A/C usage and fuel economy.

Have you really found a significant decrease in fuel economy based on A/C use? Quick summary: I purchased my Prius in the beginning of October last year here in PHoenix. Granted it was the end of the hot season but we had a week or so where it hit 100.... and it was still at least in the mid - upper nineties for a few weeks. We're hitting summer again. Today's supposed to be 95 and we may hit 100 by Wednesday.

I've only had a few tanks of fuel that were under 50mpg. By under, I mean 49-49.5. My worse fuel economy was 47mpg and that was driving up to Northern ARizona at a faster speed than I'd admit here. My very last tank was 52.5 when I filled it up at 500 miles on the tank. I was 53mpg before then. All this with the A/C on 100% of the time. I don't think I've ever turned the A/C off. I just adjust the temperature and let it do it's thing. From leaving warm weather in Phoenix and going up to Flagstaff this winter where it was 12 degrees I never touched the setting.

Anyway, so I think 50mpg is pretty good with the A/C on. With such good mileage I wouldn't even THINK about sweating my butt off in 100+ degree temperature just to increase the economy. After all, I believe my numbers are inline with others on this forum... so what would I gain?

Mike

 

The post was not an attempt to "put down" those that choose to not use their A/C. In fact it's just the opposite. My attempt was to start a conversation as to real hard facts as to what might be gained by not using A/C. I was thinking about this because in another post another member in Las Vegas stated he/she does not use their A/C. Las Vegas has the same weather as Phoenix. Perhaps he might be able to *gain* fuel economy by using his A/C? This topic has also been on my mind because my cousing just purchased an '08 Touring a couple of weeks ago and has yet to use his A/C even though we've hit around 90 degrees a couple of times. He'll drive 40 miles at 75mph (in AZ our speed limites are 75mph on the outskirts of town and then lower to 65 further in) at 2:30 in the afternoon with all 4 windows down. He of course knows that at that speed he's using more gas but he "justifies" it by not using the A/C to "save" fuel. I pointed out that he was in fact using more fuel with the windows down than he would with the windows up and A/C on due to the much increased drag.

Years ago using NOT A/C saved fuel. Within the last 10-15 years it's well known that a modern vehicle will actually use more fuel above approximately 40mph with the windows open than if the windows were closed and they were using their A/C. I have no figures on the Prius but it's very much possible that it's A/C efficiency combined with the very limited aerodynamic drag (with the windows UP) might have the mph threshold even lower.

I see this all the time with pickup trucks. Many people will drive with their tailgate down to decrease drag. It's actually the very opposite. You'll use more fuel with the tailgate down then up. This myth was further tested on an episode of Mythbusters where they found that air came off the cab and directed straight down on the (down) tailgate and cause drag. With the tailgate up air would enter the bed and create a swirling effect. This swirling effect would cause a bubble that would allow air to glide right over the bubble and slide right over the tailgate.

I'm just about the facts. If anyone has determined the effects of A/C vs No A/C Windows Down then this might keep a few members, particularly in the SouthWest a bit cool(er) this summer.

Mike