We went for a drive to Ottawa on Sunday. Well, to IKEA. I filled the van with gas. When I pulled out of the gas station the DTE (Distence Til Empty) read 538 km. By the time we hit, oh about, Odessa, it read 561. It stayed there (562-563) until we got off the 401 onto the 416. Which was at least an hour and a half later. Say what? Now, I know Big Green ('94 Grand Caravan, 3.3L V6, 358,000km) is better on the highway than in town but, really? As soon as we started going up the 416 the DTE started dropping. As much as I would like to say that the van magically uses no gas on the 401, I can't back that up.
Sadly, this is what I believe happened. The DTE sensor reads how much gas is left in the tank and calculates based on current efficiency. It said that a full tank should take me 560km. Most (older) fuel sensors work on a ball system. The ball cannot go right to the bottom of the tank, nor the top. That's why you can still drive when the meter reads empty. The ball is down as far as it can go but there is still gas in the tank. It works the same at the top. The ball is up as far as it can go and you can still put more gas in. So, I was burning the gas that was "above the ball". And, as coincedence may have it, that gas was gone just as I got onto the 416. After that point the DTE went down as I had expected. I still drove Big Green to Ottawa (and around a bit, lost) and back to Napanee on about 2/3 of a tank. Roughly, 40L. Not that bad really.