I "built" an ebike 7-8 years ago. Got the cheapest bike walmart had with 29" wheels and disc brakes. Replaced the rear wheel with a motor, strapped the battery and control box, and it was off to the races. This was back when lithium was really high and SLA batteries made a lot of sense. The battery is heavy for what it is.Its time to replace the battery now. I can replace the entire 48v 9ah battery for $100. Or I could go with lithium for like $500.
Fastest I ever got, measured with GPS, is 38MPH, going downhill and pedaling and the motor wide open. It was so fast that I ran out of gears and couldnt pedal anymore. If it were geared differently, I could break 40MPH. Don't tell the cops, because here in GA, if they can go over 25MPh, its legally a motorcycle.
When I'm just riding around, I'll stop at red lights beside cars. And off the line, its faster than most cars. It launches real quick. But once it gets up around 30-35, it runs out of steam and the cars blow past. It goes through rear tires really quick from all the torque.
It really is good exercise. Because you can pedal as little or as much as you want. The difference is you can go further. You can actually get across town and run errands on a bicycle like this and it doesn't take you 4 hours. So in reality, I found myself riding the bike more with the motor than I ever would have without.