So I am anal about checking my Jeep Wrangler's trip odometer and fuel gauge because it gets such piss poor mileage.

Due to the cost of fuel being so volatile and constantly creeping up I switched back to my brick as my daily driver. (I generally drive a minimum of 100 miles per day during the week.)

Today I decided to fill up when the tank was very, very low so that I could start calculating my mileage. The gauge needle was resting on the red zone but not really in it yet. I was trying to figure whether I could make it to the next town down the road with what I had. I ended up just filling it up then because I am not certain how much is in the tank when the needle indicates that it is empty.

I have read here that the fuel tank on my 1990 740 holds 15.8 US gallons.

I had what looked to be a good bit less than 1/16 of a tank. But I could only just get 12 gallons pumped into my car. Guessing that the space on the gauge indicated that I had one gallon in the tank I ought to have been able to pump about 14 to 14.5 gallons.

How accurate are these gauges and sending units? Should I not be trusting them on long trips? Or does my car have an un-displayed "reserve" of one or two gallons? Or is my tank actually smaller than 15.8 gallons?

The reason I ask is that in earlier year models, the Wrangler came with two sizes of fuel tanks: 15 and 19 gallons. After a while, DC decided that it would be less expensive to make and install only one tank size and limited the "15" gallon one by means of something on the fill tube that prevented users from filling all the way and sticking a 15 gallon sending unit on the tank. Later DC decided to eliminate the choices of tank size altogether. I have a 19 gallon tank with the 15 gallon sending unit. The fill tube can easily be altered to allow full filling and the sending unit for a 19 gallon tank is not all that expensive. I will do this eventually. But right now I can fit 17 gallons in the nearly dry tank. I am certain that I do not have an extra, hidden, two gallons because the fill tube has not been altered as best as I can tell. So it should not be able to hold 19 gallons without the pump reading it as being full to the top. The sending unit is a 15 gallon one because it reads at different "speeds" during the process of emptying the tank. It looks great for the first half and then drops like a stone during the use of the last "half" tank. I know how it works well enough to drive cross country and not get into trouble.

But does my vintage of 740 have anything odd in its history that would cause me to have a nearly empty tank on my gauge but only be able to fit 12 gallons in the thing?

Just how many gallons does my gauge display out of my supposed 15.8 gallons? How much is actually in there when the pump cuts off and how much of that is actually displayed by the gauge? When I am calculating mpg on the road and doing it by quarter tanks consumed against my trip odometer, how many gallons should I use for a quarter tank? Yada, yada, yada.

I thought I had my car figured out but today really surprised me with the nearly empty thing only accepting only 12 gallons.

Maybe all of the parts in this system are just fried from age.

Oh well. Go figure.