Will you get the same horse power
and torq reading with 3.55 rear gears
as you would get with 4.56 rear gears?

You think you would, but the 3.55 gears will read higher....

I'm basing this off motorcycle runs that I know of.

5th gear runs give higher results than 4th gear runs.

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

I have always wondered that myself.

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Official Cookie and Mater Tormentor.

That's with inertia dyno's that base the HP on how fast the mass of the rollers spin up. Faster accleration takes more power, so with the lower rear end, it spins up faster. (remember you're also spinning up the crank, transmission, rear end, tires, and the dyno doesn't know those masses, it only know it's own mass)

A brake dyno should read the same since it can hold rpms.

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes

Aren't you getting more HP at the wheels since it's "easier" to turn everything?

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"Nothing in all creation is hidden from God's sight. Everything is uncovered and laid bare before the eyes of Him to whom we must give account."

Yes there's torque multiplication at the wheels, but the dynos are looking at what the engine does. It compares the rpm of the roller with the engine rpm and divides the result for the torque. HP actually is a derived from TQ, so it stays the same....

Example.

Engine 100 HP & 100 TQ at 5252 rpm (I'm making it easy on myself, as 5252 is when HP = TQ)

Assume the rollers have the same diameter as the rear wheels (makes this easier as I don't need to take into consideration the tires)

1:1 rear end with 1:1 transmission output......... the roller turns same rpm as engine, and will register 100HP & 100TQ at 5252.

Change the rearend to 2:1, the roller now turns at half the engine speed, but you've multiplied the torque by 2, so at the roller you have 200 TQ at 2626 rpm, and guess what, you still have 100 HP (200 * (2626/5252)).

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Ed
64 Valiant 225 / 904 / 42:1 manual steering / 9" drum brakes