[moontower69]

I've been down this (lets say optimistic) road of "converting" a 64 or 65 GTO into a gas sipper that gets 20+ MPG. Save yourself the aggravation and don't believe the fantasy.

It won't get 20+ MPG.

Its a matter of physics, weight, drag coefficient and volumetric efficiency of these old motors such that NOTHING is gonna give you a 20 MPG set up. I've tried the 2.56 rear end and had the 700R4 with the lockup and the Q Jet "tuned" for high gas mileage, etc.

TRUTH = only marginal gains. Def not a 20+ MPG highway cruiser. At the end of the day, you're gonna improve mileage from a stock 7-10 city and 12-14 highway about 15-20% max.

Sure, with properly set up highway gear and/or overdrive tranny you'll be able to feather it and get 16-18 cruising at 60-65 on a flat midwestern highway. But that's it - at best.

It's a matter of physics (a 3600-3800 pound mass), bad aerodynamics/drag coefficient (by modern standards) and the fact that Pontiac 400 is just not gonna sip gas, no matter how low you get the revs on the highway. And even with the low rpm, then you have to open the throttle more to get it moving no matter how you have it jetted - it's just a unwinnable and ungainable equation.

JUST enjoy your car and don't worry about saving 20-50 bucks a month on gas if its a daily driver. If it's a weekend car, there's just no point IMHO. You wont reach your 20+ MGP fantasy goal and it would take a decade to recoup the investment in a Gearvendors overdrive, or 700R4 or rear end swap or new carb, etc..

Just my real world experience - I tried this with my 65 GTO (not attainable) and I daily drive my 74 Lemans.