Gearing change... rear end or transmission?

[Just Carbs]

I put 2.76's in my New Yorker and would not recommend it, unless you're just driving cross-country. I checked the fuel mileage a couple times back when I was driving it regularly and it dropped at least 2 mpg from around 11 to 8-or 9 mpg at best after the gear swap. You're just heating up the environment with the torque converter getting it up to highway speed with those gears. Might be a little better with the stick.

I had the same result going from a 3.23 to a 2.76. Lost 2 MPG suburban driving.
My shift points were too low for the 2.76 rear gear.
I moved them up to 25 and 40 light/part throttle. Completely different suburban driving experience after that. Now it takes great advantage of all that nice easy-going low rpm torque. Gets 14 suburban, don't know about full on stop and go light to light city, never seen it in this car.
MPG went from 14 at 70 with the 3.23 to 18 at 70 with the 2.79.
1964 New Yorker 10 to 1 compression 413. Too bad it only likes 93 octane.