Replace the 318 with a Slant

I had an 86 D100 w/6 it got about 15mpg when it was new,after about 60,000 miles the best it would get is 12mpg,I tuned that thing every way I could with no help.I put 180,000 miles on tha t truck before I traded it in on my 01 Quad Cab 1500 with 5.2 and it gets better mpg,16 consistantly.Keep the 318 just my .02