My plane, a IO-360/CS equipped RV-8, had me questioning the accuracy of the tach. Before I did my panel upgrade, the old round dial, first generation, Westach electronic tach would show just below 2700 rom on takeoff. After installing the Dynon, it showed 2540 rpm max rpm. Ok, which is correct? I was going to have a friend bring his prop balancing equipment and get him to check it even though I HATE doing full power run ups on the ground. The thought occurred to me that perhaps there's an app that can do it. Well, there is! It's called Video Tachometer and it works perfectly. What I did was set the app to 2540 (max rpm observed on Dynon with prop control full forward) then took off and once at altitude, I pushed the prop control full forward. The Dynon showed 2540 rpm. I then held the iphone up to the windscreen and the prop was stationary on the iphone screen meaning the rpm set on the app exactly matched the rpm shown on the Dynon! Now, I know that the Dynon is correct and I avoided the full power ground run. Next up, governor adjustment!