Idle Fuel Consumption | A/C On vs. A/C Off

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I was curious about how much gas was actually saved when idling my car with the A/C off, so I gathered some fuel efficiency numbers using the MPGuino I made for my 2002 Honda Civic. The MPGuino can display the instant Gallons Per Hour (GPH) consumed, and this measurement was used for these tests.

Testing Notes:

  • All tests were performed on a hot summer day, with the car warmed up after driving around
    • In colder temperatures, the gas consumption may be even more
  • The instant GPH was averaged over 10 seconds for each test
  • The gear shifter was either in Park or Drive
  • In all tests, the car was stationary, with the brakes on
  • The fan mode was either in Recycle (re-circulate) or Vent (pass-through)
  • The MPGuino may not be 100% calibrated for the exact GPH, but it is close so the relative difference between the tests shouldn’t change much
  • For reference, cruising flat at 55 MPH on the highway is about ~1.4 GPH, for my Civic.

Idle Fuel Consumption Analysis Notes:

  • The baseline for the bottom graph was the car idling with the A/C and fan off
  • I was surprised to find that the car A/C and fan is more efficient when in Park/Neutral vs Drive.
    • This is probably due to the torque converter being engaged when in Drive
  • While these numbers are for just the car idling, I suspect that the relative differences would also translate while driving.
  • Idling with the A/C on uses ~50% more fuel than with the A/C and fan off

4 thoughts on “Idle Fuel Consumption | A/C On vs. A/C Off”

  1. Thank you for the research and numbers, it does seem that AC consumes ~50% more fuel on idle, approximately .11 GPH, so at a rate of 1.4 GPH at 55 MPH on flat terrain, it would suggest that ~8% more GPH is used for the AC in that scenario.

  3. Thanks for this. I am currently sitting in the car on a hot day while my young child sleeps in the back seat. Was trying to calculate how much it will cost to sit here with AC on for a couple of hours. From your data and other stats I found about idling fuel consumption I think it will be about $5. Worth it for a bit of peace and a well rested child!

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