The fuel level reading in a 2WD Vanagon is not very accurate—and that's putting it nicely! The reason for this inaccuracy is simple: the fuel tank of the 2WD Vanagon is very shallow. The difference between a full tank and an empty tank is a matter of less than about 6" of fuel height:
In other words, only about 3" of fuel drop represents about ½ the total fuel supply.
There were two fuel level sending units used by VW: one with a part number ending in "E" and one ending in "K." The "E" was used on fuel tanks with a 7mm fuel outlet (1980-85) and no rectangular depression:
The "K" was used on tanks with a 12mm fuel outlet (1986-91) with rectangular depression:
Both sending units read the same resistance at both extremes of the angular travel. The only difference between these "E" and "K" sending units is the length of the arm onto which the float is mounted—the "K" version being shorter. Here is a photo of the "K" sending unit:
By shortening the arm, the "K" has a greater angular range of motion compared to the "E" for the same vertical motion of the float. Since both read the same ohms resistance at each extreme, this means the "K" offers more resistance change per degree of angle, which translates into more resistance per inch of fuel drop. That's why VW made this change.
It's easy to erroneously deduce that the reason for the change in the sending unit had something to do with the depression in the tank that appeared at the same time—and that only the "E" should be used on the early tank without the depression. However, logic dictates that simply installing the shorter-arm "K" sender will improve accuracy in a fuel tank with or without the depression. This is because the depression does not actually allow the float to travel farther down, since allowing it to do so would give a false reading of having more fuel than you actually have. The photo above—the one with the tape measure sticking out of the fuel sending unit hole and reading 5¾" is of a later tank with the rectangular depression, which is about 5/8" deep:
Indeed, an early tank without this depression is only about 5 1/8" deep. If the sending unit was designed to take that extra motion into account, it would falsely read 12% more fuel than the tank can actually hold.
We are not sure why VW added this depression. Maybe it helped keep the float from banging on the bottom of the tank when the fuel is running low. Or maybe it was added so a damaged tank could be deformed up to 5/8" and still not affect float travel (and, by extension, fuel level reading). One thing is for sure: Its absence does not make any difference on the "K" sender on an otherwise-undamaged early fuel tank.
We have installed a "K" sending unit in an empty, early-style, undamaged fuel tank (without the depression) and measured the resistance with the tank right-side up (sending unit float all the way down=condition for EMPTY) and upside down (sending unit float all the way up=condition for FULL). We have verified the depression is NOT necessary for the "K" sender to read properly. Long story short: This is why we sell only the "K" sending unit for all years of 2WD Vanagon. It reads more accurately, and it's less expensive, to boot!
This is not to suggest that replacing your fuel tank with a later-model unit is a bad idea. On the contrary, updating to the later-model tank gets you the 12mm fuel outlet and the depression (read: both the extra feature in the fuel tank bottom and the feeling you get after spending the money!). Also, these fuel tanks are commonly damaged from improper lifting of the vehicle, which can result in quite a bit of lost fuel volume—even if the bottom of the tank is only dented up by one inch! Damaged tanks also have a tendency to rust on the outside and the inside. And if the dent (of the 1" deep variety or thereabouts) just happens to be right under the fuel level sender's float... well, that would explain why you keep running out of fuel when your gauge says you have 1/6th of a tank left! In other words, if you are getting what seems to be a ridiculously-short range out of your 2WD Vanagon tank and/or whacky fuel gauge readings, make sure to check for fuel tank damage!
All that said, even a perfectly-functioning Vanagon fuel tank/sending unit system is not as accurate as the typical modern car or truck that is carefully calibrated (and often tied into the computer to compute fuel range, miles/gallon, etc.). Your Vanagon's system is never going to be that sophisticated, no matter what you do!
Tip: Any time you purchase a vehicle or make a change to the fuel tank or sending unit, we recommend proceeding as follows:
Now you'll know 1) exactly how the fuel gauge reads when the tank is full, has two gallons left, and when you're SOL!