I aasume you are talking about a carbon fiber prop, correct? Never tried one but I heard they spin faster than a wood or nylon. It does supprise me because Horizon claims 8900 on an APC 16x8. An 18x6 would have a lot more load, even though the pitch is less. I may just have to pick up one of those props

I use a $130 laser tach accurate to 50rpm in 10,000.