Why the fuel reserve?
 

What is the logic behind the fuel reserve tank?

There isn't one.

So you don't get stranded when you run it down to empty, of course :)

There are times when you get stuck out in the middle of nowhere miles from the nearest filling station when the gauge hits empty.

Like getting stuck on the freeway between towns when the propane truck a few miles down the road ahead of you rolls over and catches fire and the fire crew stops all traffic well away while y'all wait for it to burn off, hopefully slowly, but possibly very, very fast (see "explode").

FYI, it takes ~5 hours for it to burn off slowly.

Why not a larger single tank?

I am pretty sure that's the point. It IS a larger single tank. They're just calling it a reserve for the psychological reason that if "empty" really meant "empty" and the car quit moving the moment the gauge bottomed out, some people would never refuel until too late. If you make people think they've already practically bottomed out but still have a "reserve" capacity to get them out of trouble, they'll watch for that moment when they feel like they're on borrowed fuel and start looking for a refill sooner.

"Reserve" just means "You've reached the point at which we think you should be filling up NOW if possible but we've decided to build in a little extra capacity just in case."

I usually just fill up when it gets down to a quarter tank.

There is no reserve tank. The reserve light illuminates when you're down to the last 2-3 gallons............giving you ample warning of impending doom.

I fill up when it gets down to half tank (usually every two weeks on payday).

I run mine approx. 525 miles on the trip odometer which gets reset at every fillup and then start desperately looking for stations.

My gauge hits R at 450 miles or so on the tank but I have yet to EVER see the little orange light glow when it's supposed to so I don't trust the reserve system.

Based on my miles per gallon figures (never lower than 23, never higher than 26.5, checked at every fill up) I should be safe to take it 550 or 600 miles every single time. But having the gauge on R for THAT long makes me too nervous so I fill sooner. Invariably the car takes between 19 and 22 gallons when i finally stop to fuel, indicating that I always quit 50 or 75 miles or more before I'd have to have. I do remember a few tanks that I have run 580 miles before fueling.

Why do I stop so early now? Because one time I hit the road side completely dry at 528 miles showing, which just shouldn't have happened even with diminished economy due to some random cause. All I can figure is that that one time I must have filled up on a slope going the wrong way and not gotten the tank topped off like I usually do, but it's made me paranoid enough to call 525 my "better get fuel" point.

If you are filling at 525 then you are running right to the last drop every time! :eek: I ran my car out on the trip home from buying it(faulty gauge still said nearly half!), and it only made it 514.4 miles. I've daringly gone as high as 465....and it took 20.3 gallons or so....real near empty. The tank only holds about 22-23 (maybe) on a SD/SDL.....maybe a bit more if you cram it. The gassers have a slightly larger 26-27 gallon tank.

My 300E lays down on "R" with only around 290-315 miles on the trip gauge.....and takes 14 gallons or so at that point. Meaning there's about 3-4 left since it can hold 18....

I'm 100% sure I remember running 592 miles on one tank I drove for work one day a few summers ago. I admit it was on fumes when I got there.

*going out to check owners manual, because I'm curious now and don't want to post misinformation.*

According to the book, I've got a 23.8 gallon tank.

So at 25 mpg, 23.8*25 = 595 mile tank range which is just barely possible with every possible optimistic assumption made, which explains that one tank I remember. I got desperately lucky, and just happened to have a station nearby as it started to hesitate.

If we reduce that assumption down to 23 mpg, which is on average .5 to 1.5 lower than I usually record, that still gives me a 547 mile range.

So maybe 525 is a pretty good estimate after all. I DO fill up sooner if I don't know what area I'm in. Most of where I drive, I know where the stations are and there's normally diesel within twenty miles regardless.

Reserve Tank
 

I am surprised no one mentioned carrying can in the trunk?
I keep a one gal in the '99 & 2 1/2 gal in the '83. Used once in between fillups and twice to refill filter replacement done on the road. Poor fuel in the last instance. Even in the summer months, with the jugs capped tightly, I get no fumes inside. Would never do this with gasoline.

Reserve light...and...
 

Man, you guys never sleep! (looking at the post times)

I'd get away from the computer for a couple of minutes (or hours) and fix the sending unit reserve light system......MUCH better than gettin' stranded and it's usually a easy, cost free fix due ti accumalated dirt inthe sending unit. Couple of threads on here how to do it.

The light and gauge work independent of each other. They are separate circuits. The light serves as a warning so you don't have to guess what the gauge is really reading.

Jaime
-who found out the light in his 79 300SD doesn't work, and had the engine stall right in front of the diesel pump a few weeks ago!