[Half-Inch Stud]

Yep. Long ago our 70 400 GTO 3-speed stickwith 2.54:1 Type "O" rear would pull >20 MPG Highway. crazy good MPG from 16 heads and 068 cam.

Efforts to beat that all failed, except my former 455 (48 heads) & big 252 @.050 cam, Th400 into 3.31:1 rear would shoe excelleng highway MPG yet terrible backroad MPG.

I finally figured on OVERLAP EGR as the great equalizer. So my 68 GTO (455, 9.0:1) 236/244cam, Th400 into 3.54:1 rear do both backroad & highway MPH terribly.

And that is why the OVERDRIVE trans is in process.

[goatwgn]

With the Gear Vendors, TH400, Hughes GM25 convertor, 3.42 rear,27.4 inch tall tires- on my '73 455 with 041 Melling cam, I consistently get 18 mpg on the highway. I did actually get 21 one time, but that was when I had an 066 style cam in it. Haven't checked it around town, but it is mediocre, because I keep my foot planted in the "secondaries" of the Q jet from time to time. This winter, I plan on stepping back a little on the cam, as well as installing a stock convertor, I figure my mileage will go up on the highway and around town at that point. A good convertor can affect gas mileage more than a lot of people may realize as well. Nice to hear of a Pontiac 400 with factory gears and tranny doing over 20mpg.

[dataway]

68 GTO, original 400 with 100k miles, Holley 650 spread bore, 2.93 rear, TH400, AC removed, windows down, at a steady 65-70 mph it would do 18 mpg on the highway. Around town ... 12mpg, driving like an 18 year old back in 1977.
JohnnyB