My manual says that my fuel tank is 21.1 gallons, and I'm trying to get an accurate gauge on my mpg for my 300D.

That said, a good friend who owns his own shop a few hours from me, checked the car out when I first bought it. He told me several times to never let the car run out of fuel, b/c it's a bear to get started again.

So, I'm hesistant to let it run TOO empty before I fill up. When my gauge light comes on, I fill up immediately. However, the last few times I filled up, I'm only putting in 17/18 gallons or so.

Does that mean the warning light comes on with 3 gallons remaining? That seems weird...and frustrating, since I can't exactly tell how much fuel I've used between fill ups. I get about 425-450 miles before I get nervous and fill it up.

Or, I could just be terrible with math and not know what the heck I'm doing.

__________________
1984 300D: 275,000 miles (soon to be for sale)

In search of my 1987 300D in anything but white.

21.1 including a reserve of 3 gallons

For best accuracy, fill the tank the same way each time.

I don't throttle the nozzle on the pump at all - insert nozzle, squeeze lever fully, engage lock. Let the fuel flow until the nozzle clicks off. Wait a few seconds, then squeeze nozzle again until it clicks once more - that's all.

However, I'm driving a gasser, and here in Houston, we have vapor recovery on the gas pumps that caution against overfilling.

I don't think the same system is in place with diesel - why not just fill it completely full all the time. I mean right up to the neck on the filler?

If you fill it the same every time - what you use between fillups is what you put in the tank.

It's a good habit to never let a car go below 1/4 tank

You don't have to worry about crap building in the bottom 1/4 with these cars, since the fuel pickup is on the bottom of the tank

__________________
$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment

soon as my car hits R I fill her up.