I think your basic error was mentioning that the speedo didn't work.
You now can't argue with whatever the cop decided your speed was.
In the circumstances a flat out denial may have served you better, not that I would ever suggest people tell lies. 
Sometimes the officers like to play little games, they will ask what speed you thought you were doing, any answer except the speed limit or below is the wrong answer.
This isn't theory, I have experienced this, so have friends.
A recent variation I encountered was, "I thought you were doing 120 here what would you say to that?"
The right answer was along the lines, "Nooooo, never!"
If I'd said, "I don't know, my speedo isn't working."
That'd likely have cost me more than just a new speedo...
There is a grey blur, and a green blur. I try to stay on the grey one. - Joey Dunlop
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