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How to use this calculator#
- Use the distance you will actually driveTake the routed distance, not the straight line, and decide up front whether you are pricing one way or the round trip. Detours, diversions and hunting for parking typically add 5–10% to a long journey.
- Match the efficiency unit to your dashboardUS mpg, UK mpg and L/100 km are three different measures. 30 UK mpg is only 25 US mpg. Enter the number your car actually displays and set the matching unit rather than converting in your head.
- Use your own observed economy, not the sticker figureOfficial figures come from a laboratory cycle. Fill the tank, reset the trip meter, refill and divide miles by gallons. Do it over two or three tanks — one tank is distorted by how full the pump cut off.
- Enter the price you will pay on the routeMotorway service stations routinely charge well above local stations, and prices vary by tens of cents between neighbouring states or counties because of differing fuel taxes.
- Read the cost per mile, not just the totalCost per mile is the number that lets you compare cars, routes and modes of travel. It is also the figure to use when deciding whether a more efficient car pays back its price premium.
The formula#
Trip fuel cost
Fuel used = distance ÷ mpg • Cost = fuel used × price • L/100 km: fuel = distance ÷ 100 × L/100km
- distance
- Miles or kilometres actually driven, one way or round trip
- mpg
- Miles per gallon — specify US (3.785 L) or imperial (4.546 L)
- price
- Cost per gallon or per litre at the pump
- L/100 km
- Litres consumed per 100 kilometres — an inverse measure, so lower is better
mpg and L/100 km are reciprocals, so they cannot be converted with a single multiplier: US mpg = 235.215 ÷ (L/100 km), and imperial mpg = 282.481 ÷ (L/100 km). Because the relationship is inverse, equal steps are not equal savings — going from 15 to 20 mpg saves more fuel over 10,000 miles than going from 30 to 50 mpg does. The other trap is the gallon itself: the imperial gallon is 20.1% larger than the US gallon, so a UK mpg figure applied to US pump prices understates the bill by about a sixth.
mpg, L/100km and why they are not interchangeable#
US and UK mpg use different gallons: a US gallon is 3.785 litres and an imperial gallon is 4.546, so 30 UK mpg is only 25 US mpg. L/100km runs the other way, where lower is better, and converts as 235.2 divided by US mpg. Pick the unit your dashboard actually shows — using a UK figure against US pump prices understates your fuel bill by about 20%.
A worked example#
A 420-mile drive in a car returning 32 US mpg burns 13.1 gallons. At $3.45 a gallon that is $45.25 one way, $90.50 there and back, and 10.8 cents per mile. Split four ways it is $22.63 each for the round trip. Fuel is only part of the picture though: tolls, parking and the wear the IRS values at around 67 cents a mile are the real cost of driving.
Why your real mileage is worse than the sticker#
Official economy figures come from a lab cycle. A steady 55 mph on flat highway can beat them, but short cold trips, roof boxes, a fully loaded car, air conditioning and 80 mph cruising all hurt. Expect 10-20% worse than the rated figure in winter, and more in a hybrid where the engine has to run just to make cabin heat. Average your own trip computer over a few tanks instead.
Worked examples#
A long weekend road trip
420 miles each way, a car returning 32 US mpg, fuel at $3.45 a gallon, four people sharing.
- Fuel one way = 420 ÷ 32 = 13.13 gallons
- Cost one way = 13.13 × $3.45 = $45.28
- Round trip = $45.28 × 2 = $90.56
- Cost per mile = $45.28 ÷ 420 = $0.108, or 10.8 cents
- Per person = $90.56 ÷ 4 = $22.64
$90.56 for the round trip, $22.64 each. Fuel alone though — tolls, parking and vehicle wear are on top.
Does a more efficient car pay for itself?
12,000 miles a year, choosing between a 25 mpg car and a 40 mpg car costing $3,000 more, fuel at $3.50.
- 25 mpg: 12,000 ÷ 25 = 480 gallons × $3.50 = $1,680 a year
- 40 mpg: 12,000 ÷ 40 = 300 gallons × $3.50 = $1,050 a year
- Annual saving = $1,680 − $1,050 = $630
- Payback = $3,000 ÷ $630 = 4.8 years
The premium pays back in just under five years on fuel alone — sooner if prices rise or you drive more, later if the efficient car costs more to insure or service.
