I've often heard that automatics use a certain amount or percentage of power on the way to the rear wheels. 904's use less than 727's, etc. Example: I've heard before that with automatics you can expect a 15-20% loss of crank horsepower that gives you the rwhp. Do manuals use as less, more, or the same? What are the percentages for both types of transmissions?

Not as much as mine when I miss 3rd.

I have always figured it out at about 10% for manuals, 15% for autos with that are built, and 20% for a non-built auto. At least they are what I think are close. Autos will always lose more though the whole driveline. That is why I am always so amazed at the numbers people throw out there for horsepower as opposed to how their cars seem to run. Every chevy 350 out there runs 500hp yet many are no faster than 12.8. We figured my Pontiac Trans Am running a 400 bored 30 over, 10.5-1, headers, Crane 284 cam, 750 carb and Holly Street dominator was making close to 425 engine(hp), sans 15% puts it at 360rwhp. She moves pretty quick in the high 12's so that shows you how the true numbers actually work out. I don't think it would work out any different for a chevy so obviously a non-NOS chevy isn't making much more.

Just swapped from a 518 4 spd overdrive auto to a NV3500 5 spd in my 87 Ram.

Hard to say drag times because they dates were separated by 2 years but I gained 3mph with the manual.

The biggest place I can note a difference is a hill I climb everyday on the way to work.

With the auto I had to switch (manually in my application) from OD and Lock Out to Drive to climb this hill. I could do it in 4rth but it took about half throttle.

With the manual the same hill can easily be dealt with by only adding 1/8" more throttle to climb the hill.

Mileage seems about the same and I'm not sure why. The only thing I can blame is the chrome Hurst shifter sticking out of my floor.

http://www.nhra.com/stats/stk_record.html

Here is a list of the current NHRA records for both manual and auto.