1 window down vs 2 windows down

[mikehallbackhoe]

going down the freeway at 65 mph with both winfows down in a crx. noticed stuff trying to fly around in the back. rolled up drivers window and left passenger window down, and stuff settled right down. has anyone done a comparison check on mpg . 1 window versus 2? I wonder if it is like a pickup bed, where the tailgate up gives better mpg due to the air pressure in the bed. had the vents on , but when my wife finally rolled up her window, that was when I started feeling air from the vents.

[user removed]

Last Saturday I drove 150 miles to Chantilly Va. to a show, then the exact same route back home. It was cooler in the morning, about 65 degrees and as high as 84 on the trip back. On the return trip we rode with the front windows down to avoid using the AC. My brother had the passengers window all the way down and there was some buffeting so I rolled the drivers window down a couple of inches and the buffeting stopped.

The difference in mileage was .4 MPG between the two trips with the first leg at 47.5 and the return at 47.3 indicated without a reset of the readings. Traffic coming back was considerably heavier than going there and that could have been the cause of the change or at least a major factor because I managed to time the lights better after we got off the Interstate than I did coming back on the same route.

Final consumption calculation was right at 45.5 MPG, right at 2 MPG lower than the factory instrumentation.

regards
Mech

[Thymeclock]

Originally Posted by Old Mechanic

Last Saturday I drove 150 miles to Chantilly Va. to a show, then the exact same route back home. It was cooler in the morning, about 65 degrees and as high as 84 on the trip back. On the return trip we rode with the front windows down to avoid using the AC. My brother had the passengers window all the way down and there was some buffeting so I rolled the drivers window down a couple of inches and the buffeting stopped.

The difference in mileage was .4 MPG between the two trips with the first leg at 47.5 and the return at 47.3 indicated without a reset of the readings. Traffic coming back was considerably heavier than going there and that could have been the cause of the change or at least a major factor because I managed to time the lights better after we got off the Interstate than I did coming back on the same route.

Final consumption calculation was right at 45.5 MPG, right at 2 MPG lower than the factory instrumentation.

regards
Mech

That sounds typical. It's a trade off - but you're probably better off having the windows up and the A/C on at highway speeds. At lower speeds, it probably doesn't matter as the A/C saps engine power and the aero effects are less.

Me, I drive a convertible in summer, and I don't mind losing 1-2 MPG of FE for the fun of having the top down.

[California98Civic]

Originally Posted by Thymeclock

you're probably better off having the windows up and the A/C on at highway speeds.

It all depends. The wikipedia entry on the relevant 2005 mythbusters re-test of this idea concluded that:

Going less than 50mph it is more efficient to leave the windows down, but going greater than 50mph it is more efficient to use the A/C.

So yes. But there are other options than just windows all the way down or all the way up. I like my method best: I crack the windows just 1"-3" and sometimes also run the fan on lowest vent. I kick my shoes off.

With a small motor like a CRX 1300 or my 1.6, the AC definitely has an FE effect.