The kit accelerometer returns a voltage proportional to acceleration.
It's described on page 3 of the
2008 Sensors Manual
There is also a link there to the
Product Webpage.
Bottom line: 2.5V output = no acceleration and it deviates from that at roughly 1000 mv/g.
I would not recommend using accelerometer if all you want is speed. Shaft encoders + the yaw-rate sensors will probably find you more success for autonomous navigation.
There are a large number of other threads on this exact topic. The search function is wonderful
