How many teeth?

The easiest way to get 100% accuracy is to figure the exact error in percentage, then transfer that error to whatever tooth count is in your trans.
In your case, a reading of double is hardly logical. If you look at the chart, the minimum tooth count is 21, and to slow your speed-o down to half would require a tooth count of 42 . While this scenario is possible, it is hardly likely.
So the most logical answer is that under your car, someone has installed a speed-O doubler. so go look. There should not be any adapter in the speed-o cable, anywhere in it's length. If there is. just take it out, and roadtest again. Those adapters come in at least two ratios that I have seen.