Lol, its not that important!!!
I have a schwinn speedo from walmart. I have not checked it with a car, so it may have been off a bit on 50, but I will GUARANTEE that I hit 40 on a flat. I ride with traffic on almost every ride on a busy road that has a speed limit of 35, and most people drive 40+. I take off faster than all of the cars that are driving normally, and I can take the lane because I match their legal speed with no problem ( I could use a bit more acceleration between 20 and 35, and long uphills aren't nice either)
FYI, I have a schwinn Ranger mountain bike, with a 68cc motor and a jackshaft shift kit.
Again, its not as important to me as it seems to be to you, I was just sharing. But if you want to feel what I feel, you go get yourself a schwinn ranger with a 68cc motor and a shift kit too!