My 1955 TO-35 has the original " tractor-o- meter" ( or some name close to that) in the dash, but it doesn't function since the generator has been replaced by an alternator. I bought a small digital readout tachometer for under $30 that I attached to the dash . I use the same type tach on my riding mower, and it also shows engine hours. ( Tiny Tach is one name brand , but there are similar less expensive versions , on , for instance, Amazon.com) . ) If, however, your dash has an empty hole where the original tractor-o-meter was, the VDO 333-199 mentioned in the above post by K7LM would be a nice choice. There is also an alternator available ( last seen on eBay) that has a mechanical drive at the rear so that an original type cable operated tractor-o-meter can be used. That's getting a bit expensive, however. Or, you can buy a reproduction tractor-o-meter to put into the dash to look nice, but use the small digital tach for the actual rpm reading.