If you just need to test something then that seems a bit over the top as personally I see no need for a rev-counter unless perhaps you go racing. An alternative quick and cheap solution just to test something quickly, if you have a smart phone, there is an App called “RPM Guage” for and Android and I am sure there is probably an equivalent for Iphone.
This reminds me when I was younger, I used to ride smaller bikes, instead of having a rev counter they used to mark the speedometer with maximum revs for each gear. Might be another way to solve your problem, depending on what you need it for.
