Fuel or electricity is the energy part of your cost per mile. With the same fuel economy or energy consumption, a higher price increases the cost of each mile and the monthly total.
The calculation, step by step
- Fuel economy or energy consumption: use mpg for a fuel-powered car or mi/kWh for an electric car.
- Energy per mile: divide the price in US$/gal or US$/kWh by the mpg or mi/kWh figure, respectively.
- Monthly energy: multiply that cost per mile by your monthly work miles, including the drive to passengers.
If you use L/100 km or kWh/100 km, divide consumption by 100 and multiply by the price per liter or per kWh. With km/L, divide the price per liter by fuel economy. Those results are per km; with mpg, the result is per mile and the price is per gallon.
Two examples
For a fuel-powered car with 30 mpg at US$3.40/gal, the math is 3.40 ÷ 30 ≈ US$0.113333/mi. For an electric car with 4 mi/kWh at US$0.16/kWh, it is 0.16 ÷ 4 = US$0.04/mi.
Use the same miles as in the cost per mile article: five days a week, 125 miles a day. The average month has 5 × 125 × 52 ÷ 12 ≈ 2,708.33 miles.
| Example | Fuel economy or energy consumption | Price | Energy per mile | Energy per month |
|---|---|---|---|---|
| Fuel-powered | 30 mpg | US$3.40/gal | US$0.113333 | US$306.94 |
| Electric | 4 mi/kWh | US$0.16/kWh | US$0.040000 | US$108.33 |
All figures are fictional examples, not market prices or spending forecasts. The month is an average; calculations use unrounded values, including monthly miles. The per mile column uses six decimal places.
The calculator shows US$0.11/mi for the fuel-powered example and US$0.04/mi for the electric one. Money is rounded to six decimal places, then to two for display. To calculate the month, use the cost per mile before rounding.
If the price rises
Keep the same 30 mpg and change only the price from US$3.40 to US$3.60/gal. Energy now costs 3.60 ÷ 30 = US$0.120000/mi. The difference between the unrounded costs per mile is approximately US$0.006667/mi.
For the same 2,708.33 miles per month, energy rises from US$306.94 to US$325.00, an extra US$18.06. If you keep the US$250.00 in fixed costs from the cost per mile article, total car costs rise from US$556.94 to US$575.00 per month. With the same amount received, you have US$18.06 less left, as in the what is left calculation.
The logic is the same for an electric car: with the same energy consumption and miles, a change in price per kWh changes cost per mile and the monthly total. These figures are still fictional.
Use the price you actually pay
Enter the price you pay per gallon or per kWh, including the discounts and fees that actually apply to you. If you refuel or charge at different prices, divide the total paid by the corresponding gallons or kWh to find the average price.
Count energy once, through fuel economy or energy consumption and price. Don't repeat it under “Other”. The result is an estimate: redo the math when your price, fuel economy, energy consumption or work miles change.
Use your own numbers
In Radr's fuel and charging calculator, enter distance, fuel economy or energy consumption, and price. To include maintenance, insurance and other car costs, use Full cost.
General information, not tax or legal advice.
