greasy is almost always pronounced as [gree-zee] /ˈgri zi/ (Show IPA)
with a medial [z] /z/
in the South Midland and Southern U.S. and as [gree-see] /ˈgri si/
with a medial [s] /s/
in New England, New York State, and the Great Lakes Basin. Speakers of New Jersey and eastern Pennsylvania are divided, with some using [s] /s/
and some using [z] /z/ .
Standard British English reflects both [z] /z/
and [s] /s/
pronunciations and British folk speech is also divided regionally, with [z] /z/
heard in the eastern counties and [s] /s/
in the central and western ones. Both pronunciations were brought to the colonies, where the present U.S. pattern emerged.