Increasing public acceptance of power accessories and particulary of factory installed air conditioning units coupled with bigger engines and lower hood lines on the newer automobiles necessitated a reappraisal of the hot fuel handling characteristics of automobiles. Twenty-nine 1956 automobiles, of which nineteen were equipped with factory air conditioning units, representing twelve makes were evaluated for fuel volatility requirements.In 1956 approximately four per cent of all new cars were air conditioned. By 1962 it is expected that about 25 per cent of all new cars will be air conditioned and that one car of every nine on the road will be air conditioned. These figures illustrate the rapid trend to nationwide acceptance of air conditioning in cars of all price ranges rather than the past localized acceptance in high-priced cars.The data obtained in the tests showed that the volatility limits of the cars were reduced on an average of 0.6 lb. RVP upon installation of an air conditioning unit and lowered still another 1.3 lb. RVP on the average when the units were operated. It was further noted that cars with 4-barrel carburetors had a volatility advantage of about 5 lb. RVP over the same cars with 2-barrel carburetors. Comparisons of these data with those obtained in 1955 show that the overall effect of air conditioning is the same 1.9 lb. RVP, and that the 1956 cars are somewhat more critical than their 1955 counterparts. The increasing use of 4-barrel carburetors tends to offset this trend to some extent.An opinion survey of the owners and drivers of the cars tested indicated that the car drivers are surprisingly sensitive to minor engine manfunctions but are reluctant to have them corrected, and that the average driver is unable to distinguish between true vapor lock and mechanical difficulties. Where true vapor lock was known to exist there was a 90 per cent correlation between the survey and the test results.This test illustrates the overall trend toward increasing vapor lock tendencies of modern automobiles. That the know-how exists to combat this trend is illustrated by the fact that each manufacturer has at least one car that offers excellent vapor lock protection. It is to be hoped that the improvements in various individual cars can be transferred to the companion makes in each given company.