I do it in my head... divide the number of litres by 9 to fill, brim to brim, (easier than 4.5, most people know their 9 times table) then double it to get your gallons, and divide the mileage by that figure.
e.g.
63 litre fill, divided by 9 = 7
Double it = 14 gallons
200 = 140miles (10mpg) + 60 miles (~4mpg)
= 14mpg
^^ Clearly that's for a V8
I find it faster to do that than anything else TBH, plus you can do it whilst driving.