Look at Navman 3200 diesel
Works the same as the Navman 3100 but with a second flow meter fitted on the return diesel line to the tank .
One flow meter sensor measures the flow from the tank to the engine and the second sensor measures the amount returned to the tank.
The 3200 unit calculates one from the other and gives the total burned.
With a NEMA 2 link to your GPS the 3200 unit can calculate total fuel used or miles / knots per gallon / litter.
I got my 3100 petrol unit from CH. Also available from most UK on line shops.