DivinylsFan I don't know exactly how speed is converted, but it seems like it might be a bit more than 1/10 of the tuning value, or maybe there are defined boundaries. Whatever the setup, the rating clearly reflects the approximate speed relative to other approximate speeds. The child bicycle's speed of 15 is converted to 2, and the adult bicycle's speed of 20 is converted to 3.
For the acceleration, it's just how quickly the vehicle gets up to speed... maybe how much speed increases per in-game minute? Try using Retuner to change a very fast car's acceleration to something small, like maybe this one which I think is the §105k car:
CarExpensive1: 100, 33
The full path:
Sims3.Gameplay.Objects.Vehicles > CarExpensive1 > kCarTuning > Acceleration > [value]
And see if you can observe how much slower the acceleration is. I've never tried this myself, but I would guess the value means something—if it were taken out of the game, the later vehicles wouldn't have had separate values here, either all the same or nothing at all if the value wasn't being read.