I always wanted a method to measure RPMs on stuff that don't normally have a tach connected to them. I considered one of the old contact type analog units but never bought one. The other day I decided to act on this want so I looked them up at HF and saw they had them for $38. Being the cheapo I am at times I decided to look on ebay. As I normally do I put the item name in the search field then set the search parameters to US Only and sort by lowest price. Found one in California for $9.74 shipped so I bought it. Just arrived today and I tested it on the PTO shaft of my tractor and on the flywheel of a lawn tractor. Seems to be VERY accurate.
With the tractor tach reading just over the PTO set point I read 547.4 RPMs at the shaft.
On the lawn tractor it read 3266 RPMs.