I can push the bike on a level, tarmac surface, at less than walking speed. Add any incline and I'm leaning hard into it and inching the bike along. I was used to much lighter bikes (150-200lb), so I'm hitting the gym now, lol.
My daily ride is only 2.5mi each way, I average 20-25mph due to bends and ever changing pot-holes, gritty patches, leaves, lichen, dead things, branches, etc., except one spot I can get to 35mph before decelerating.
So short of driving in a low gear most of the way, I can't imagine it gets a decent charge. The bike does charge fine, I just don't provide it with the best of conditions. Once I hook up a tachometer and ammeter I'll have better information.
1986 Rebel 250 / N. California