How is the mechanical efficiency of a machine usually expressed 1. As a fraction 2. As a percentage 3. As inches or feet 4. As one of three classes

The efficiency of any contraption that uses energy to

do a job is defined as

(amount of useful energy output) / (amount of energy input).

In describing or talking about the efficiency f the machine,

it's most often expressed as a percentage.

It's important to notice that the output is taken to be the useful output.

Why is that important?

Here's a simple example:

If you use an incandescent light bulb as a source of light, then

the useful output is the light, and its efficiency is quite low.

If you use an incandescent light bulb as a source of heat, then

the useful output is the heat, and its efficiency is quite high.