacturers calibrate them to slightly flatter the driver.

Consumer Reports, AAA, the German ADAC and the British Auto Express have all tested trip-computer accuracy independently. Across roughly 50 modern car models, the trip computer consistently overestimates fuel economy by 5 to 15 percent compared to brim-to-brim measurement. A dashboard showing 32 MPG corresponds to roughly 28 MPG in reality.

There is no federal regulation in the US (or any equivalent in the EU) that requires trip-computer calibration accuracy. The federal CAFE and EPA tests use external dynamometer measurements, not the car’s own readout, so manufacturers have no incentive to make trip computers honest.

The single exception is fully electric cars: battery-management systems measure energy outflow very precisely through dedicated current sensors, and EV trip-computer numbers tend to be within 3 percent of reality. Conventional and hybrid gasoline cars are not in that league.

Why does US gallon vs UK imperial gallon cause a 20 percent trap?

The US gallon is 3.785411784 liters. The UK imperial gallon is 4.54609 liters. The imperial gallon is exactly 20.09 percent larger. Both definitions have been frozen by international agreement since the 1959 international yard and pound treaty.

Direct consequence: any MPG (UK) figure is 20.09 percent higher than the same fuel consumption in MPG (US). A British review of a Honda Civic that quotes 50 MPG combined is reporting 41.6 MPG in US terms. A US review of a Toyota Yaris quoting 36 MPG combined corresponds to 43.2 MPG in UK terms.

The most common error in international car coverage is taking a UK number and printing it as US without conversion (or vice versa). Look closely at British YouTube car reviews: when they say “the new diesel does 65 to the gallon,” they mean 65 MPG-UK, which is 54.1 MPG-US. Posting that as “65 MPG” on an American forum starts arguments.

This calculator solves the trap with the convert mode: pick the unit your source uses, type the value, see the other two. The active card highlights so you don’t lose track of which one was the input.

How do speed, tire pressure and load affect fuel economy?

Speed dominates. Aerodynamic drag rises with the square of velocity, and the engine power required to overcome drag rises with the cube. In practice: cutting highway speed from 75 to 65 mph drops drag by 24 percent and fuel consumption by roughly 12–15 percent depending on the car. Cutting from 80 to 65 drops consumption by 20 percent or more. The Department of Energy estimates this effect adds the equivalent of $0.20–0.40 per gallon for every 5 mph above 50.

Tire pressure is the second-biggest lever. Every 1 PSI under spec costs about 0.2 percent in MPG, per the Department of Energy. A car driven on tires 6 PSI low (a common Spring-time problem after Winter pressure drops) loses about 1.2 percent MPG and wears tires noticeably faster. The fix takes five minutes at any gas station.

Weight and cargo. Every 100 pounds removed from the car gains about 1 percent in MPG. Roof boxes and bike racks hurt MPG even when empty because of drag — a typical roof box costs 10–20 percent MPG at highway speed. Removing the box between trips is essentially free MPG.

Cold starts. Engines deliver 50–80 percent of their warm-engine MPG in the first 5 minutes after a cold start. Trip-chaining (running multiple errands in one drive) and avoiding 1–3 mile commutes have outsized effects on yearly fuel consumption.

Idling. Ten minutes of idling consumes about a third of a gallon. Modern stop-start systems eliminate most idling at red lights, but long drive-throughs and air-conditioned waits in parking lots are still pure waste.

How do fuel economy and CO2 emissions relate?

Gasoline contains roughly 19.6 pounds of CO2 per US gallon when fully combusted (the EPA standard figure for transportation-fuel accounting). Diesel contains 22.4 pounds per US gallon — more carbon per gallon, so higher emissions per gallon but also higher energy content per gallon.

A 30-MPG gasoline car emits about 0.65 pounds of CO2 per mile, or 295 grams. A 40-MPG car drops that to 222 grams per mile. A 50-MPG hybrid sits around 178 grams. The EU’s 2026 fleet target for new cars is 93.6 grams per kilometer, which corresponds to roughly 56 MPG-US for gasoline — a target that requires hybridization or partial electrification for almost every body class.

