TodFitch said, "Turns out you can get car performance apps for smart phones. At least you can for Android based ones and I expect you can for iPhones, etc."
I had to check it out and I downloaded a free app for my Droid X2 that is really neat. I tried it out in my OT truck that I know is off by a couple mph on the speedometer and the app confirmed it. I will now be able to remark my existing speedometer in the 41 without making multiple runs past one of those roadside radar speed signs as I had planned. Thanks for the heads up Tod!