Most of the Mopar gears are about 9% change between them, except 3.73s, which are 5% up from 3.55s, and 4.8% up to the 3.91s, so about a half a step. And so, it is the one gear that I have never actually run..
In the Mopar manual trans game, the gears are about 39% spread apart which equals about 4 rear gears
What I mean is this;
2.76 x 2.66 Low = 7.34 Roadgear, whereas
3.73 x 1.92 Second= 7.16
and from 2.76s to 3.73s is 35%, pretty close.
In the Mopar Auto trans game , the convertor acts as an infinitely variable ratio between about 1.1 and 1.8 or a lil more, making it act like; a deep low, fully independent gear. For example, at zero mph your 2.45 gear will act like a 4.40 low gear. However this TM(Torque-Multiplication) immediately begins to change as a function of two things, 1) the amount of torque entering the convertor on it's input side, and/or 2) the amount of resistance that the load is presenting on it's output side. So then, as soon as the car starts to move, the TM ratio is declining, whereas, if engine Torque is simultaneously increasing, then the Ratio is increasing. These two conspire together so that at the top of first gear the TM ratio may still be close to 1.3ish, making your A904 ratio act like a 3.19. and/or your 3.23s acting like 4.20s, one or the other, the engine doesn't care.
This gives you a way to understand what acceleration will feel like between any 3 to 4 pumpkins apart, and to estimate those in between.
What this also means, is that you can estimate how much more torque it will take to feel like you had the next bigger size rear gear.
Say you start out with a 318, an A904 auto, 2.76s and you are thinking of getting some 3.23's. Well from 2.76 to 3.23 represents a torque multiplication factor of plus 17%. So if you can get a torque increase of plus 17% from your engine, this will achieve the very same result without the cruise-rpm penalty.
To get such a torque increase is reasonable easy, if not always cheap. Your choices are;
1) 17% more cubes, or
2) 17% more engine torque
3) or a combination of the two.
#1 leads to 372 cubes,
#2 leads to a higher stall convertor; or more cylinder pressure. More cylinder pressure leads to a higher Scr requirement, an earlier-closing intake valve event, greater air thruput, or supercharging. The cheapest of all of these is a Higher stall convertor. But that is mostly only helpful in First gear, and/or the bottom of Second gear.
Of course, you can always install a lower first gear, into your A904 trans, or just pick up an A998/999. Mopar has available, a 2.74low which is almost 12% greater, leaving you with a shortfall of just 5% . and you can get that in one of the other ways, and again, the higher-stall is the way to go.
To recap:
in this case, swapping to a 2.74 low, and more stall, can easily get you that 17% low-gear performance increase, without the penalty of the higher cruise rpm. But, that same trans that carries the 2.74 low, also comes with a 1.54 Second gear which is 6.2% better than the 1.45 stocker. By adding a lil more stall than "normal", you can carry your 318 performance increase into Second gear as well. And if you pay attention, the A999 comes with a lock-up, that will reduce your cruise rpm, up to about 150 rpm.
And finally is;
the A500/A518 overdrive, which opens up a whole new exciting world for the 318. Especially for a streeter, that is usually limited to just two gears before getting into speeding territory. In this case, you can gear the car to rev out, at or a lil past, the power peak, at or near 60 mph.This works out to about 3.91s, depending on your combo, and the .69od will reduce the 3.91s to 2.70s, the best of all worlds for a streeter. and Because this combo is now running so much TM, it no longer needs the high-stall.
Now your problem becomes one of tuning, to give the engine the ignition advance that it will be begging for.
With 2.70s, 65 in loc-up becomes ~2200, and your combo may be asking for 56 degrees or more. That's gonna be a new challenge for you.
