on the dash
The other definition is false.  On the dash does refer to the dashboard of a car, specifically the gauge panel.  The number refers to the highest speed on the speedometer.
The Boss - Rick Ross
I've got a Chevy with a 454 in the hood and it's got 125 on the dash. 
(Most classic muscle cars displayed a top speed of 125 on the speedometer.)

T.I. - Big Things Poppin'
Think I won’t pull up in something with 200 on the dash. 
(High-end supercars, particularly of Italian origin, commonly have the top speed of 200 on the speedometer.)