If you're trying to figure MPG, don't worry about how much is in your partially full tank. Start off by filling the tank and resetting the trip odometer. The next time you fuel the vehicle, fill it up again, and see how much fuel was required to do so and how many miles you've traveled. Figure the apparent fuel economy for that tankful. Reset the trip odometer. If you're really interested in your fuel economy, do this every time you fuel, and write it down.

As noted by nevremj, the average of the results over a number of tankfuls will be more meaningful than the apparent fuel economy for any one tankful.

Even easier than keeping a notebook and logging several fillups - do the following:
fillup- reset trip meter to 0
fillup - don't reset rip meter, but keep track of fuel volume
fillup ditto
if trip meter goes to zero, try to remembeer, it's now 1000 miles plus trip reading

repeat
right before 2nd rip meter rollover, fullup . add up all the fuel and the 1000 + trip meter reading and you have almost 2000 miles of readings to calculate from.

just for snicks at the b20 pump i filled and burped and filled and burped several times. amazing how much air is trapped in the tank and how far the fuel runs back into the tank after burping. anyhoo i did the same procedure when i got off the i and came away with 30 mpg. 60 miles 2,01 gallons. 100kph to 65 mph and a cool sunday night going north with no traffic.

__________________
currently
[1981 300 td tdidi 165500 dark brown/palamino-Brownie-mine-3k miles of ownership
1983 240d 162+++ Anthricite grey w/ henna red interior and hella lights-wifes car-Red

the above two cars are for sale
and can be seen on the cars for sale thread here. pix also available.


240d-144+ Manilla Yellow w/ palmino interior-greasecar kit-Blondie-the college kids car

23" gt 21 speed still on original tires-still got the nubs
21" khs tandem