Just a wild guess for what it's worth, there are no .40 sized engines on this planet that will turn a 10X6 APC 25,500.

I would say your tach is malfunctioning, or you are running a 3 blade prop with your tach set on 2 blade.

Something is up since 17,000/.6666=25,500 seems like a coincidence....and then again there are no .40 sized engines on the planet that will turn a 10X6 3-blade 17, 000. So back to the malfunctioning tach.

I'd go ahead and run it knowing that if your prop is a 10X6 it won't over-rev...17000 is a pretty good speed for a 10X6 on a .40 if this is what it is really turning I'd be a little surprised.

It will be fine for a trainer...your transmitter should have a stick to control the throttle, that will be what you will need to learn to use.