From empty I fill the tank. It takes ten gallons I drive it until the little gas pump shows up, I fill it up, ten gallons. The manual says 15.9. So what's up?

 

Was the low fuel warning light (yellow) showing when you assumed it was empty?
The gauges themselves are not always accurate. It may also be a sticking float switch.

 

It's just a really big reserve. I can only guess Volvo did not want anyone running out of gas.

I have read one thread where the poster bent the tank float arm to read more accurately.

Since my daughter drives my 2001 I just leave it alone knowing she will run it until the low level light comes on. She once remarked that the car took 11 gallons to fill once. Uh huh, lucky you. Any other car and you would be walking.

 

Must have been the sticky float thingy, it was on empty today and I put 10 gallons in it and it only showed about 2/3 of a tank which would be about right for 10 gallons.

 

Hey difflock54, are you the same difflock I know from Benzword or is it peachparts?

 

No Robert I'm not that one.
I am from way down under, in New Zealand.
Have never had anything to do with Merc's or their forums.