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Track real fuel economy: tank-to-tank

By RoadCost Editorial5 min readUpdated

The dashboard is a useful guide, but it is not always the same as the fuel you actually buy. The tank-to-tank method gives you a real number you can use in a cost estimate.

Quick answer

Fill the tank to the same level, record the odometer, drive normally, refill to the same level, then divide the distance by the fuel added. Repeat over several tanks for a stable average.

In this guide

  1. 1. The method
  2. 2. Common errors
  3. 3. Use the number
  4. Questions

Key takeaways

  • Use the same fill level each time.
  • Distance divided by fuel added gives real economy.
  • One tank is noisy; several tanks are more reliable.

The method

Start with a full tank and write down the odometer. Drive as you normally would. At the next fill-up, fill to the same level and record both the odometer and the fuel added.

The fuel added is the fuel used since the last fill-up, assuming both fills were at the same level. Divide distance by fuel to get MPG, or fuel by distance to get L/100 km.

MPG = distance travelled ÷ fuel added
L/100 km = fuel added ÷ distance travelled × 100

Common errors

Stopping at the first click on one fill and topping off on the next changes the amount of fuel in the tank. That error carries into the calculation. Use the same filling habit each time.

A single tank includes one weather pattern, one traffic pattern and one driving style. Three to five tanks give a more useful average.

  • Do not compare a partial fill with a full fill.
  • Record the odometer at the pump, not later from memory.
  • Keep tyre pressure and load reasonably consistent.

Use the number

Once you have a real MPG or L/100 km figure, use it in the fuel-cost calculator instead of the brochure figure. Your commute and road-trip estimates will be closer to reality.

Questions

How accurate is the car's fuel-economy display?
It can be close, but it is not a substitute for measuring fuel added at the pump. Some cars read slightly optimistic.
How many tanks should I track?
Three to five tanks is a reasonable start. More tanks give a better average across seasons and routes.
What if I cannot fill the tank?
Use a consistent partial fill and record the exact amount, but expect more error. A full-to-full method is easier to trust.

Sources

Try the calculator

Fuel cost calculator

Use your measured MPG to estimate real trip costs.

Related guides

How to calculate commute fuel cost

A practical method for turning your daily commute into a fuel budget, including round trips, working weeks and real-world consumption.

Fuel economy vs consumption

MPG and km/L measure distance per fuel. L/100 km measures fuel per distance. Here is why the distinction matters for comparisons and costs.

Tyre pressure and fuel economy

How under-inflated tyres affect rolling resistance, how to check pressure correctly, and why over-inflating is not a free fix.

A note on prices and rules

This guide explains arithmetic and units. It does not publish live fuel prices, electricity tariffs or changing tax rules. Use your own price and check official sources for anything that changes.

Page last reviewed: . Formulas are covered by automated tests. Prices are inputs, not live data.