Alternator sizing question

[dontlifttoshift]

It's the idle output that matters and the 225 should do better there. You would have to work pretty hard to need 225a ever but needing more than 120 at idle is a possibility. Picture 850w fan, hot as balls outside, AC on full blast, etc. That said, the 170 is usually fine in most applications.

Yes, you can upsize the fuse but remember the fuse is there to protect the wire and it should be also upsized accordingly.