Electronic Control Modules (ECM) on Heavy Duty truck engines have the ability to control or monitor important engine and vehicle parameters. Examples of these parameters are fuel consumption, cruise control operation and oil pressure. Under certain conditions, many can also record parameters that are useful in accident reconstruction. Cummins engines are commonly found on highway tractors on the road today. In a heavy vehicle equipped with a Cummins engine, a hard brake application or a collision with an object can create a Sudden Deceleration Data (SDD) Report on the ECM. Since this data reports second-by-second speed information, it can be very useful in the analysis of an accident. Establishing the reliability of this information is important, if it is to be used by an accident investigator. This paper explores the data recorded on a Cummins engine ECM created during a series of acceleration, cruising, and braking tests.