Research octane ratings do not adequately define the antiknock performance of gasolines in cars on the road; gasoline sensitivity, composition, and other fuel properties also have effects. Specific correlation equations can express the effects of the fuel properties, but a different equation is needed for each car and speed. With the aid of simple statistics, the different equations can be combined to yield the Customers' Antiknock Rating, or CAR. The CAR of a gasoline is uniquely related to the percentage of cars the gasoline will power without knocking; even in simplified form, CAR is much better than Research rating. The use of CAR improves comparisons of gasolines, quality control, evaluations of processes, and projections of future antiknock quality. Although the CAR is more complex than Research rating, it has many advantages; we are using it and hope the automotive and petroleum industries will consider adopting something like it in the future.