WHY IS MY VOLVO USEING SO MUCH FULE ITS A S40 V40

Have you keep up with maintenance?
1) Air Filter
2) Tire Pressure Check
3) Oil change & Coolant change
4) Fuel System Cleaning (see below)

Run some Lucas Oil "Upper Cylinder Cleaner & Fuel Injector Treatment" in your gas tank. 3oz. treats 10 gallon. So if your Volvo has an 18 gal. gas tank, use about 5 to 6 oz. of that when you fill up next time.

This Lucas Oil brand is the best in this kind of cleaning... all other brands are "water down" versions... AdvanceAutoParts AutoZone, Wal-Mart all sell them for about $6 for a small bottle. Or $12 for a 32 oz. bottle which you can save and use for your other cars to improve fuel economy.

I do this kind of fuel system cleaning once a year.

I noticed earlier that you said your check engine light was on, did you ever figure out why?, or is it still on?  How many miles on it, did you get the timing correct after changing the timing belt, what error codes is it throwing and have you done a tune-up recently?