Hi!
OK! You see we modelers don't usually set our engines by any gadgets like tachs...we simply use our ears to listen to how the engine sound and set the engine accordingly.
This is very easy when you know how. The human ear is a very reliable instrument if you know how to use it right.

You seem to think that the manufacturers recommended max rpm is what you should get (aim for) with every prop you use on your engine ...like if the OS LA. 46 you have got should be able to rew 16000rpm with every prop...Nothing could be further from the truth. An OS .LA .46 engine will only rew around 10000-12000rpm with a 11x6 APC ...or MA (Which is the worst prop you could use)...but with a 9x6 it will rew around 13500-15000 rpm. So which prop is the best then?...In your case definitely the 11x6.

What we seek when we choose a prop for an engine /airplane is to get as good performance as possible. Different props perform differently. But we never try to prop the engine to the manufacturers max teoretical rpm as this would be useless and require a much too small prop for most airplanes which would not give us any performance at all.

So just listen to what your ears tells you, and set the high speed needle accordingly!

Regards!
Jan K
Sweden