Slant Six mpg's?

I get 15-22 mpg around the city and 25-27 mpg highway. 5W-30 castrol motor oil and a 2.76 rear end with gear problems and bad bearings up the ying-yang (gonna get an 8-1/4" soon anyways), oh yeah, the tranny is getting tired too...

177K miles or maybe 277K miles hell it could even be 377K miles, some 40K miles on the engine. Original transmission and rear though...