Question:

I have a light tunnel with a slightly modified 50 cu in stinger. I am running a 11 3/4" X 23" cleaver 3 blade. I am only getting 72 mph gps at 7200 rpm's..this calculates out to roughly 15% slip. I am surprised the slip is that high. Is my issue the small diameter I have? A good 12-12 1/2 X 23 is hard to come by it would seem. Any thoughts??? Thanks.