this page shows a variety of RPM gauges. The most common type is the 2:1 used on inline engines.

 

Deuta1a
Deuta1a
Deuta1b
Deuta1b
Deuta drive1a wwi aviation
Deuta drive1a
Morell1a
Morell1a
Phylax1a
Phylax1a
Phylax3a
Phylax3a
Phylax4a
Phylax4a
Phylax2a
Phylax2a
Phylax5a
Phylax5a
Field repaired Phylax1a
Field repaired Phylax1a
Phylax6a
Phylax6a
Pylax detail1a
Pylax detail1a