It dont work like that waiting for petrol light to come on. Have seen mine come on with 60+ mile left and still register 35 left after 32 1/2 mile.
Petrol light on mine comes on at 30 mile left usually even though it has conked out several times at 11, 13 and 0.
Every Sunday i reset trip computer. I drive on the instant economy if theres a hill coming up i build up speed before like an old diesel and still be doing over say 30 mpg at top where as if not down to sub 20 mpg, certain things like that. I can tell you it starts to drink it above 4000 rpm, 5k barely double figures.
Needs a certain amount of fuel in to reset something, try it. Run fuel almost out carry a can :clap: then slap £6 in.
As said depends upon driving, i can easilly get mine down to sub 20mpg or over 36 mpg on same run and i aint hanging about.
Even so £10 will not get me to work and back at £1.22/ltr, put £66 in the other day £59 going to fund the last government so triple taxation.