Gear vendors OD

I have run the GV in my 68 Barracuda/ 4-speed, with almost every gear you can think of from 2.76 to 4.88. and 5.38s besides. My favorite ended up being 3.55s. My 367 gots torque.
I actually liked 3.23s better with my 4 speed, but she traps to 60 slightly better with 3.55s and splitting gears, and traps at 93mph in the Eighth, running four tight-split ratios...

Depending on your camshaft, sometimes the engine will not get any better fuel economy, or say not significantly better, in overdrive as it might with say 3.55s ..So if fuel economy is your goal, then you have to gear your car to be in the engine's sweetspot, not just slam the rpm into the basement.
And then there are the costs involved. Depending on your annual mileage, it can take 5 to 10 years to break even, and you'll probably have to get it rebuilt at least once during that time. which will add maybe 5 years to the break-even. So
where will you be in 10 to 15 years?
As to fuel-economy, perhaps just broke even.
So, IMO, if fuel economy is your goal, there are more cost effective ways to deal with that.

Yes, I did install a GVod, but not for fuel economy. I split gears with it, to keep my small-cammed engine on the power peak for more average power to the ground per unit of distance. This makes my combo quicker than it should be given the cam that is in it, I trade away gear for a reduced cruise-rpm.
Between the small cam, and the overdrive, that is where some of the economy comes from. Another place is cylinder pressure; my combo runs ~180psi., Another is cruise timing. With the factory timing controls, it is impossible to give your engine the cruise timing it wants,at 2000 to 2200say; and if you don't, the fuel economy will just not be there.

I can tell you this; with a HFT cam of 230*@.050,and geared for 65=2240rpm, I have long-ago stopped trying for big-number fuel economy, It actually gets better with more speed.... At 80=2760, it's doing all right.... but I can only cruise at that speed when I go stateside.
And you know what's funny? 2760 in direct with 3.55s is 62.6 Mph,which is the speed limit in Manitoba everywhere except on certain parts of the Trans-Canada Hiway,lol. I must admit tho, that cruising at 2240 rpm trumps 2760, for NVH; that is Noise,Vibration, and Harshness.

If I was able to start over, this is not the path I would choose. It's been a lot of fun, but we have IMO, better options today than I had in 2002 or 2003, whatever year it was that I installed the GV.

Whatever you decide;
Happy HotRodding.