Chas. is right. Because of this thread, I looked at my speedo this weekend for the first time in a long time (the Smiths speedo is in the middle of the dash). At 45MPH it was not holding perfectly steady, but was "wiggling" up and down about a half inch or so. I hardly ever look at it -- I've used the "tach" method forever. 7, 11, 17, 21, and 34 -- just do the quick math in your head. Cops tend to cut me more breaks in the Cobra than they do in my DD cars anyway.
