The band is wider than the gap
The opening figures are deliberately ordinary: 2,720 miles, 191.5 gallons bought, 16.5 mpg expected, 26 hours of idling. Expected fuel comes out at 180.4 gallons, so the period looks 11.1 gallons heavy — about $41 worth. Then the band gets calculated and it runs from 156.2 to 206.8 gallons, which is a spread of fifty gallons around an expectation of a hundred and eighty.
An eleven gallon gap disappears into that. Not because the gap is fake, but because two odometer readings, an economy figure known to plus or minus eight per cent and a tank that might be a quarter fuller at one end than the other cannot resolve anything finer. The page reports it plainly rather than dressing it up as a result.
Making the method sharper
Three things narrow the band and they are in order of effect. Knowing the economy better is first — measure it fill-to-fill on the actual vehicle in the actual work, over several tanks, and the eight per cent might come down to three. Reading the odometer at the pump on the last fill of the period is second, because it collapses the tank-level slack to almost nothing. Running longer periods is third and it helps less than people assume, since the economy uncertainty scales with the miles while only the tank slack stays fixed.
What actually catches something is repetition. One period inside the band says nothing. Six consecutive periods all leaning the same way say a great deal, because ordinary variation changes sign and a systematic difference does not.
Idle fuel has no miles on it
On the defaults, 26 hours at 0.6 gallons an hour is 15.6 gallons — 8.1 per cent of everything bought, and about $58. It is completely invisible to an odometer. Divide the miles by every gallon on the statement and the van appears to do 14.2 mpg; take the idle fuel out and it does 15.5. Neither figure is wrong, they answer different questions, and a fleet report that quietly uses the first one will make every van with a lift or a reefer look like the worst vehicle on the list.
Related pages
Vehicle idling fuel cost takes the standing-still side out to a year and a fleet. The mileage log is where clean start and end readings come from, and petty cash reconciliation does the same kind of expected-against-actual arithmetic for money rather than gallons. Fuel cost converts miles and economy to dollars directly.
Questions people ask
Why does the calculator not tell me whether fuel is being taken?
Because the arithmetic genuinely cannot support that conclusion and pretending otherwise would be the worst thing this page could do. What it can do is bound the question: it shows the range the gap would have to clear before ordinary variation stops explaining it. On the default figures the band is fifty gallons wide, so an eleven gallon gap is not evidence of anything. If a gap does clear the band, the useful next step is still checking the readings and the statement, not the person.
What economy figure should I use?
One you measured on that vehicle, in that work, over several tanks — fill to full, note the miles, divide, and repeat until the number stops moving. A manufacturer figure is the wrong tool here because it was produced under conditions your van never sees, and using it makes every period read as a shortfall. If you have never measured it, do that before running reconciliations at all, because everything downstream inherits the error.
Does the tank slack get smaller over a longer period?
No, and that is the useful thing to know about it. The uncertainty from the tank level is set by how full the tank might be at each reading, so it is the same twelve gallons whether the period is a month or a year. The economy uncertainty, on the other hand, grows with the miles. Longer periods therefore help with the tank problem and not with the economy problem, which is why measuring the economy properly beats extending the period.
Should idle hours include a refrigeration unit or a lift?
Include them if that equipment runs off the vehicle engine, because then it burns the vehicle fuel and puts no miles on the odometer, which is exactly the case this page is trying to account for. Use your own gallons-per-hour figure for that mode rather than a bare idle figure, since the difference is large. If the unit has its own tank and its own fuel purchases, keep both the hours and the gallons out of this calculation entirely.
What if the odometer reading and the last fill are days apart?
Then some of the miles in the period were paid for by fuel bought in the next period, and the gap will lean negative for no interesting reason. It usually reverses in the following period, which is another argument for reading several periods in a row rather than one. Where you can, take the odometer reading at the pump on the last fill so the two ends of the period line up.