Cylinder head deposit (CHD) thicknesses and weights were measured for five vehicle types in 16,093 km (10,000 miles) and 24,140 km (15,000 miles) fleet tests and for the Ford Ranger in additional 8,046 km (5,000 miles) vehicle tests. Thickness measurements at 60 locations in each combustion chamber provided an accurate picture of the distribution of deposits. Deposit formation profiles measured by deposit thickness in the cylinder heads were reproducible for pairs of matched vehicles in different tests. Deposit profiles varied little for different test series even though selected test conditions were altered. Similarly, almost all tests showed good repeatability of deposit profiles for the four to six combustion chambers of a given engine. The lack of reproducibility for one vehicle type was related in part to the changing orientation of the combustion chambers along each of the cylinder heads. Extreme changes in dynamometer test cycle resulted in irreproducible deposit formation which is indicative of non-steady state conditions.For the Ranger engine at 8,046 km (5,000 miles) and 16,093 km (10,000 miles) there was a good correlation between CHD weight and thickness. Four of the five vehicles reached steady state in deposit formation by 16,093 km. Data for the Ford Ranger suggests that steady state in deposit thickness is attained by 8,046 km. Deposit formation profiles obtained in 200 hour dynamometer testing with two of the five engines showed CHD profiles essentially identical in nature to those obtained in the corresponding vehicle road tests. Deposit thickness profiles may provide an excellent measure of whether deposit formation has reached a steady state condition in a series of vehicle or dynamometer tests.