Definition
Full-tank-to-full-tank mileage is the distance driven between two full-tank fills divided by all the litres filled after the first full tank, up to and including the second. It ignores what was in the tank and only uses what you paid for.
Why measure mileage between full tanks?
Because a tank is only a known level when it is full. Between two full tanks, every litre burned has been bought, so kilometres divided by litres bought is the true mileage. Measuring fill by fill mixes in whatever was left in the tank and swings widely, which hides real diesel loss.
Can you show a worked example?
A tipper is filled full at 12,000 km. It takes 80 litres at 12,350 km (not full) and is filled full with 160 litres at 12,900 km. Distance is 900 km and litres since the first full tank are 240, so mileage is 3.75 km/l.
- Distance: 12,900 − 12,000 = 900 km
- Litres: 80 + 160 = 240 (the first full fill is not counted)
- Mileage: 900 ÷ 240 = 3.75 km/l
- Expected 4.20 km/l; 10% below is 3.78 km/l, so this fill is flagged as low.
What causes low diesel mileage?
Low mileage has honest causes and dishonest ones. Honest causes include heavy loads, hill or ghat roads, long idling, poor tyre pressure, a clogged air filter or an injector problem. Dishonest causes include diesel drained from the tank or fills billed but not put in. Check the vehicle first, then the pattern by driver.
- Same vehicle, different drivers: compare mileage by driver.
- Fills close together with few km in between deserve a look.
- Fuel bought while GPS shows the vehicle parked is a strong warning sign.
How does software help track mileage?
Software records every fill with odometer and photo, checks the odometer is higher than last time, works out full-tank mileage automatically and flags fills more than 10% below the vehicle’s expected figure. A report lists suspect entries by vehicle and driver, so you see a pattern instead of a single bad day.
FleetLite does this on every plan. Premium’s cost-leak report adds duplicate fills and, with GPS, fuel bought while the vehicle was parked.
How to measure diesel mileage between full tanks
Fill the tank full
Fill the tank to full and note the odometer reading. This starts the measurement.
Record every top-up
For each partial fill, note the litres and the odometer. Do not reset anything.
Fill full again
At the next full-tank fill, note the litres and odometer. This closes the measurement.
Work out the distance
Subtract the first full-tank odometer from the second.
Add up the litres
Add the litres of every fill after the first full tank, including the second full tank.
Divide and compare
Divide distance by litres for km per litre, and compare it with the vehicle’s expected mileage.
FAQ
Frequently asked questions
How often should I fill the tank full?
Often enough to close a measurement every week or two. The more full tanks, the sooner a drop in mileage shows up.
What expected mileage should I set?
Use the vehicle’s own history on its usual routes and loads; a new vehicle can start from the manufacturer’s figure and be adjusted after a month.
Does AdBlue or engine idling affect this?
Idling burns diesel without kilometres, so long idling lowers mileage. AdBlue is a separate fluid and should be recorded as its own expense, not as diesel.