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 calculate based on the pump output? 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...
Are all 1st gens like this (really strange fuel tank) or is it just me?

 

1) fuel milage is figured with control conditions. which really are hard to meet on the road. Such factor as road conditions hills bends, inclines ect.. all factor in as does the type of fuel used. Not only is the grade of fuel a factor but so is where, from who and time of year a factor. There is roughly 100 diffrent formulas of gas all of which produce different results. condition of the engine, milage,oil weight are also factors.
2) you could consider the bit of fuel being left in the tank as sort of a safety net but don't rely on it. Floats in the fuel tank used to measure the level are like any other component they are subject to wear and tear. The way that i check fuel milage manualy is to check milage and check again when I fuel up and use current milage minus old milage then by how many gallons i put in.

 

thats the way Im doing it. Just subtracting the refill amount from the tank capacity to find how much I had left at the empty mark. I try to use only BP or Exxon, but around here the best I can find is Hess. Its only been one full tank, I will get a more accurate picture of my car's fuel economy as I go along.

 

Also keep in mind that is an average with varying conditions which will flucuate to some degree depend on how, where and when as factors.