Well let's see. If you're only putting in $10 worth at a time, you're only talking about 3-4 gals each time.
If most or all of your trips are under 10 miles, the engine is NEVER getting hot.
So you're running with the choke on all the time and mileage is going to suffer.
I'm not that familiar with that year Volvo, but if it's not a bad gauge and there are no visible leaks,
and the gauge reds let's say 1/2 tank when he gets to work and it's down to let's say 1/4 when he goes home, then
the only other thing I can think of is does your roommate have a locking door over his gas cap?
One that he has to pull a lever inside the car to open? if not I'd suggest your roommate buy a LOCKING gas cap.
Someone might be siphoning some gas while he's at work. :eek: