That would be the better way of doing it as at the moment I keep my defaults on gallons, input my data, switch the default to litres (kpl) and edit the liters I put in the tank, then switch back to gallons. Another thing I've noticed while doing this is that the cost doesn't get calculated, for example when I first input my figures it's in gallons and I set the cost to ?1.339 (or whatever it is at the time) which is the cost in litres; when I switch to litres the cost is the same whereas it should divide by 4.54 for the correct ? per litre based on what is being input is supposed to be in ? per gallon.
I think it would be better if the developers sorted it out as otherwise you'd have to re-code it each time they produced an update; or just don't update
The main code for the fuel is in one file. There are many other files to do with other areas in the garage. I'd have to keep my copy of the fuel update code. Do the update and re-patch in my mod. It looks like the updates don't come around often. I would probably avoid the update for the garage, or be very careful when I do it.
I could post my mod on their forum for other people in the UK wanting to use fixed units of litres and miles and MPG.