I have seen two low-gear numbers bandied about, namely 2.54 and 2.74. I don't know which is correct.
But I do know that;
From 2.45 to 2.54 is an increase of ~3.7%..... which also just happens to be ~3.7% of a typical gear-swap in the 3.55 and lower sizes.... IMO, for a 360, not worth the time to swap it in, even if you have it.
and
From 2.45 to 2.74 is an increase of ~7.9% which is closer to ~7.9% of a sub 3.55 gear swap, and about double the gain of an advertised 2.54. It makes a 2.76 rear feel like a 2.98 ...... in first gear.
I have done this swap to both a slanty and a low-compression 318, and by the feel of it, I think the low gears in these transmissions are actually closer to the 2.74 ratio; which is what I was expecting and the reason for the swap..
For an engine, cranking 240 ftlbs@2000rpm, at the start-line;
the 2.54 is worth about 9ftlbs from the crank; whereas
the 2.74 is worth double that.
9ftlbs at 2000rpm is 3.4hp; for a 360, are you gonna feel that?
19ftlbs at 2000 is 7.2hp; That's something I can feel.
Like I said, IDK which number is gospel;
but I'm leaning hard towards the 2.74.
For me, having gone thru this, if it wasn't for the LU feature, I wouldn't bother hunting for a 2.54 low-gear. But I would hunt for a 2.74/LU.
That LU feature is worth;
1) ~3>5%less rpm at cruising speed. At 2600rpm this comes to 80>130 rpm. See note-1
2) ~4>5% fuel economy. for a car making 18mpg, this translates to about 1mpg.
note-1
Do not trust your tach . I have had cars that cruised at UNDER the calculated rpm. So when I tested the cruise rpm gain from cruising to accelerating, the tach would jump 400 to even 500 rpm! But the calculated slip was just 200rpm on the flash .
What I mean is this;
A very long time ago, I had an all-stock 71 Monaco, with 360 lo-stall/ 2.94s, that, by the math should have cruised at 65=2380rpm. On the tach, it showed ~2200rpm. When I stomped it, the Rpm flashed to ~2600. So you might conclude that the convertor was slipping, 400rpm. When in fact, from the calculated cruise rpm, while cruising, it was only slipping 180rpm but in the reverse direction! ; Not slipping but gaining!
How this is possible IDK, but after that, I watched for it with other combos, and often saw it.
So like I said, don't trust your tach. It may not be lying, but sometimes, you just gotta think about stuff. I run a manual trans now, so I don't think much about these kinds of things anymore.
But I just gotta say something;
If your 66 is a streeter, running street tires, inside the factory tubs; I sure wouldn't waste my time hunting for any non A904. But if one just fell into my lap, well ok then..... lol.
