I usually see spur gear ratios stated as best for a range between 1:1 and 6:1 (with efficiencies of 98-99%).  What happens as you go to higher numerical ratiosn, e.g. 9:1, do efficiencies go down (a little, a lot, enough to make the increased complexity of a 2-stage reduction a better choice)?  Where's the sweet spot assuming 20 degree pressure angles?

I don't have the AGMA standards in front of me but if I remember correctly they try to keep ratios below 5:1 for parallel shaft gearing.

With higher ratios you get pinions that are quite small and gears that are relatively large.  This could mean that your pinion ends up with significantly less strength rating than the gear.  A gear and pinion with balanced rating means you aren't "overdesigning" one of the elements of the gear mesh.

Also, Big Gear = More Expensive Gear.  Mesh efficiency does go down with an increase in ratio.  This I believe has to do with an increase in the Specific Sliding Ratio of high ratio gear meshes relative to lower ratio gear meshes.  There is always sliding action in a gear mesh but you want to keep it to a minimum since the sliding action creates wear and I believe also contributes to noise.

 

Usually the rule of thumb is 6:1. If you go above this, pinion gets so small, it might be hard to manufacture or will have little strength. So, try to keep below 6:1 and you will be ok. http://www.gearsandbearings.com

The problem is not the ratio.  The problem is the number of teeth on the pinion.  When you get low tooth counts, the gears tend to become more undercut and weaker.  Achieving contact ratios above 1.0 under all tolerance conditons may prove to be problematic.

My recomendation is to keep your pinion tooth counts high enough to avoid these types of problems.  The actual tooth count that you can get depends on the diametral pitch, pressure angle, mating gear number of teeth, loading, and quite frankly how good a designer you are.

Generally with 20 degree pressure angle systems, I recomend novice designers to start with a minimum of 16 teeth on the pinion and work your way down as your can develop your design.  In most cases I can accomodate 12 teeth in my designs.  You may get to 10 depending on the diametral pitch of the system.  If you need to go lower - consider helical gears to increase contact ratio.