Best way to determine the actual fuel consumption is to brim the tank, drive till nearly empty then brim the tank again.

Then you have an exact volume of fuel and a specific distance.

Repeat over three or four tanks and you will have an accurate fuel consumption figure based on your actual use.

The gauge is not linear in performance and not designed to be accurate - it is an indicator. Many gauges will gain or loose a bit on bends or going up, or down, hill. One thing that is done to hide this is to make the gauge slow to respond... Owners manuals usually cover this by saying fill up at 1/4 tank.

Also as pointed out in the other answer the tanks are not designed with constant volume through the depth of the tank. One reason is to avoid sloshing of fuel and picking up air, the other is that the space for the tank usually has to accommodate drive shafts or exhaust pipes etc.

The best way to determine the actual fuel consumption is to brim the tank, drive till nearly empty then brim the tank again.

Then you have an exact volume of fuel and a specific distance.

Repeat over three or four tanks and you will have an accurate fuel consumption figure based on your actual use.

The gauge is not linear in performance and not designed to be accurate - it is an indicator. Many gauges will gain or lose a bit on bends or going up, or down, hill. One thing that is done to hide this is to make the gauge slow to respond... Owners' manuals usually cover this by saying fill up at ¼ tank.

Also as pointed out in another answer the tanks are not designed with constant area through the depth of the tank. One reason is to avoid sloshing of fuel and picking up air; the other is that the space for the tank usually has to accommodate drive shafts or exhaust pipes etc.