What kind of mileage are people getting with the first gen TBI 4.3's?

I seem to be consistantly getting only 13mpg, mix of city and highway and non agressive driving. Thats pretty poor, i believe the factory rating was 18 city 22 highway.

Tune-up was done before i bought it (ive only had it for approx. 800miles) Ive since replaced the O2 sensor (had a code), removed and cleaned the throttlebody, Chevron Techron injector cleaner in the tank twice and a top-end carbon cleaning. It also has a newer cat on it and high-flow muffler.

I was expecting to average around 18-20 when i bought it. As it stands now it gets worse gas mileage than my 96 Caprice (with stall converter,gears etc.) and slightly better than my 472 1969 Cadillac. Is 13mpg what these normally get or did i just buy a dud?

Oh and its an 88 2 door 4.3 TBI 4x4 Auto

 

Take a look through this thread:
Official MPG Poll
The EPA rates your vehicle at 15/20 mpg.

 

Yeah, my car has never gotten what it is estimated to get even after a rebuilt motor and transmission and it has had a complete tune up.

 

I get 20-21 MPG in my 89 4.3 2WD. Michilin X Radials, Mobile 1 Oil, K&N Air Filter, Injector Spacer. Factory Exhaust.