Well.....just for reference I had an 81 g-body in college with that engine. The rear gears were something in the 2.90 range and I kept it under 70. MPG was about 18 or so if I remeber correctly. Lots of variables though in these age of cars.
I put 3:42's in it and while it woke it up, it was way to tight at highway speeds with a turbo 350 transmission. Overdrive or a lock up convertor would been a great thing to have.
Put some 255-60-15's on it and took a few rpm out of it.
If you do nothing but city driving, 3:42's would be cool.
Finally got my 85 Bonnie inspected and on the road.
Ran a tank of gas threw before started keeping track of MPG , had to fix the trans etc.
Anyway , getting a hair under 17 mpg.
That is suburban, drive to work, driving, rarely sees over 50 mph. Did get the a/c working and used that 1/2 the time.
What are you other 3.8 V6 guys getting?
The last time I checked, Darwin's anything but stock 4.3 was getting 16-18mpg in mixed driving even with a leaky 4160 and 2.41 gears.
Her best mpg was recorded during my only major road trip back when she had 2.73 gears, a Comp 252H camshaft and a freshly rejetted Holley. I saw mileage in the low 20s running 65-70mph with the windows up and the AC running. My best mileage was 24mpg at 65mph