Running-cost and performance formulas for cars, each computed for a worked example and matching the calculators. Prices, rates and tax rules change, so the dollar figures are examples to replace with your own.
Formulas
| Measure | Formula | Example |
|---|---|---|
| Trip fuel and cost | fuel = distance / efficiency, cost = fuel × price |
300 miles at 30 mpg and $3.50: 10 gallons, $35.00 |
| Fuel economy conversion | L/100 km = 235.215 / mpg |
30 mpg is 7.84 L/100 km, or 12.75 km/L |
| Mileage reimbursement | miles × rate per mile |
1,000 miles at an example rate of $0.67: $670 |
| Tire diameter | rim in + 2 × width mm × aspect % / 25.4 |
225/45R17: 24.97 in; 245/40R18: 25.72 in |
| Speedometer error from a tire change | actual = indicated × new diameter / old diameter |
Indicating 60 mph after moving from 225/45R17 to 245/40R18: 61.8 mph |
| Horsepower from torque | hp = torque (lb-ft) × rpm / 5252 |
300 lb-ft at 5,000 rpm: 286 hp |
| Horsepower from the quarter-mile trap speed | hp ≈ weight (lb) × (mph / 234)³ |
3,000 lb at 100 mph: about 234 hp |
| EV charging cost | kWh = battery × (to% − from%) / efficiency, cost = kWh × rate |
60 kWh battery, 20% to 80%, 90% efficient, $0.15/kWh: 40 kWh, $6.00 |
| Speed | speed = distance / time |
100 km in 1 hour is 100 km/h, or 62.1 mph |
Try them
Costs: Fuel Cost Calculator, Gas Mileage Calculator, Mileage Calculator and EV Charging Cost Calculator. Wheels and speed: Tire Size Calculator and Speed Calculator. Engine output: Horsepower Calculator. To weigh buying against leasing, see How Different Loans Work.
Notes
- Electric versus petrol per mile. With the numbers above, the EV costs $0.048 a mile against $0.117 for the 30 mpg car, before accounting for the vehicle's price or home charging setup.
- Bigger tyres read slow. A larger overall diameter means fewer wheel turns per mile, so the speedometer under-reads: it shows 60 mph while you travel 61.8.
- Why 5252? Horsepower and torque lines cross at 5,252 rpm in lb-ft and hp, because 1 hp = 33,000 ft-lb per minute ÷ 2π.