The arithmetic, in full
Gallons equal distance divided by MPG. Cost equals gallons times the price per gallon. That is the whole calculation, and it is worth doing once in the abstract rather than per trip, because the useful number that falls out of it is cost per mile: price divided by MPG. At $3.50 a gallon and 30 MPG that is 11.7 cents a mile, and once you have that figure you can price any distance in your head without a calculator at all.
Scaled up, 12,000 miles a year at 30 MPG is 400 gallons, and at $3.50 that is $1,400. Halving the economy doubles it: the same 12,000 miles at 15 MPG is 800 gallons and $2,800. This is why the difference between a mid-size sedan and a full-size truck shows up as roughly a hundred dollars a month rather than the few cents a gallon people argue about at the pump.
Use observed economy, not the sticker
The EPA combined figure comes from a standardised test cycle, and your commute is not that cycle. Short trips where the engine never reaches operating temperature, stop-and-go traffic, sustained speeds above 70, a roof rack, a full load, underinflated tires and winter-blend fuel each cost something, and they stack. The honest way to get your own number is to fill the tank, note the odometer, drive normally for a tank or two, fill it again and divide the miles by the gallons the pump put in. Do that twice in different seasons and you will have a summer figure and a winter figure that may differ by four or five MPG.
| What changed | Typical direction |
|---|---|
| Trips under a few miles, cold engine | Well below the sticker; the warm-up is the whole trip |
| Steady highway at moderate speed | Often above the sticker |
| Highway at 80 rather than 65 | Meaningfully worse; drag rises with the square of speed |
| Roof box or loaded rack | Worse, and it stays worse until you take it off |
| Winter blends, cold air, warm-ups | Worse for months at a time, not a bad tank |
Reading the EV comparison
Electric efficiency is quoted two ways and they are the same fact inverted. Miles per kWh is the one that behaves like MPG, so it goes in the field above; kWh per 100 miles is what most window stickers use, and the page shows it alongside. An EV doing 3.5 mi/kWh over 12,000 miles draws 3,429 kWh at the wall, and at 16 cents a kWh that is $549 against the gas car's $1,400.
Two cautions on that comparison. The rate matters more than anything else in it: home charging overnight and public DC fast charging can differ by a factor of three, so an EV that is cheap to run in a garage is not necessarily cheap to run from a parking lot. And efficiency falls in the cold, sometimes by a third, because heating the cabin comes out of the same battery that moves the car — a gas car heats the cabin with waste heat it was making anyway. Neither of these is modelled here; both are reasons to enter your own winter figures and see what the year really looks like.
What a fuel number does not cover
Fuel is the visible cost and it is nowhere near the whole cost. Tires wear, oil gets changed, brakes and belts and batteries get replaced, insurance and registration are due whether the car moves or not, and the car is quietly losing resale value the entire time. Depreciation alone usually exceeds fuel by a wide margin on anything bought new. Put fuel on top of those rather than mistaking it for the running cost, and the full picture is on the cost per mile calculator and the car ownership cost calculator.
Questions people ask
Should I enter the EPA rating or my own MPG?
Your own, every time, unless you are reimbursing at a rate that a policy defines in terms of the rating. The EPA figure is a repeatable laboratory result that lets two vehicles be compared on the same basis; it is not a prediction of what any particular driver will see on any particular route. Drivers who mostly do short suburban trips typically land well under it, and drivers who mostly do steady highway miles sometimes beat it. Fill the tank, reset the trip meter, drive a tank, fill again, and divide miles by gallons pumped. The trip computer's lifetime average is a reasonable second choice and is usually optimistic by a few percent.
Why does the same car cost so much more to fuel in winter?
Several effects arrive together. Winter gasoline blends carry slightly less energy per gallon. Cold oil, cold transmission fluid and cold tires all raise friction until everything warms through, which on a short trip may be most of the trip. Idling to defrost burns fuel while covering no distance at all, so it divides into the miles as pure loss. Cold air is denser, which increases aerodynamic drag. Tire pressure drops about a pound for every ten degrees Fahrenheit the temperature falls, and underinflated tires cost economy. Individually these are small; together they routinely account for ten to twenty percent, and more on a route made of short hops.
Is the electricity rate on my bill the right one to use?
Use the all-in delivered cost, not the generation charge alone. Divide the total dollar amount on the bill by the kWh it covers and you get the rate that actually applies to charging at home, including delivery, distribution and fixed charges spread over your usage. If you are on a time-of-use plan and charge overnight, the off-peak rate is the relevant one and can be far below your average. Public charging is a different market entirely: DC fast charging is typically priced well above residential rates, sometimes by the minute rather than the kWh, and if most of your charging happens away from home the comparison on this page needs that rate instead.
Does driving slower actually save enough to matter?
On the highway, yes, because aerodynamic drag rises with the square of speed and it dominates above roughly 50 MPH. Dropping from 80 to 65 on a long interstate run is one of the few changes a driver can make that shows up clearly on the next tank. In town it makes far less difference, because at low speed the losses are mostly rolling resistance, accessories and the energy thrown away in braking rather than air. The city-driving version of the same advice is to accelerate gently and look far enough ahead to coast to lights instead of braking, since every hard stop discards fuel you already burned.