Huey, you might be on to something. I did not think about it, but most of these tachs use photo cells that measure movement based on disruption of the light. I will try a solid colored background.

As far as using it in front of the prop, it should not make a difference. From a safety perspective though, I would always hold it behind the prop. Never know when something could turn loose and the plane lunges forward. Good comments though. I have a friend with a different tach than mine, and it is erratic +- 1500 rpm. Maybe these things are just not that accurate and are intended to give you a ball park reading.

Thanks guys.