That doesn't sound very good. I've been averaging around 20 mpg with my 88 with 32's and stock gearing. I get 23 on the highway. I just filled up yesterday and it was 225 miles on 11.63 gallons and I was still a little above 1/4 on the gauge. Sounds like you have an issue somewhere that is messing up your engine's efficiency. I have the Jardine 2-1/4" cat-back that is mandrel bent. Are you still running the stock cat to muffler pipe? That pipe necks down to under 1-3/4" at the bend. If you still have it, get a 2 or 2-1/4" pipe put on.