tach·o·graph 
(tăkə-grăf′)
(tăkə-grăf′)n.
A machine that records the measurements of a tachometer, especially one in a vehicle recording its speed and the times at which it was driven.
[Greek takhos, speed + -GRAPH.]
- on the money
- on the move
- on the nose
- on the off chance
- on the onehand
- on the order of
- on the other hand
- on the outs
- on the part of
- on the prowl
- on the qui vive
- on the rack
- on the rag
- on-the-record
- on the right foot
- on the road
- on the rocks
- on the ropes
- on the run
- on the same wavelength
- on-the-scene
- on the shelf
- on the side
- on the sly
- on the spectrum