'PEDAL TO THE METAL'
When the accelerator is pushed so far it cant move any more- in some cars it hits a metal bar, or the end of the footwell (which is hopefully made of metal).
Hence the term pedal to the metal. Obviously if you have the accelerator all the way forwards you're gonna be gathering speed...or going fairly fast.
Crap! Cops, 6 o'clock, pedal to the metal Jerry!