In order to properly calculate fuel consumption, Fleetio assumes that every Fuel Entry returns the tank to FULL fuel capacity, regardless of how much was in the tank before fueling.
In some circumstances, a Fuel Entry may NOT completely fill the tank to full capacity. To accommodate these exceptions, Fleetio fuel transactions offer a Partial Fuel-up flag.
In the Flags section of the New Fuel Entry form, select the Partial Fuel-up checkbox.

NOTE: Fuel Entries that are flagged as Partial Fuel-up will not be factored into MPG or cost/mile calculations for the transaction. However, fuel costs and volume on Partial Fuel-ups will be factored into aggregate metrics like Total Fuel Cost, Cost per Gallon/Liter and Total Gallons/Liters consumed. Complete calculations for MPG and cost/mile will resume on the next Full Fuel Entry.
Partial Fuel-ups are identified by an icon in the Fuel History list. There also won't be any data available in the "Fuel Economy" and "Cost per Meter" columns.

Once a Partial Fuel Entry has been logged, subsequent Fuel Entries determine how Fleetio uses that data.
Partial Fuel-up followed by a full Fuel Entry
If you log a Partial Fuel-up, then log a full Fuel Entry next, Fleetio will include the miles traveled and gallons purchased from the Partial Fuel-up in the fuel metrics calculation of the full Fuel Entry.
Multiple Partial Fuel-ups in a row
If multiple Partial Fuel-ups are entered consecutively, and then a Full Fuel Entry is logged, Fleetio will only use data from the single latest Partial Fuel-up in combination with the Full Fuel Entry to determine MPG and CPM metrics.
All data from each Partial Fuel-up will still be included in the aggregate metrics, such as Total Fuel Cost.
Partial Fuel-up follows by a Reset Fuel Entry
If you log a Partial Fuel-up, then enter a Reset Fuel Entry, fuel metrics won't be calculated for either transaction. However, all data will still be included in the aggregate metrics.