My first HWY test in 2005 49.94 mpg with no EFI
 

In 2005 after porting my engine, I had the need to travel 236 miles in my 1984 Nissan Bluebird Wagon to retrieve a child from a country police station in Australia I used approx 3.7 Gal of fuel.

That works out to be 49.94 mpg but I was only doing 49.7 mph on the trip on flat roads.
I have found out since then that the driveway slope effects how much you can put in your tank due to air pockets in it. I have since found that I can fit 1 g more fuel in the tank depending on the angle of the car so that's why when I do my tests now I use the same gas pump to fill up and top up to gauge how much fuel I have used, so my 49.94 mpg could be as low as 44.13.

I just wonder how many people have found that out or taken into account when quoting there figures.

That's why we log our mileage over time. I know that if I have an especially good tank followed by an especially bad tank, I probably didn't get a complete fill on that first tank.

I would love to do my tests with several full tanks over time like most people do but I simply do not have the money to do such a test I only live a short distance from the shops and my odometer is broken so I have used whereis.com to look up my mileage and produce a tacho graph eg record rpm at 1 second intervals to prove I did not have a tow rope attached etc.

But anyway Clev thanks for the advise.

I'm not suggesting that you make an unnecessary trip multiple times, but if you make a trip, for instance, and then return to your normal driving and find that you return to your normal mileage on the next couple of subsequent tanks, I think that's evidence enough that your trip mileage was probably accurate. If the next tank after returning to normal driving is unusually low, then you probably got a short fill on the trip.

But yeah, not having an working odo makes that whole process harder. :-)

I try and buy all of my fuel from the same pump. I always let it click off 3 times.

I fill mine right to the top of the spout at the same pump this seems to be more accurate for me.

That is very good