My driving instructor taught a drill of
every few seconds flicking your eyes to the mirror , then to the speedo - after 20 years of driving I do it instinctively so i'm always aware how fast i'm travelling
I think ive only once had the "oooh crap am i really going that fast" moment , which was in my mate's shelby mustang when i booted it to overtake and glanced at the speedo to see that i was momentarily doing 90mph

after years of driving a tdci that much umph from the loud pedal was a novel experience
I'm not saying i never creep up to 75 on a motorway or dual carriageway but i'm always aware of how fast the car is going (and 75 indicated probably isnt much more than 70 actual anyway)
I can't see any way someone could get to 140mph without realising they were going far to fast unless they were completely oblivious in which case they'd be driving without due care and attention anyway