Consumer use vehicle fleet testing is the ultimate test of a fuel additive. This real-world evaluation of how different types of engine designs and various driving conditions react to gasoline additives provides very useful information. A carefully designed employee fleet test consisting of 26, currently in-use, cars from four different makes and five different models was conducted throughout a twelve month period. The vehicles were split into matched pairs to provide two roughly equivalent fleets. Each fleet had the opportunity to run each of the two additives that were being tested. The unleaded base gasoline used throughout the test was similar to a reformulated fuel with 11% by volume MTBE, low aromatics, low T90 and low sulfur and was designed to provide good driveability throughout the seasonal changes associated with an entire year of operation. A fiberoptic borescope was utilized to evaluate intake valve and combustion chamber deposit levels. The CRC merit rating scales were used to quantify the fuel related deposits.Both fuel additives resulted in a statistically significant reduction in average intake valve deposits, however only one of the two fuel additive packages tested showed a statistically significant decrease in combustion chamber deposit levels. Data was also collected to allow changes in deposit level to be correlated with mileage accumulated during the test phases. Additionally, emissions data was collected and is discussed in relation to changes in deposit levels.