Green’s Dictionary of Slang

(Aus.) a speedometer.

1995		P. Vautin Turn It Up! 109: ‘Look at the speedo Gibbsy, you’re my witness, 101 kilometres per hour’.
1998		R.G. Barrett Mud Crab Boogie (2013) [ebook] Les looked at the speedo. He waas sitting on 170 klms an hour.