accuracy

Thinking about buying one of the cheap tachometer/speedometers ebay but concerned on the accuracy of both readings. I know at lesat a couple of people here are running them, care to comment ?

I purchased the Carbtune several months ago and set it up as specified in the instructions. I sync'd the carbs and got the guages damn near perfect. Just to see how accurate it was, I switched the hoses around and the gauges were all over the place. Has anybody else noticed this? I made sure all...