Both cylinder head deposits (CHD) and piston top deposits (PTD) can be accurately characterized using deposit thickness profiles. Fleet test data using 1994 Lumina vehicles demonstrates the utility of this technique for both the qualitative and quantitative comparison of test results. This particular vehicle is able to differentiate between the relative deposit forming tendencies of different fuel additives. Repeatability of tests measuring deposit formation are discussed. The relative magnitude of deposit profiles and the scatter in such profiles both have to be considered in the statistical evaluation of performance differences.