The EPA requires a formal statement that the test laboratory "shall have no financial interests in the outcome of these tests other than the fee charged for each test performed". The Federal Test Procedure comprises a simulated city drive trace, and a simulated highway drive test. The tests are conducted in triplicate using two different vehicles. The vehicles should be from two different manufacturers and should be examples of the largest selling engine/transmission combinations on the US market. Tests are conducted in a "back-to-back" fashion. In the case of fuel additives that don't require an engine retune, then triplicate tests with and without the additive are required.