Regarding the fuel tank, check this out

The S40 Fuel tank is something that's very odd on this car. While Volvo claims it is a 16 gallon tank if you own a 2001 S40 you'll notice that when you fill up it's only about 12.5 gallons. No Volvo didn't install the wrong tank in your car, while Carlos was convinced this was what happened to him, after consulting with Don at Volvoparts.com we made an interesting discovery. Due to the way the tank is shaped and the type of sensor it uses to measure the fuel, there is only about a 12 gallon capacity, or is there? Well, all in all, the way we look at it is, if you only fill up with 12 odd gallons, that must be all your using. Check out the pictures below to see the goofy shaped tank.

There's more at http://www.classiccats.cc/s40fueltank.html