Fuel Guage Playing Tricks?

I know fuel guages are infamous for their inaccuracy, and mine likes playing games. I was trying to test the fuel economy of the strat and filled the car up to a full tank about a week ago (According to the owners manual is approx 16 gallons). It spent some time above the full mark, then descended down to the 1/4 mark in less than 200 miles.
After resting the car, the guage jumped back up a bit, and I expect to reach the 300 mile mark on a full fill. Thats only about 19 MPG Highway...

How do you guys go about testing fuel economy? Do you let the car keep going till it runs dry and stops? Do you carry a tank of extra fuel?

Tires are new and balanced.
Fuel System was inspected and flushed recently.
Doesnt seem like the fuel/air mixture is very rich, but 19 MPG seems low for a little V-6. Any suggestions?

Heres something kind of interesting. I kept driving the car trying to run into the Empty marker, and it made it all the way to 280 miles and still the light didnt flash by the time I got to a Hess gas station. The needle was right above the red line, but I got tired of waiting.

280 miles /16 gallons = 17.5 MPG. Pretty terrible right?

So I let the car start filling up as I clean off the windshield... I figure it will go to about 15 gallons since I probably had a gallon left in the 16 gallon tank...

The pump didnt even reach 11 gallons and the tank was full. So if the owner's manual is correct, I had 5 full gallons left in my tank when it was reading nearly empty?

So lets say I had only spent 11 gallons instead of 15.

280/11 = 25.45

Seems much better, but it seems unrealistic that a guage could show nearly empty when it still had over 5 gallons in it... Confused...

My '05 5.7L Ram 1500 does this as well.

I had a quarter tank left in the AM, and I drove 4 miles to a store to run some errands, and when I came out to start it up, the low gas ding came on and the fuel indicator hovered just a tiny bit above the empy line.

Knowing it's terrible inaccuracy, I drove for another 60 miles before I pulled into a gas station, and the indicator didn't move either way, nor did the low gas light turn off. I did keep glancing down nervously as I was driving wondering if it'll start sputtering any moment. I hate that feeling.


--Setherial

I've learned to pay very little attention to what the guage tells me. However when trying to figure out what kind of gas mileage Im getting, it would be useful to have a fuel guage that actually makes sense.

Hia

My manual say's: When fuel gauge pointer is on "E" there is a reserve fuel capacity, witch will correspond to aprox 10% of tank volume.

On my 1500 short bed that would be 2,6 gallons (3,5 on 1500 long bed and 3,4 on 2500/3500 short beds).

about the tanks I think they are built up on cell like dingys inside, in every cell there is tiny holes in the buttom. That could be a reason for sometimes (after a sharp turn) the gauge points to a wrong level. After standing still (or riding smooth on a leveled road) the fuel have leveled trough the holes and the gauge is more exact again.

just some thoughts

thanks MrZ, I think it depends on the shape of the car and how much they have to pack in to these small sedans and coupes. 10% is actually a good amount to have at the E mark, but right now it seems I have close to 33% at the E mark in my car.