MPG Hit -- Top Down vs Air Conditioning

[Prospero]

Every Car takes a MPG hit for running the air conditioning.

Similarly, it's an article of faith that convertible is less aerodynamic (i.e., more wind resistance) with the top down than with the top up (hence worse gas mileage).

In fact, it's less aerodynamic even if you run it with the windows open (learned this on a Prius forum, where MPG is the Holy Grail).

So ......

Does anyone have data (or apocryphal stories, even) as to the hit we take on MPG:

1. By running the air conditioner.
2. By driving a Cabrio with the top down
3. By just opening the windows.

[nscaler]

I know from other threads that on most vehicles running the A/C gets better gas mileage than open windows - but only at freeway speeds. Slow speeds - windows open - no A/C is best. Don't remember any threads about the top down differences.

[CruisingAZ]

Originally Posted by nscaler

I know from other threads that on most vehicles running the A/C gets better gas mileage than open windows - but only at freeway speeds. Slow speeds - windows open - no A/C is best. Don't remember any threads about the top down differences.

Perfect information, this is what I have read as well.

[kannonvaggon]

Our Cabrio definitely uses less fuel in city/ suburban traffic with the TOP DOWN and windows down, rather than the A/C engaged. On the slab at cruise revs it seems to be slightly better economy with the A/C engaged than with the top down. Parasite drag increases as the square of the velocity. If ya had a 60 MPH tailwind, then go ahead and put the top down.....

In heavy stop and go traffic the little bugger will use quite a lot of fuel on a hot day with the A/C running. Not compared to a Hummer though....

:-)