Hi Graeme,
why so much variation on your 2 td5s, 510 for 20 gallons then 410 for 20 gallons ?, is it concievable you could get one with 610 per 20 litres?
i run with nice hard tyres (sometimes) and well greased/oiled and maintained transmission and brakes etc. so rollling resistance was minimised,
aircon off etc, no towbar, no junk in boot.
havn't calculated it for a wee while though, cant afford to fill the tank without good reason now
anyway the point is it is lower but not too much different from manual gearboxes, but i don't know the figure for mauals.