Miles Per Gallon (MPG) Calculator

MPG and Fuel Efficiency Guide

What's the difference between Real-World and Official MPG?

Official MPG figures from WLTP (Worldwide Harmonised Light Vehicle Test Procedure) testing are closer to real-world figures than the old NEDC test they replaced, but still typically 10–20% higher than real-world driving. Factors that reduce MPG below the official figure: cold engine starts (the first few miles of each journey are significantly less efficient), motorway speeds above 70mph, short urban trips, air conditioning, roof boxes, tyre under-inflation, and carrying heavy loads. Urban driving with frequent stop-start traffic is typically where the gap between official and real-world MPG figures is widest, since official tests don't fully replicate genuine city conditions.

What do I need to know about Typical MPG Ranges?

Small petrol (1.0–1.2L): 45–60 mpg real-world. Medium petrol (1.5–2.0L): 35–50 mpg. Diesel: typically 10–15% better than equivalent petrol — medium diesel 45–60 mpg. Hybrid (e.g. Toyota Prius, Yaris): 55–70 mpg in mixed driving. Plug-in hybrid: 100–250+ mpg on official figures (misleading — depends heavily on charging frequency). Large SUV petrol: 25–35 mpg. Large diesel SUV: 35–50 mpg. Sports car: 20–35 mpg. These are indicative — real figures depend on driving style and conditions more than any single published number can capture.

What do I need to know about Improving Your MPG?

The single biggest factor in fuel economy is driving style — smooth acceleration and anticipating stops can improve efficiency by 15–25%. Specific techniques: maintain a buffer behind traffic to coast rather than brake; use cruise control on motorways (reduces throttle hunting); shift up early in manual cars (most efficient at 1,500–2,000 RPM for most engines); avoid excessive idling (turn engine off if stopped for over 30 seconds); maintain correct tyre pressures, since 10% under-inflation adds roughly 2% to fuel consumption — a small but consistent drag that adds up over regular driving.

What do I need to know about Comparing Fuel Types?

At current UK prices, comparing cost per mile: petrol at 45 mpg and 150p/litre: 15.2p/mile. Diesel at 55 mpg and 155p/litre: 12.8p/mile. Electric (3.5 miles/kWh) at 7p/kWh (off-peak): 2.0p/mile. Electric at 25p/kWh (home day rate): 7.1p/mile. The cost advantage of electric vehicles is dramatically affected by whether you can charge off-peak at home. Public rapid charging at 70p/kWh: 20p/mile — more expensive than petrol or diesel for most cars.

What should I know about UK (Imperial) vs US Gallons?

A frequent source of confusion in MPG figures is that the UK imperial gallon and the US gallon are different sizes — the imperial gallon used in UK MPG figures is about 20% larger than the US gallon. This means an identical car's fuel economy, measured in the same real-world units of distance per unit of fuel, produces a notably higher MPG number when expressed in UK (imperial) gallons than when the same car's economy is expressed in US MPG, purely because of the different gallon size, not because the car is actually more efficient. A car quoted at 40 US MPG is roughly equivalent to about 48 UK MPG for genuinely the same fuel efficiency. This matters when comparing a car's economy figures sourced from UK versus US reviews or manufacturer specifications, or when using an online MPG calculator that doesn't clearly state which gallon definition it's using — always check which standard is being used before comparing MPG figures across different sources, since treating a US MPG figure as if it were a UK MPG figure will make a car look significantly less efficient than it actually is.

What's the key thing to understand about Why Short Journeys Give Worse MPG?

A car's fuel economy on short trips is typically noticeably worse than its average or official figures suggest, for reasons specific to how engines and fuel systems behave when cold. An engine burns fuel less efficiently until it reaches normal operating temperature, and on a short journey it may never fully warm up before the trip ends, meaning the whole journey is driven in a less fuel-efficient state. Stop-start city driving with frequent braking and accelerating from a standstill also wastes energy compared to maintaining a steady speed, and this effect is proportionally larger over a short distance than a longer one where some steady-speed driving can offset it. For this reason, drivers who do mostly short local trips — a daily school run or short commute — will typically see real-world MPG noticeably below both the car's official combined figure and below what the same car achieves on a longer motorway journey, and this isn't a sign of a fault; it's an inherent characteristic of short-trip driving that affects virtually all petrol and diesel vehicles to some degree.

