If 15/46 = 50+mph & 17/46 = 60+mph running the factory 2'000KV motor. Going up to the 21/46 with 200KV more. 50'400RPM up to 55'440RPM. I can see the potential for around 10ish-mph, running the stock tires. Because of the extra gear ratio from the tires ballooning. I have heard that the Posidon motors are pretty impressive and can handle impressive loads. I'm not really sure what to expect from the SRC Road Crusher tires. Do you have a picture of the tires? I will Google them, & am pretty curious about how much speed you can actually achieve !!!

Okay, they are 5.5"×3" so there's that possibility that you could break even on the 60MPH or pretty close to it. Because your loosing 1" of overall tire hight, and without the extra gear ratio from the ballooning. Sounds interesting though !!!