I forgot about the Tachs, I have an Ace Tachmaster that I added a 10K, a 15k and a 20k scale to the 5k and 25k scales that it was designed with along with a 2/3 blade switch. I hated the dinky slide switch it originally had. The self calibrating feature using a 60 cycle light bulb in the house still works and seems to be right on, but I never have checked this against something like yours. I am using a different prop and Tach, so this definitely is not a controlled experiment. I am in Pennsylvania at an altitude of around 1200 feet above sea level which can affect the "experiment" also. The thrust from these engines I am running would definitely pull an airplane, so who knows how accurate my Tach really is? It did give me an 8k reading with a 25cc Weedeater which surprised me when all I could get out of 21cc Weedeaters was 7k. It was accurate enough until I started this business of convering engines. What Brand of Tach are you using and was it designed for airplanes. I don't remember seeing a lot of Laser Tachs on the market.