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.
regards
gary
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