The last time I drove a rental the speedo was bouncing around between 50 and 80 (I was driving a single constant speed) I already had my 'if I get pulled over' speech ready.

"Sir do you know how fast you were going?"
"Actually officer I have no idea, the speedo was telling me something between 50 and 80 even though the RPMs never changed."

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1982 300GD Carmine Red (DB3535) Cabriolet Parting Out
1990 300SEL Smoke Silver (Parting out)
1991 350SDL Blackberry Metallic (481)

"The thing is Bob, its not that I'm lazy...its that I just don't care."