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I have a 51 3100 with 342-B15 rear end gears. Where do I find out when GM started using them, and in what trucks? Every time I search I am offered spare parts.

Is this in an original drive-train/rear-end?

PO put in Saginaw 4 speed and 342 rear. We put in T5, seems sluggish--Bill

If you're going to run the T5 you'll need to go to a lower geared rear end. The current setup is lugging the engine down below its best RPM range for good torque production. "There's no free lunch" where final drive ratios are concerned. Somewhere in the 3.70 range on the rear axle should be about right, depending on the OD ratio of the transmission.
Jerry

Just curious. What was wrong with the Saginaw 4 speed?

Have a 49 with a T5 we put in last summer--purrs...took the Saginaw out of the 51 and sold it. like the 5th gear

What are the ratios of 1st through 5th gear in your T-5?
In which gear does it seem sluggish? If it is when you are in 5th gear, don't shift into 5th until you reach 65-70 MPH.

Carl--1st is 4.03, 2nd is 2.37, 3 is 1.49, 4 1.00, and 5th is .76--think We need a 373 rear end--Bill

We do this on a regular basis and would need to know:

What donor vehicle furnished the rear end?

10 bolt, 12 bolt, Ford 8"/9", other?

What engine is in the vehicle?

Ring gear and pinion sets are not terribly expensive and save time and expense over a rear end swap.
Final drive (.76 5th gear) ratios:
3.42----2.60
3.73----2.83
3.90----2.96
4.11----3.12
For a 6 cylinder GM used a 4.11 or a 3.90 rear gear with the .67 BW OD trans. This gives good lugging power for loaded starts yet low rpm cruise speed in OD.
If a high torque output V8 then the 3.73 is fine. If a big engine and just a never loaded cruiser then one can go down into the mid two's for a final drive.

PO put in GM342-B15 12 bolt rear end, do not know from where it came. 1954 235 with Langdons 32/36 carb, rebuilt 848 head and headers. May go the ring and pinion route. Thanks Bill

Bill, it looks to me like everything should feel more "zippy" except for 5th gear. That is the one gear that stands out as the problem to me. Here is why I say that. Your original (stock) 3 speed final gear ratios were:
1st gear = 2.94 x 4.11 = 12.08
2nd gear = 1.68 x 4.11 = 6.90
3rd gear = 1 x 4.11 = 4.11

Since we don't know which 4 speed you had (ratio-wise), we'll ignore it, but we do know your final gear was 1:1, so your final overall ratio was 3.42 (apx 15% of an improvement from 4.11).

We do know the T5 ratios, so let's see how those compare...
1st gear = 4.03 x 3.42 = 13.78
2nd = 2.37 x 3.42 = 8.10
3rd = 1.49 x 3.42 = 5.09
4th = 1.00 x 3.42 = 3.42
5th = .76 x 3.42 = 2.59

So in comparison to your original (OEM or stock) 3 speed setup, your first through third gears are higher than I would say are ideal, but they'll make it seem like you have more power than you really do...meaning your engine will be spinning faster to achieve the same mph. You will need to shift out of first around 13~15 mph, shift to third around 24~25 mph, shift to 4th around 40 mph...and here we need to stop and think. It would be nice if your "new" 3rd gear matched your old one more closely, yes? That way your first 3 speeds would feel almost exactly like the stock 3 speed. But now the shift from 4th to 5th (with your current 5th gear ratio) is going to have you driving at 60 mph at about 1800 rpms (assuming you have a 28 inch tire) and that is apx 200~300 rpm slower than I think you need to be and will make that last gear feel sluggish.

My suggestion would be to find a .86 overdrive gear for the T5, and use it if you want to keep the T5 ratios as they are.

Or...if you want a setup which will mimic the standard gear ratio and give you two nice "overdrive" gears, swap your rear end for a 3.08 ratio (as I recall the deck height will allow you to do this with the 12 bolt you have) and swap the 5th gear for a .86 overdrive. Let's see what happens to your overall gear ratios if you did that:
1st would be 4.03 x 3.08 = 12.41
2nd would be 2.37 x 3.08 = 7.30
3rd would be 1.49 x 3.08 = 4.59
4th would be 1.00 x 3.08 = 3.08
and 5th would be .86 x 3.08 = 2.65
Now if you compare that to the stock gear ratios the GM engineers wanted that truck/engine to have, you'll see gears 1 through 3 are almost exactly the right spot. So your truck is going to feel the same and it will work just like a pickup should...especially in the hills of Taos. And when you get to 4th gear, you'll enjoy a 25% reduction in rpm...just slightly less than a typical overdrive 3 speed transmission which works fine in a pickup. Finally when you get to nice flat road (let's say driving to Roswell), fifth gear will be very nice because in fifth at 60 mph you'll be turning about 2000~2100 rpms (depending on your tire size...I'm thinking 15 inch wheels and something like 205/75R tires). And that in terms of the torque curve is in the lower portion but will work (unless you're in a bunch of wind). Good luck!

3.42 gears came in GM 10 bolt front axles, I put 3.42 gears in my 14 bolt SF rear to match front.

Ed