# Drop in gas mileage when heat switch is in "windshield defrost mode

**URL:** <https://community.cartalk.com/t/drop-in-gas-mileage-when-heat-switch-is-in-windshield-defrost-mode/43681>\
**Category:** Maintenance/Repairs\
**Created:** [February 2, 2010, 3:25am UTC](https://community.cartalk.com/t/drop-in-gas-mileage-when-heat-switch-is-in-windshield-defrost-mode/43681 "2010-02-02T03:25:05Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![cardude](https://avatars.discourse-cdn.com/v4/letter/c/6a8cbe/32.png) [@cardude](https://community.cartalk.com/u/cardude)\
**Post date:** [February 2, 2010, 3:25am UTC](https://community.cartalk.com/t/drop-in-gas-mileage-when-heat-switch-is-in-windshield-defrost-mode/43681/1 "2010-02-02T03:25:05Z")

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Gas mileage drops dramatically when the heater control switch is in the defrost mode. I believe the compressor runs all the time in this selection. Why would mileage drop that considerably??

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**Author:** ![Caddyman](https://sea1.discourse-cdn.com/cartalk/user_avatar/community.cartalk.com/caddyman/32/23_2.png) [@Caddyman](https://community.cartalk.com/u/Caddyman)\
**Post date:** [February 2, 2010, 3:36am UTC](https://community.cartalk.com/t/drop-in-gas-mileage-when-heat-switch-is-in-windshield-defrost-mode/43681/2 "2010-02-02T03:36:21Z")

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A/C compressors take a considerable amount of power. On a 4-cylinder car, this will be very noticeable…