Which combination has a lower gear friction? I need roughly a 1:3 reduction and I see a couple of ways to achieve it:


using a 8t gear and a 24t gear (1:3)
using 2x 12t gears and 2x 20t gears (1:2.779)
using a 16t gear and a 40t gear (1:2.5)
using a 12t gear and a 36t gear (1:3)


Now, size issues aside, and assuming all axles are held by at least two beams, how do these combinations compare as far as wasting the least amount of power due to friction? What about torque? (where the 8t gear may have an issue) 

This is for a high-RPM, low torque situation, so I am not exactly super concerned about backlash or torque forces, just efficiency and friction.

I would be open to other gears as long as the reduction is in the same rough ballpark as the aforementioned, especially if it's more efficient somehow.
According to Sariel's gear tutorial in chapter 5, Efficiency:, 

- the less gears, the better
 - the smaller gears, the better

Based on this principle the first setup looks the best, but you are right that the 8 tooth gear is the weakest of all and it has the most backlash too, if any of these are valid concerns for your build.

Note that gears can be strengthened by using them in parallel by putting some 24 tooth on one axle and the same number of 8 tooth on the other, mating them together all at once.

If you are concerned by backlash, you can experiment with springs, rubber bands or other ways to put a constant load on the system.