For an apples-to-apples comparison between a diesel and a gasoline car, CO2 per mile is the right number — not MPG, because diesel’s higher carbon-per-gallon partially offsets its higher MPG. A diesel at 40 MPG emits roughly the same CO2 per mile as a gasoline car at 35 MPG.

How does the cost mode help with car-buying decisions?

The cost mode multiplies fuel-per-100-km by annual distance by fuel price to give a yearly fuel bill. This is the right unit for comparing total cost of ownership across cars with different efficiencies.

Example. A 35-MPG sedan costs $1,200 a year in fuel at 12,000 miles and $3.50 a gallon. A 22-MPG SUV costs $1,909 at the same mileage and price. The difference, $709 a year, multiplied by 8 years of ownership is $5,672 — meaningful when the SUV often costs $5,000 more on the lot too.

Hybrid versus ICE math. A Toyota Camry Hybrid at 48 MPG combined costs $875 a year at 12,000 miles and $3.50 a gallon. The non-hybrid Camry at 32 MPG costs $1,313. Annual saving: $438. At a typical $2,500 hybrid premium, payback is 5.7 years. Drop the gallon price to $4.50 and payback shortens to 4.4 years; raise mileage to 20,000 a year and payback drops to 3.4 years.

EV vs gas. Compare yearly fuel cost from this calculator against estimated electricity cost from the same annual miles times the EV’s kWh-per-mile times your electricity rate. Most modern EVs deliver 3 to 4 miles per kWh — at $0.16 per kWh and 12,000 miles a year, that’s $480 to $640 in electricity, compared to $1,200 for a 35-MPG gas car. The savings cover the EV premium in 4–7 years for typical commuters.

What are realistic 2026 fuel economy ranges?

EPA combined ratings get closer to real-world driving with every passing test-procedure update, but real-world numbers reported by drivers on Fuelly and Spritmonitor still show modest gaps:

• Compact sedans/hatchbacks (Civic, Corolla, Forte): EPA 32–38 combined, real-world 30–36
• Mid-size sedans (Camry, Accord, Sonata): EPA 28–35, real-world 27–33
• Compact SUVs (RAV4, CR-V, Tucson): EPA 27–32, real-world 25–30
• Mid-size SUVs (Pilot, Highlander, Telluride): EPA 22–25, real-world 20–24
• Full-size SUVs and pickups: EPA 16–22, real-world 14–20
• Compact hybrids (Prius, Civic Hybrid, Niro): EPA 50–55, real-world 47–52
• Mid-size hybrids (Camry HV, Accord HV): EPA 44–48, real-world 42–47
• Plug-in hybrids with regular charging: EPA 80–120 MPGe (effective), real-world 60–100 MPGe
• EVs in cold weather: subtract 25–35 percent from EPA range, especially at highway speed

For any specific vehicle, the most representative real-world data comes from Fuelly (US) or Spritmonitor (Europe), which crowdsource fill-up entries from owners and report the actual distribution rather than a single EPA number.

Why does this tool ship no trackers?

Every calculation runs in your browser. There is no server round-trip, no analytics beacon, no fingerprinting. The unit choice and mode are stored in localStorage on your device — the same place bookmarks live — and are wiped if you clear browser data.

Most competing MPG calculators load 30 to 80 third-party tracking scripts to monetize a three-line divide-and-multiply calculation. Insurance affiliate cookies, car-comparison funnels, ad-network retargeting pixels — none of it is necessary for the math. This tool ships with none of them by design.

Further reading: Wikipedia: Fuel economy in automobiles, fueleconomy.gov — Driving Habits (US Department of Energy data-driven advice on which habit-changes save the most fuel).

Which tools Are Related?

• Hourly-to-Annual Salary Calculator — pairs with the cost mode here. If you want to see your annual fuel bill in hours of labor, run your hourly rate through that calculator and divide.