Fuel / Trip Cost Calculator

Trip fuel cost is distance divided by fuel efficiency, times fuel price. In imperial units: gallons used = miles ÷ MPG, then cost = gallons × price per gallon. In metric units, efficiency is usually expressed as liters per 100 km rather than "distance per liter," so liters used = (km ÷ 100) × L/100km, then cost = liters × price per liter.

Fuel Efficiency mode works backward from an actual trip: given the distance you drove and the fuel you used to refill, it computes your real-world MPG or L/100km — useful for checking against a vehicle's advertised rating.

Worked examples:

• 300 mi at 30 MPG, $3.50/gal → 10 gal used, $35.00
• 300 mi as a round trip at the same figures → 600 mi, 20 gal, $70.00
• 500 km at 8 L/100km, $1.50/L → 40 L used, $60.00
• 800 km at 7.5 L/100km, $1.75/L → 60 L used, $105.00
• Drove 300 mi, used 10 gal → 30.00 MPG
• Drove 412 mi, used 13.2 gal → 31.21 MPG

MPG is a reciprocal, and it lies about savings

This is the one genuinely counterintuitive thing about fuel economy, and it changes which car you should buy. Equal gains in MPG are not equal savings. Because MPG puts distance on top and fuel on the bottom, the same +5 MPG is worth wildly different amounts depending on where you start. Over 12,000 miles a year at $3.50/gal:

• 15 → 20 MPG saves 200 gallons, $700 a year
• 20 → 25 MPG saves 120 gallons, $420 a year
• 30 → 35 MPG saves 57 gallons, $200 a year
• 40 → 45 MPG saves 33 gallons, $117 a year

So replacing a 15 MPG truck with a 20 MPG one saves six times as much fuel as replacing a 40 MPG car with a 45 MPG one — and nearly twice as much as going from 25 MPG all the way to 50. If you run two vehicles, the improvement is almost always worth more on the thirsty one, which is the opposite of where attention usually goes.

The fix is to think in fuel per distance instead. Flip the same figures to gallons per 100 miles and the savings become obvious by inspection: 15 MPG is 6.67, 20 is 5.00, 25 is 4.00, 30 is 3.33, 40 is 2.50, 50 is 2.00 gal/100 mi. The steps get visibly smaller as the number improves.

This is exactly why metric fuel economy is quoted as L/100km and not km/L. It is already a fuel-per-distance scale, so it is linear in the thing you are actually buying, and the illusion never arises. For reference, 20 MPG is about 11.8 L/100km, 30 MPG is 7.8, and 40 MPG is 5.9.

What actually moves your real-world number

Speed, more than anything else on a long drive. Aerodynamic drag rises with the square of speed, so fuel use climbs steeply once you are past roughly 50–60 mph. A car returning 32 MPG at 65 mph might manage 27 at 75. Over 500 miles at $3.50/gal that is $54.69 against $64.81 — about $10 more — and it saves you 62 minutes. Whether that is a good trade is a real decision rather than an obvious one, but it is worth knowing you are buying roughly an hour for ten dollars, and that the trade gets worse the faster you were already going.

Anything on the roof. A loaded roof box is the single worst thing most people do to their fuel economy, costing on the order of 10–25% at highway speed, because it attacks the drag term directly. Empty roof bars still cost a few percent. Take them off when the trip is over — the penalty applies every mile, not just the loaded ones.

Short trips, more than cold weather itself. An engine below operating temperature runs rich and its driveline is stiff with cold fluid, so the first few miles can be dramatically worse than the rated figure — often on the order of half. A winter of two-mile errands will wreck a tank average in a way that no amount of gentle driving fixes, and it is the usual reason a tank measured in January looks nothing like one measured in July.

Tyre pressure and load, in that order. Under-inflation costs something like 0.2–0.3% per psi below spec across all four tyres — small per psi, not small when you are 8 psi down all round. Weight matters most where you accelerate and brake a lot, so clearing out the boot helps in town and barely registers on a motorway cruise. Air conditioning versus open windows is close enough either way not to be worth the argument.

Measuring your own efficiency properly

Fuel Efficiency mode works backwards from a tank you actually burned, and it is worth more than the manufacturer's figure — but only if you measure it cleanly. Fill to the first automatic click of the pump, note the odometer or zero the trip meter, drive, then refill to the first click again and use the pump's litres or gallons as the fuel figure. The first-click rule matters more than which pump you use: topping off past it adds an unknown and variable amount that goes straight into your error.

One tank is noise. Tank shape, parking slope and how hard the pump clicks off can move a single measurement by several percent. Take three or four consecutive tanks and average them, and keep summer and winter figures separate — they are genuinely different numbers rather than the same number measured badly.

Expect your result to come in below the official rating. Published figures come from standardised test cycles, and real driving has hills, traffic, headwinds, roof bars, passengers, and cold starts that the cycle does not. A real-world figure 10–20% under the sticker is normal, not a fault — and it is the number you should be putting into the trip cost above.

What the estimate leaves out

It is a fuel calculator, so fuel is all it counts. The full cost of driving a mile also includes tyres, servicing, and depreciation, which together often run to more than the fuel on a modern efficient car — worth remembering when comparing a drive against a train fare rather than against another drive.

A hilly route, a heavily loaded car, or a long stretch of stop-start traffic all deserve a lower efficiency figure than your usual average, rather than a contingency added at the end.