I raised this same fuel gauge issue some time ago with MMC through my dealer. Their reply seems to be Morgan speak for 'put up with it'.
Amongst a number of problems with my Plus Four (now mostly sorted but some still ongoing) my comments on the fuel gauge were as follows
My comments " Fuel gauge and low level warning light calibration clearly needs some work. Last weekend the low level light came on when the gauge was reading exactly 50% full. Exactly 11 miles later when I stopped for fuel the gauge read 25%. I could then only get 26.75l of fuel in the tank. Needless to say both gauge and the warning light are currently close to no use and need sorting"
MMC's reply was, and I quote "The total amount of fuel that is USABLE in the tank is 33 litres. The actual gauge reading is 27litres From full to empty. There is 5 litres above the float when the tank reads full. This is why it takes a while for the gauge to start to read. Once it starts to read you have 27 litres left. The low fuel light comes on when there is 6 litres remaining. Once the gauge hits the bottom red the tank will only be able to take 33 litres. There is fuel in the bottom of the tank, but this is not useable".
It certainly wouldn't be acceptable on a normal car but in Morgan's view of the world it clearly is justifiable ...
So it looks like you have
- a 38 litre tank according to the Plus Four handbook.
- of that 38 litres only 33 litres are usable (according to Morrgan's reply to me).
- The sender is then calibrated to illuminate the level warning light when you have 6 litres of the 33 litres left.
- meaning you can only use about 27 litres before the light comes on.
That 27 litres correlates with my experience and I have only ever managed to get 28 litres in afetr teh light had been on for a bit. Currently if I am driving normally I aim for fuel every 250 miles. As Spanner Juggler indicates the odometer is still the best gauge in a Morgan.
On a more positive note fuel consumption is pretty good. Mine is an automatic and normally gives 42/44 mpg in mixed driving going with the traffic flow. Although a couple of weeks ago on the Isle of Man for the Motoring Festival it averaged 26 mpg on one tank in Sport+ mode on the unrestricted roads.
