It's a BAD idea to EVER tell a cop you knew how fast you were going, that's instant ticket. And you could always argue that you had a faulty speedo when you go to court and show that you put in a "new" 140mph calibrated speedo, show the judge some pics of the old and new...and with a little luck, ticket gets thrown out
Never heard this from anyone in the know. I would not tell the cop I had a faulty speedo. I would tell them I was going slower than I was, but never that my speedo is broken.