MPG with & without AC... tested!

Hello,
I'm new to the Prius chat forums, but I've been reading the forum and learning for a while. Anyway, I noticed the effect of AC on fuel economy has been discussed off and on without much testing. Here are the results of a simple test I conducted. I drove a round trip loop that's a total of 20.3 miles long, twice (just over 40 miles). The loop was driven back to back in my 2010 prius II with 28,000 miles. The car was fully warmed up and I had 6 bars on the battery starting each time. The route has 8 stops total, and speed limit is 45 MPH. Pretty much steady cruising at 45 MPH with only gentle hills and almost no traffic (for consistancy). Outside temp was between 84 and 88 degrees F (temp varied along the route). AC was set to 78, except when the temp dropped I lowered the setting by 4 degrees to keep the same temperature differential. I drove gently (a nice Sunday drive), but I did NOT hypermile. First trip was with AC on. I got 59.3 MPG. Second trip was with the AC off and I got 67.9 MPG. An 8.6 MPG difference. FWIW, I have the Yokohama AS530's that reportedly cost 4-5 MPG vs. stock tires.

Of course the AC may have very different effects on fuel economy depending on the type of driving, but at least we have a bit of a starting point now.

Any thoughts?

 

Understood. But if you read the threads about ac's effect on mpg the answers range from negligable, to 2-3 mpg, to a 10% loss. Here we have a resonably well documented loss of around 13% and almost 9 mpg!

 

Well, I have no intention of doing it even 4 or 5 more times. Next time my wife is out of town on business I may do it again if the temps are mid 80's or higher. I'll share the results if it happens.

FWIW, my 15 mile roundtrip commute to work typically nets 47-51 mpg with ac (usually 48'ish). It's stop and go traffic for all but the last mile. Without ac I typically get 56-57 mpg. There is more variation with this route because of differing traffic, so I consider it less scientific. It also includes ICE warm-up which would tend to make the figures appear closer than a fully warmed up trip would yield. Yet it shows a similiar difference in actuall mpg, but a larger percentage of difference. This route has been tested way more than 10 times in each configuration. YMMV.

 

I agree about the belt. FWIW, I have an '03 Honda Civic Hybrid I just sold. I put around 90,000 miles on the car. I could hypermile some excellent economy out of that car (60 mpg+), but the A/C was a killer! My 52 mpg trips would become 38-39 mpg trips (41-43 mpg if I still enabled "auto stop"... no AC at stop lights though ). A government study tracking hybrid battery performance (320,000 miles of data from two '03 HCH) put the mpg loss of the HCH at 13 mpg with AC use, which certainly mirrors my experience.

Interestingly, the belt is always being pulled around the pulley, AC running or not, and turning the AC on still made a huge difference in economy. So while I expect the all electric AC is more efficient than a belt, I would guess the belt itself was a rather small percentage of energy loss. Every little bit helps though!

More efficient or not, being able to shut the ICE off and have AC is worth it!

 

Exactly! I chose the 45 mph route instead of interstate cruising because I guessed that overall drag would be near it's low point at 40-45 mph, which would (hopefully) let the AC energy consumption come through over the noise. I would imagine at 70-75 MPH the percentage of mpg loss and actual MPG loss would be even less (same rate of energy consumption for AC but a higher rate of speed, so the AC energy would be consumed over a larger number of miles...).

In the city it gets confusing... it would depend on if you could do enough regen braking to power the AC or not. But my experience is that I take a bigger hit in the city. Though it's much less of a hit than my '03 HCH took!

 

That's a good suggestion... I'll crunch the numbers tomorrow. My average speed for each trip was exactly 38 mph each time (per the trip computer). I was wanting to get a steady state gph figure for running the ac while idling anyway. It'll be interesting to compare the numbers.

 

1) Eco mode the whole time... eco mode is all I ever use.
2) car was pre-cooled during ICE warm-up. Warm-up (and interior cool down) lasted about 20 minutes.
3) No fan. Windows were cracked less than an inch to keep it tolerable. At 45 mph the drag would be of little concern. Fan would probably have been more efficient.

 

I don't suffer for better economy... unless I'm seeking data from time to time, if its uncomfortably warm, I run the ac. I bought a prius so I didn't have to suffer! I'm curious about the effect on mpg for comparison sake mostly.