If you can get a dipstick like a wooden dowel into the tank you can calibrate that by marking it every 10 litres when you fill up.
Fill the tank. Figure out your mpg. Set the trip odometer and drive until you have a gallon (approx. 25 miles) left to go and then fill it again.
If the neck is relatively straight, you could use a dipstick.
Not really possible!
The fill up and travel a set difference is a quaint idea but not really practical as you would have to drive exatly the same way every time and the same type of road.
Otherwise you get differing answers each time due to traffic and road conditions.
JUST GET YOUR FUEL GAUGE FIXED
The only surefire way would be to drain and measure. But I doubt that's what you have in mind.
If you know the capacity of the tank you can estimate whats left in the tank.
Let's pretend your tank hold 15 gallons. Go and fill the tank completely and at the same time reset your trip odometer to zero.
Now after driving maybe 150 miles go and fill the tank again, but this time record the gallons used (7.5 gallons in our case) and the numbers on the trip odometer (150 miles in our case), then reset it to zero again so it's ready for next the time.
Now we know that when we next have traveled 150 miles we probably have used 7.5 gal and there should be 7.5 gal remaining in the tank. Over time you may go farther distances and recalculate, eventually fine tuning your figures.
The problem with this method is that it always will be different because MPG will always be different.
Of course, it's much easier and less hassle to just fix the gas gauge.
run it empty, put 5 gallons in it.run it empty,then figure your miles pre gallon.
