How to Track Fuel Economy: The Complete Guide (2026)

Tracking your fuel economy is the simplest way to understand what your car actually costs to run — and the first step to spending less at the pump. This guide covers everything: the metrics that matter, how to calculate them by hand, how to log fill-ups so the numbers are accurate, and the proven habits that improve them.

What "fuel economy" actually means

Fuel economy is how far your vehicle travels per unit of fuel. It's expressed two ways, and they point in opposite directions:

• Distance per fuel — miles per gallon (MPG) or kilometers per liter (km/L). Higher is better.
• Fuel per distance — liters per 100 km (L/100km). Lower is better. This is the standard across most of Europe and Australia.

Same physical reality, different convention. Which you use just depends on where you live.

The metrics, and how they relate

A few things that trip people up:

• US and UK gallons are different. A US gallon is 3.785 L; an imperial gallon is 4.546 L. The same car shows a higher MPG figure in the UK than in the US — about 1.2× higher — purely because of the bigger gallon. Always compare like with like.
• Quick conversions (they work both ways):
  • US MPG = 235.2 ÷ (L/100km) — and L/100km = 235.2 ÷ (US MPG)
  • Imperial MPG = 282.5 ÷ (L/100km) — and L/100km = 282.5 ÷ (Imperial MPG)
  • L/100km = 100 ÷ (km/L) — and km/L = 100 ÷ (L/100km)

How to calculate fuel economy by hand

The reliable method is the full-tank (brim-to-brim) method. Partial fills add noise to any single reading; a full tank gives you a known quantity every time.

1. Fill the tank completely. Let the pump click off. Note your odometer reading, or reset your trip meter to zero.
2. Drive normally until you next need fuel. Don't change how you drive just to flatter the number.
3. Fill completely again. As long as you fill to the same cut-off each time, the amount the pump dispenses is effectively the fuel you used since the last fill. Record that, plus the distance traveled (trip meter, or new odometer − old odometer).
4. Do the math:
   • L/100km = (liters added ÷ km driven) × 100
   • US MPG = miles driven ÷ US gallons added
   • Imperial MPG = miles driven ÷ imperial gallons added
   • km/L = km driven ÷ liters added

Worked example: you drove 520 km and the pump put in 38 liters. 520 km on 38 L → (38 ÷ 520) × 100 = 7.3 L/100km (≈ 32 US MPG, or 39 imperial MPG).

One reading is a snapshot. Your true economy is the running average across many fill-ups — which is exactly why consistent logging matters.

How to log it consistently

Whatever tool you use, the discipline is the same:

• Log every single fill-up. One skipped tank breaks the chain and the average.
• Always fill to full (or always use the same cut-off) so each entry measures a complete tank.
• Record the odometer at every fill-up, not just the trip meter — it protects you if you forget to reset.
• Note the price and total cost too. Once you have fuel and cost in one place, you can see cost-per-km and spot price creep.

A simple per-fill-up log captures everything you need:

Notebook vs spreadsheet vs app

• Notebook — zero setup, but no automatic math, no trends, easy to lose.
• Spreadsheet — flexible and free, but you build and maintain the formulas, and it's clumsy to update standing at a pump.
• A dedicated app — enter liters + odometer at the pump and it calculates MPG/L/100km, keeps the running average, charts the trend, and handles multiple vehicles. This is where My Car fits: log a fill-up in seconds and it calculates your mpg, L/100km, or km/L, keeps the running history, and charts the trend.

What affects your fuel economy

If your numbers drift, it's usually one of these:

• Driving style — hard acceleration and late braking are one of the biggest controllable factors. Smooth, anticipatory driving can improve economy by roughly 15–30% on the highway (and more in stop-start traffic).
• Speed — aerodynamic drag rises sharply above ~80 km/h (50 mph); steady cruising beats stop-start.
• Tire pressure — under-inflated tires add rolling resistance and waste fuel.
• Weight and drag — roof boxes, racks, and a trunk full of cargo all cost fuel; aerodynamic drag from a roof rack is significant at speed.
• Climate and AC — cold engines are less efficient until warmed up; AC draws power, though at highway speed windows-up-plus-AC is often a better trade-off than open windows (which add drag).
• Maintenance — worn spark plugs, the wrong oil grade, a failing oxygen sensor, and dragging brakes all quietly reduce economy. (A clogged air filter mostly hurts performance, not mpg, on modern fuel-injected engines.)
• Fuel and conditions — short trips, idling, and winter fuel blends all lower real-world figures.

How to improve it

Track first, then act — you can't tell what's working without a baseline:

1. Smooth out acceleration and braking; anticipate traffic.
2. Hold a steady speed; use cruise control on open roads.
3. Keep tires at the manufacturer's recommended pressure.
4. Drop excess weight and remove roof racks when not in use.
5. Cut needless idling — for modern engines, switching off when you'll be stopped for more than ~10 seconds usually saves fuel (not in traffic, and don't over-cycle the starter).
6. Keep up with servicing (oil, plugs, brakes).
7. Combine errands into one trip so the engine stays warm.

Re-check your running average a few fill-ups after each change. The log tells you which habits actually moved the needle.

Tracking for tax or business

If you claim mileage or run a vehicle for work, your fuel and mileage log doubles as supporting evidence — though exact requirements vary by country and tax authority, so check yours. Keep dates, odometer readings, fuel cost, trip purpose, and the business-vs-personal split — then export a PDF report when it's time to file or expense. A per-vehicle log makes this painless at year-end.

Electric vehicles: the 2026 equivalent

EVs don't burn fuel, but the same tracking logic applies — you're just counting electricity:

• kWh/100km or Wh/mile — energy used per distance (lower is better).
• mi/kWh — distance per unit of energy (higher is better).
• MPGe — "miles per gallon equivalent," a US EPA measure where 33.7 kWh is treated as one gallon of gasoline, so you can compare an EV to a combustion car.

Log charging sessions the way you'd log fill-ups: energy added + odometer. For running cost, use the kWh drawn from the wall/charger (it includes charging losses); the energy that actually reaches the battery is a little lower. If you run both an EV and a combustion car, track them side by side to compare real running cost.