My solution was to fill the tank, one gallon at a time, to get an idea of the actual readings in one gallon increments.
Guage readings vary for several reasons. The most significant is the fact that when you turn on the ignition the guage reading is utilizing battery voltage, around 12.5 volts. When you start the engine your voltage rises to about 13.5volts, so the guage reading will be slightly higher.
Any motion of the vehicle causes float oscillations.
Any motion or g forces both longitudionally or laterally will cause oscillations.
Changes in the angle of your vehicle, uphill, downhill, or sidehill, will change readings.
All of these considered I find my guage to be fairly accurate, when you understand the various outside influences to the guage reading. My guage reads exactly to the center slash (1/2) when it takes 5 gallons of fuel to fill the tank.
Another way to measure fuel level would be to install an ohm meter so you can read the sending unit directly, but you will have to take readings under identical conditions in order for it to be fairly accurate. I did this by parking in the same spot, allowing things to stabilize for 15 minutes, then reading the sending unit with the ignition on, but with the engine not running. I even did this without sitting in the car so my body weight would not create a splashing effect on the fuel in the tank.
On all the Hondas and Acuras that I've owned, the needle takes forever to reach the middle mark, but then once you pass that mark, it falls to empty quite rapidly. For instance by the time you reach the middle mark you will have right around 250 miles on your tripmeter. But by empty you will have scarcely over 300 miles.
The engineers did this intentionally as people have already mentioned.
The fuel gauge on my VX fluctuates wildly. When going around right turns it lowers; on left turns it reads higher. Going up or down hills also affects the reading. I make sure to zero out my trip odometer at each fill up, then make sure to refill when the odo reaches about 350 miles or so regardless of what the gas gauge says. I then simply divide the number of miles on the trip odo by the exact amount of gas put in on the credit card receipt and calculate my mpg per fill up (worst mileage=42 mpg, best mileage=47 mpg).
Simplicity is the glory of expression.-Walt Whitman