Thats a good question. They both have the same bore and stroke and the same carb. I bought mine last month from Webraus.com. That is the only 55 he has listed on his site. Maybe Dave from that site or one of the other gurus can chime in. Actually CustomPC my P-5 has a larger bore carb than my Speed 50. I think, but not sure, that the Speed 61 and 50 both use the same carbs.