I think its best to think in terms of direct drive, in other words, whats the biggest roller a 3.5 hp engine will practically use. If its say 3" and you go 4 you will have to under drive it, so the only advantage is the efficiency of the larger slower turning roller. If you ran a 4" reduced 2:1 assuming an engine rpm of 5000 top speed is right at 30mph, not bad. Assume a clutch engagement of 2000 and its a pretty tall pull off as the roller speed is 11.5mph from stop. My direct bike with the 2.5" roller pull off speed is 9mph with a top speed of just under 40. Rollers are a trade off, takes some experimentation to find the most practical speed envelope for the riding desired. If top speed is the goal then a big roller, just dont plan to slowly ride down a gravel road say. This of course changes with an auto clutch, it will slip until the roller speed matches road speed. My belt driven bike runs a 2.25 roller, with that it has a decent top speed, around 32mph at 4800rpm. climbs hills well and pulls away very nicely with out pedaling. Thats at a 1:1 drive, I can still over or under drive it as necessary. Wayne has the hot lick with the 2speed, thats what you need with a bigger than direct drive roller.