It could be a combination of things. The engine operating at too low of a speed my contribute. Unlike today's engines that are tuned to opearate efficiently at low rpm od speeds, the firebreather wasn't designed to operate at 1.7k.

I have a 3:23 rear that has collected around 400k miles in 2 birds. I don't have a tack, and I have never payed attention to mileage. It seemed that the best mileage was around 45 to 60. I don't know the exact sweet spot was, but over 65 and under 45, the gas poured through the car like it had a leak in the tank. Over 65 notwitstanding, it seemes that about a 30 mph seady cruising speed, was the real gas suck.

There could be other issues too, but someone might wish to see if they can find a formula to determine engine rpm @ 30 mph. 3:23 rear through a 350 slush box. Wouldn't it be interesting if the rpm was about 1.7k?