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.
- All tests were performed on a hot summer day, with the car warmed up after driving around
- 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
- 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.
- 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