[Donut]

Probably you want an encoder. Accelerometers can be used, but tend to have more error than encoders (both because use must integrate acceleration from it to get velocity, and because accelerometers tend to get a decent amount of noise in their output).

Many teams use the Greyhill encoders that can be obtained from Digikey. Another available for encoders this year is to buy the encoders designed to hook right up to the Banebots kit transmissions (available at the Banebots website).