I have been running the 5.5, 6.5, 7.5 and the 8.5.
Burned two 6.5 in a DEX410, maybe because of undergearing and burned the 8.5 in a DEX210. I`m using the 7.5 in all of my cars now and its very impressive! Tried the 5.5 in my DEX210 on carpet and it was stupid fast. I think that will be my DEX410 motor when the outdoor season comes.