Reference tables#
| US mpg | L/100 km | UK (imperial) mpg | km per litre |
|---|---|---|---|
| 15 | 15.68 | 18.0 | 6.38 |
| 20 | 11.76 | 24.0 | 8.50 |
| 25 | 9.41 | 30.0 | 10.63 |
| 30 | 7.84 | 36.0 | 12.75 |
| 35 | 6.72 | 42.0 | 14.88 |
| 40 | 5.88 | 48.0 | 17.01 |
| 45 | 5.23 | 54.0 | 19.13 |
| 50 | 4.70 | 60.0 | 21.26 |
| 60 | 3.92 | 72.1 | 25.51 |
US mpg = 235.215 ÷ (L/100 km); imperial mpg = 282.481 ÷ (L/100 km). Both constants come from the exact gallon and mile definitions.
| US mpg | At $3.00/gal | At $3.50/gal | At $4.00/gal | At $4.50/gal |
|---|---|---|---|---|
| 20 mpg | $15.00 | $17.50 | $20.00 | $22.50 |
| 25 mpg | $12.00 | $14.00 | $16.00 | $18.00 |
| 30 mpg | $10.00 | $11.67 | $13.33 | $15.00 |
| 35 mpg | $8.57 | $10.00 | $11.43 | $12.86 |
| 40 mpg | $7.50 | $8.75 | $10.00 | $11.25 |
| 50 mpg | $6.00 | $7.00 | $8.00 | $9.00 |
Multiply any cell by your trip length in hundreds of miles. A 420-mile drive at 30 mpg and $3.50 is 4.2 × $11.67 = $49.01.
| US mpg | Gallons a year | At $3.50/gal | At $4.00/gal | Extra vs 40 mpg |
|---|---|---|---|---|
| 20 mpg | 600 | $2,100 | $2,400 | +$1,050 |
| 25 mpg | 480 | $1,680 | $1,920 | +$630 |
| 30 mpg | 400 | $1,400 | $1,600 | +$350 |
| 35 mpg | 343 | $1,200 | $1,371 | +$150 |
| 40 mpg | 300 | $1,050 | $1,200 | — |
| 50 mpg | 240 | $840 | $960 | −$210 |
Note the diminishing returns: moving from 20 to 25 mpg saves $420 a year, but moving from 40 to 50 mpg — a bigger jump in the headline number — saves only $210.
Common mistakes#
- Using an imperial mpg figure against US pricesA UK car quoted at 48 mpg is doing 40 US mpg. Price a 12,000-mile year at 48 mpg when you are really getting 40 and you budget $875 instead of $1,050 at $3.50 a gallon — a sixth short, every year.
- Treating mpg as if it scaled linearlyBecause L/100 km is the reciprocal of mpg, improving from 15 to 20 mpg saves 167 gallons over 10,000 miles, while improving from 40 to 50 mpg saves only 50. The biggest fuel savings come from replacing the worst vehicle in a household, not from upgrading the best one.
- Budgeting the trip on the manufacturer's rated economyLab cycles do not include a roof box, a full load, air conditioning, cold starts or 80 mph cruising. Expect 10–20% worse in winter and more in a hybrid, whose engine must run simply to produce cabin heat. Use your own trip-computer average.
- Calling the fuel figure the cost of drivingFuel is typically only a fifth to a quarter of the true per-mile cost once depreciation, insurance, tyres and maintenance are counted. The US IRS standard mileage rate — around 67–70 cents a mile in recent years — is a far better proxy for what a mile really costs.
Frequently asked questions#
How do I convert L/100km to mpg?
Divide 235.21 by the L/100km figure for US mpg, or 282.48 for imperial mpg. So 8 L/100km is 29.4 US mpg or 35.3 UK mpg.
What is a good cost per mile?
For fuel alone, 8-15 cents per mile is typical for an efficient petrol car at current prices. Once depreciation, insurance and maintenance are included, the true cost is nearer 60-70 cents per mile.
Does driving slower really save fuel?
Yes. Aerodynamic drag rises with the square of speed, so easing from 80 to 65 mph usually cuts highway fuel use by 15-20%.
How should we split fuel on a road trip?
Divide the round-trip fuel cost by everyone in the car including the driver, which is what the per-passenger figure here shows. Tolls and parking are normally split the same way.
Key terms#
- US gallon versus imperial gallon
- A US gallon is exactly 3.785411784 L; an imperial gallon is exactly 4.54609 L, 20.1% larger. Any mpg figure is meaningless until you know which gallon it uses.
- L/100 km
- Litres of fuel consumed per 100 kilometres, the standard measure across most of the world. It is an inverse measure — lower is better — which is why it cannot be converted to mpg by multiplication.
- Cost per mile
- Trip cost divided by distance. For fuel alone it is typically 8–15 cents for an efficient petrol car; including depreciation, insurance and maintenance the total is several times higher.
- Rated versus real-world economy
- Rated figures come from a standardised laboratory drive cycle so that different cars can be compared fairly. Real-world economy depends on speed, load, terrain, temperature and driving style, and is usually lower.
- Aerodynamic drag
- Air resistance, which rises with the square of speed and dominates fuel use above roughly 50 mph. Easing from 80 to 65 mph typically cuts highway consumption by 15–20%.
Sources#
- Gasoline and Diesel Fuel Update — weekly retail prices — US Energy Information Administration
- Fuel economy ratings and driving tips — US Department of Energy / EPA
- Frequently asked questions about fuel and energy prices — US Energy Information Administration
Figures last checked .
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