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Engine performance

Does a gasoline engine consume more gasoline when a car is coasting downhill in gear or out of gear.

with manual transmission:
It uses more gasoline to keep the engine running at idle, out of gear. When in gear, the drive train drives the engine, and the fuel is cut off (injectors).

Fuel is also cut off automatic transmission when decel above about 1500 rpm.
On long downhill at idle, fuel injectors are periodically activated by PCM to maintain cat temp.

Great information. Thanks for the help.