How Do You Calculate Fuel Cost Per Mile for Trip Budgeting?

Converting a car's MPG figure into a cost-per-mile figure makes trip budgeting much more intuitive, since it directly answers "how much will this journey's fuel cost" rather than requiring separate mental steps. The calculation starts with current fuel price per litre, converted to price per gallon (multiplying by 4.546 for UK imperial gallons), then dividing that gallon price by the car's MPG figure to get cost per mile — for example, a car achieving 40 MPG with fuel at £1.50 per litre (£6.82 per UK gallon) costs roughly 17p per mile in fuel alone. This cost-per-mile figure can then be multiplied directly by a planned trip's total mileage to give a realistic fuel budget, and it's a far more useful figure for comparing the running cost of different cars or trips than MPG alone, since MPG doesn't scale in an intuitive linear way (the fuel saving from improving from 30 to 40 MPG is proportionally larger than the saving from improving from 50 to 60 MPG, even though both represent the same 10 MPG improvement). It's worth using real-world rather than official MPG figures for this calculation, since official test-cycle MPG figures are typically optimistic compared with real driving, particularly for the mix of urban, motorway and varied-speed driving most actual trips involve. For a longer trip, also factoring in whether driving conditions (motorway cruising versus stop-start urban driving, or a heavily loaded car on a family holiday) will push real-world MPG meaningfully away from a car's typical average gives a more accurate budget than applying a single generic MPG figure to every type of journey.

MPG vs L/100km: Why a Straight-Line Conversion Is a Common Misconception

A frequent mistake is assuming that MPG and litres-per-100km convert in a straight line — that gaining 5mpg always saves the same amount of fuel regardless of the starting figure. It does not, because MPG measures distance per unit of fuel while L/100km measures fuel per unit of distance, and the two are related by an inverse formula: L/100km ≈ 282.481 ÷ MPG (imperial gallons). Because of that inverse relationship, the same fuel-economy improvement in MPG saves very different amounts of actual fuel depending on where you start. Going from 20mpg to 25mpg (a 5mpg gain) takes L/100km from about 14.1 down to about 11.3 — a saving of roughly 2.8 litres per 100km. Going from 40mpg to 45mpg (also a 5mpg gain) takes L/100km from about 7.1 down to about 6.3 — a saving of only about 0.8 litres per 100km. The identical 5mpg improvement saves nearly four times as much real fuel at the low end of the scale as it does at the high end, which is precisely why fuel-economy regulators and climate researchers generally argue that improving the least efficient vehicles on the road (getting a gas-guzzler from 15mpg to 20mpg) saves far more fuel overall than squeezing another 5mpg out of an already-efficient car (getting 45mpg to 50mpg) — even though both look like the same '5mpg improvement' on paper.

Worked Example: Why the Same Car Reports Different MPG in Different Countries

Take a car with a real, measured fuel consumption of 6.5 litres per 100km — a single, unambiguous physical fact about how much fuel it uses to cover that distance. Converted to UK imperial MPG using L/100km ≈ 282.481 ÷ MPG, this car works out to roughly 282.481 ÷ 6.5 ≈ 43.5 mpg. Converted to US MPG, using the equivalent US-gallon formula (L/100km ≈ 235.215 ÷ MPG), the same car works out to roughly 235.215 ÷ 6.5 ≈ 36.2 mpg. A UK motoring review and a US motoring review of the exact same car, with the exact same real-world fuel consumption, would therefore report '43mpg' and '36mpg' respectively — a 7mpg gap that has nothing to do with the car being more or less efficient in one country than the other, and everything to do with the imperial gallon (4.546 litres) being about 20% larger than the US gallon (3.785 litres).