Atilla the Fun

7.625" = 3165 ft-lbs. Dana 44 = 4400 ft-lbs. Dana 60 = 6000 ft-lbs. Dana 70 gears in a Strange S60 housing ( Jana 76 ) = 8000 ft-lbs. Ford 8.8" 28-spline = 4600 ft-lbs. same but 31-spline = 5100 ft-lbs. Dana 50 gears in a Dana 44 housing ( Jana 54 ) = 5000 ft-lbs. Easy to figure what you need if using manual trans. For automatic, multiply by 1.6:1 for stock torque converters, no more than 2.55:1 for high-dollar converters. There is no such thing as a 3:1 converter, Yank's old advertising to the contrary is false. Don't include frictional losses of more than 5% for manual or 10% for automatic, since most of the drivetrain losses are after the pinion gear's input shaft, which is what's really at heart of the above ratings, though the above factors in how well both the ring gear and pinion gear are supported. Do include the torque losses due to thinner air at high elevation. Roughly 5% per 1500' elevation will be close enough. You can find your elevation by searching your city in Wikipedia. Yes, elevation / thin air reduces drag, so calculated power loss based on dragstrip MPH shows less loss, but there's no aero drag at launch, so use the corrections I just gave you. Note nothing for the ford 9", really no longer practical considering all the variations now possible.

AlkyIROC

Trouble is, you can't compare all axles as the same. A factory 28 spline axle is not as strong as an aftermarket replacement axle. A 28 spline Chev axle doesn't have the same strength as a 28 spline ford axle.

I use Moser axles in my car and using Moser's calculations.

Engine torque x First gear ratio x Diff ration x 0.9 (efficiency) = Maximum torque the engine can put to the ground providing you can get perfect traction.
Torque converter multiplication isn't even taken into account. Engine torque would be at sea level or an engine dyno. As mentioned above, the higher in elevation, the less power you make.

Moser axle rates per axle so there's a 200% safety margin

30 spline = 6200 pound
31 spline = 7000
33 spline = 8200
35 spline = 9600
40 spline = 12,000

So if your engine make 500 foot pounds of torque, you use a 700R4 and have 3.73 gears

500 x 3.06 x 3.73 x 0.9 = 5136

Since they don't have a rating below 30 spline axles, chances are 28 spline would be a bare minimum and a factory axle is probably below minimum. For maximum performance, you only need enough axle so that one axle is capable of withstanding the total torque put to it giving it a 200% safety factor.

My car only uses 31 spline axles because it's all I need
800(estimated) x 1.76 x 4.86 x 0.9 = 6158 which is still below the 7000 pound capacity of the 31 spline axles. Even if my engine was making 900 pounds of torque, the 31 spline axles would still be just enough at 6928. If I was using a 3 speed transmission then I would need 35 spline axles.

If I used a 2.0 multiplier for the torque converter, I should need 40 spline axles which I don't.

When I bought my axles, I painted a line down each length. I've pulled the axles every few years just to inspect them. The line is still straight and the splines are still straight even with all my wheels up launches. There's always a possibility I could break one but it's highly unlikely.

Atilla the Fun

We're not on the same page. Look, these ratings are the design specs, and even changing brands or materials ( or both ) of rings and pinions doesn't affect the numbers I posted. Nor does choice of axle ratio, or type of differential. If you buy 4340 shafts that double the shaft capacity, that doesn't change what I posted. It just means you can use a bigger-number gear ratio before risking the shafts. What I posted is considering the pinion size, pinion length, pinion offset, and a few other such factors that you can't change by buying aftermarket upgrades. To get more strength in this aspect, you must upgrade the whole deal. The only thing you can do to help these numbers is adding a load bolt, to reduce ring-gear deflection. And you might be able to find all you need to load-bolt a D60, but none of these others. Again, for clarity, putting 99-spline 4340 shafts in a 7.625" ( were it possible ) will never do a thing for the input torque capacity