but I'm wondering how much I'm leaving on the table for low end torque. The rpm's at 65 in lock-up run at 1900. I'm thinking there's plenty of room for deeper gears but need some help finding what might be optimum for my application.
Any help is soo appreciated. I love it here and you guys are my go to when I can't find it in a search.
Every bigger rear end will leave something on the table for the next bigger size until you run out of sizes.
The thing is; sooner or later, you will get more torque multiplication than your chassis can handle, and it all goes to tirespin; OR you can't use all of your throttle.
I have run a bunch of different rear gears searching for some happy medium. I found that for my combo; 367/4-speed, the perfect rear gear is different under different circumstances.
For instance;
>a 4.88 is great for straight-line acceleration; banging thru the gears keeps the engine in the powerzone.
>But around town, starting off with 4.88s, I am shifting into second before even finished crossing the road! And all the time; shifting,shifting,shifting so busy shifting.
>When sliding around corners, the engine needs to be in a sweetspot on the power curve for three reasons; 1) so I don't have to shift mid-corner, and 2) so I can steer the car with throttle, and 3) look good doing it. So what I discovered was that 3.55s are a really sweet gear for my combo, and driving style.
>In my combo, the starter gear is 3.55 x3.09=10.97 and no help from any TC. An automatic could use less starter, and in fact using a manual trans, the engine is married to the road, and so there are many many times that the engine might be down below 2000rpm. Whereas an auto with a 2800 only needs a lil pedal and pow the rpm climbs up to the stall rpm. Whereas the manual will have to downshift.
Are 3.55s right for you?
IDK your driving style. So I can't say.
I'm thinking there's plenty of room for deeper gears
With the overdrive, yur right, there is plenty of room. About 500 rpm from 1900 is still a good place to be. So that offers you plus 26% to work with ,which is a full gearsize in the transmission, or at the back; from 3.55s to 4.48s.
The thing is once moving at higher than stall speed, all you have accomplished is trading away one top gear to get one super-low race-type gear.
Lemme show you this relationship with numbers. First I'll list your current roadgears, then I'll try to match the new gears to 3.55s without overdrive gears;
your trans ratios are 2.45-1.45-1.00-.69od. With 3.55s this will produce Road-gears of;
.........8.70-5.15
-3.55-2.45,.......then with 5.13s;
12.57-7.44-5.13-
3.54........
Now look at the numbers;
3.55 and 3.54 are close enough to be considered to be the same. So in this example I traded away the 2.45 overdrive, to get the extra 12.57 first gear. In the process the cruse became 65=2890
From experience, I can tell you that 12.57 is a terrible street gear. Yeah ok, it'll be fun for about an hour. After that not so much because now, when yur in Second gear;
look at that 7.44 second gear versus your current 8.70 first gear. That's about a 15% drop in TM (Torque Multiplication) . That doesn't sound like much, but it is in fact a rear gear difference equivalent of going from 3.55s to 3.04s..
So second gear, the gear that you will be spending MOST of your time in as a streeter, with this 5.13 combo, will feel like it has a 3.04 rear gear.
Ok so forget 5.13s, lol
>>Lets consider something that will get you ~65= 2300. That would be 4.10s, for 65=2310 in LU.. lets compare roadgears again; first the current ones; In first gear at WOT; (the (*) is at 10% slip.)
.....8.70-5.15-3.55-2.45, and 30 to 40 window is 3610* to 4810*, Next;
10.05-5.95-4.10-2.83, with 4.10s and the window is 4170* to 5555*
So look at the rpms in the 30 to 40mph window; which is right for your engine?
So examine your driving style, and see where you want the power to be different. Maybe you can fine-tune it a lil. Is it worth the gear change? IDK. I've run nearly every ratio from 2.76s to 5.38s; and I keep coming back to 3.55s. For me, it's the right gear from 30 to 40 mph, and works with my driving style.
Only you know your driving style. Only you can choose the rear gear swap; if any. You already said that you were happy with the current performance........ If your current combo already has at least some First-gear tirespin, more first gear TM will only get you more tirespin.
EDIT
I should add that; your engine does not care where the TM comes from, whether from the trans or the rear gear; it only cares about the roadgear.
So whether you have a 4.0 in the back and a 3.0 in the trans, to the engine, it is the same as a 3.0 in the back and a 4.0 in the trans; the roadgear is 12.0 in either case. 
