I thought this was going to be about my aunt who got out of a few tickets by claiming she didn't know the difference between the tachometer and speedometer. Evidently, it worked for her in a late 60's red GTO to state:
"But officer, I thought I was only doing 45, 50 max, that's what my speedometer was showing!"
I'm sure she knew she was doing one hell of a lot more than 50!