87 B2000 gas mileage?

Hey guys...I know we have about a 14.5 gal. fuel tank...and new should be getting 25 MPG highway. Thats about 360 mi. a tank.

The thing is, when I bought it about 1 1/2 months ago I got about 24.4 MPG highway. I drive about 90% highway...about 70 mi. round trip a day. I was able to get 330 mi. out of a tank once. I have to fill up twice a week now and have noticed that my MPG is dropping.

I filled up just yesterday, put in 12 gal. to full and only had 240 miles on the tank. That is down from 24.4 to 20. The times before it had gone down to 22, then 21.5 and now 20.

Could this just be the winter blend of gas? I would like to change my fuel filter which I think would help as well as running a synthetic tranny fluid. I also want to remove my carburetor and give it a good cleaning. I sprayed some carb/intake cleaner on it and a brown residue wiped off the throttle plate. I definetly want a K&N filter to help since I think my filter is old. I also think I need to check my tires to make sure they are inflated properly. I already changed my oil with a synthetic blend 10W40.

If I do this do you think I could get back up to 24 to 25? Would a fuel injector cleaner in the tank help to clean the main jet in the carb or would it damage it? (I doubt it would hurt)

This just sucks because gas has just risen to $3.06 a gal here and its costing me $40 a trip to fill it.

Be sure your tires are properly inflated,be sure your plugs are ok and gapped properly (.032),be sure your fuel filter and air filter are ok and finally be sure ur pcv valve is ok(pull it out and shake it real good,if u hear no rattle,replace it).FI additive will not help ur carb,use a bottle of STP gas treatment instead. Hope this helps.BJ