Aluminum Driveshaft Calculation

So just sitting around......

and looking into the force of rotational inertia of an aluminum driveshaft v.s the stock oem driveshaft. You know the ole saying of it not producing more hp's but the idea that it takes less hp's to spin the lighter one faster to the desired rpm, but basically WHAT IS THE NUMBER DIFFERENCE of the amount needed or used to rotate the heavier weight of the stock one?? So I found basic horsepower calculation and plugged in the numbers for a 5800rpm load. Believe I plugged them in correctly but here is the site where I found it so anyone can correct this if I did it wrong. Thought it was just cool to try and figure out a potential difference in the two rotational forces needed to spin each one.

http://generatorjoe.net/html/motorformula.asp

http://1985mustanggt.com/Weights.htm

so from the formula I got :

HP = ["radius" x "2(3.14)" x "rpm" x "lb."] ÷ [33,000]

Which got me...

stock = 3" diameter, 19.6lbs
aluminum = 3.5" diameter, 13.5lbs

64.90 hp's needed for stock shaft at 5800rpm
52.15 hp's needed for aluminum shaft at 5800rpm
for a difference of 12.75 hp less needed to spin 5800 rpm.

Which really just declares one thing, save a little weight and
the aluminum shaft really just revs a little faster

I think set ups with 3:55's and lower (could have a better result in difference) and could help give an illusion of a higher gear and reving through the power band faster. It would also help without having the final resting place in rpms so high in 5th gear on the highway, other than having the actual higher gear installed. Hmmmmmmm, I got 3:55 DD I just may look into this now .....lol

Oh crap I used the diameter instead of radius, but the numbers are really...

stock: 32.45
aluminum : 26.07

But yes they are very close anyways....