I had 3.23's with a 340 4-speed and 26" tires and it got 15 mpg at 70 mph on the freeway, which is where I drive it most of the time. I pulled them and put in a poor-man's overdrive - 2.76's with 28" tires and now it gets 19-20 mpg. Big improvement and I can actually talk to my family too. Only problem is starting off the line is pathetic and hard on the clutch. I think 2.94's are probably best for mostly freeway driving with a 4-speed.
I am running 3.23's in my B-body with a GM auto-overdrive and it is heavenly.
7 1/4 sure grip 2.94's currently, 13 inch pizza cutters. Ill see if i like the 2.94's in my 8.25 sure grip to be swapped in with 225/60/15's...if not, 3.23 swap. good enough gear to keep the rpms in a manageable range cruizing (i have a lockup 904) but still "tall" enough to get crazy with.