After recently having my AC repaired, I calculated my gas mileage after having the AC on about 80% of the time. I had been getting 23-24 MPG (no AC, all city driving) and this time my gas mileage was about 19 MPG. Is this normal for city driving or should I be concerned? I'd be lucky to get 17-18 MPG at this rate when Summer hits. New compressor is a Four Seasons (Nippondenso type) and the car is the Vibe in my signature. Any thoughts?
Significant drop in MPG with AC on. Normal or something wrong?
- Thread starter Jonas
- Start date
On a side note, I can get ethanol-free gas in my town at a good price and it increases my MPG by about 2. Is the non-ethanol stuff still regulated with all the detergent requirements and everything? Would you run it if the extra cost was justified or is it a bad idea long-term?
The detergency does not depend upon the ethanol content.
How could it ever be a bad idea?
We were travelling and for the first time we saw and used ethanol free gas. I think it was 85 or 87 octane. We always use 92 or 93. the price was same as 93. The mpg went up approx. 2+ mpg but the car felt sluggish to me and not as lively. My wife said there is no difference. I can feel the diff between 93 and 87 in my old truck going uphill.
As far as mpg with ac on, I think we got about 1-2 mpg less but were doing 80 mph which was the speed limit.
6 mpg drop with AC sounds very high if all else is equal. At least based on my experience!
As far as mpg with ac on, I think we got about 1-2 mpg less but were doing 80 mph which was the speed limit.
6 mpg drop with AC sounds very high if all else is equal. At least based on my experience!
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I thought so too. I am doing all low speed delivery driving with lots of idling, but that's always how I drive. The AC worked when I first got the car and, though I didn't monitor MPG that closely, I don't think it was as bad.
Would an undercharged a/c system require less up to spin? Maybe it's been low on refrigerant for a while and got better mpg. Now is charged and operating may take a bit more power to spin?
5-6mpg is far, far, far too much to be attributed to running the a/c, IMHO.
I've never been able to discern a difference of any sort in my car, which has lots of ways to view the MPG (as graphs, etc.), or any of our cars for that matter. Most of my best tanks in the Camry (39 or better mpg) have involved significant a/c use. I'd be extraordinarily surprised if the difference even approached 1mpg in your Vibe. I'd say you either have another mechanical issue (say, a dragging caliper), a measurement error/issue (e.g. you had the first fill-up be short a gallon or so, and maybe the second was a gallon or so more, throwing off your numbers), or your driving style changed in some way (more traffic, idling, long drive-throughs, etc.) Or you could've gotten a tank of higher-ethanol gas accidentally (perhaps E-85 vs E-10). I've tracked every tank of gas going into my car, and watch the MPG readouts quite a bit, so have a pretty good sense of what impacts fuel mileage in my driving. Any of the other options could cause such a MPG drop, but a/c shouldn't even make a noticeable impact on your fuel economy, much less 6mpg.
I've never been able to discern a difference of any sort in my car, which has lots of ways to view the MPG (as graphs, etc.), or any of our cars for that matter. Most of my best tanks in the Camry (39 or better mpg) have involved significant a/c use. I'd be extraordinarily surprised if the difference even approached 1mpg in your Vibe. I'd say you either have another mechanical issue (say, a dragging caliper), a measurement error/issue (e.g. you had the first fill-up be short a gallon or so, and maybe the second was a gallon or so more, throwing off your numbers), or your driving style changed in some way (more traffic, idling, long drive-throughs, etc.) Or you could've gotten a tank of higher-ethanol gas accidentally (perhaps E-85 vs E-10). I've tracked every tank of gas going into my car, and watch the MPG readouts quite a bit, so have a pretty good sense of what impacts fuel mileage in my driving. Any of the other options could cause such a MPG drop, but a/c shouldn't even make a noticeable impact on your fuel economy, much less 6mpg.
After getting the AC repaired, I drove for about a week with it off because it wasn't hot outside and I was getting 23-24 MPG. I'd imagine the belt tension wouldn't have changed. I did change the oil a few days ago (Magnatec 5w-20 + Liqui Moly oil saver) but I can't imagine that making a huge difference either.
I had a 7mpg drop once. Turned out that my tires all dropped in pressure. I keep them at 38-40psi, but due to not checking for a while and significant changes in ambient temps often (thanks NC climate) my tires were all at 26-28psi. Also soon after I had to replace my alternator, so maybe the old one was on the way to fail and was causing the extra drag as well. Got all the MPGs back now though.
Edit: And as mentioned earlier - a sticky caliper is more than enough to cause such a drop. Toyota/Lexus vehicles (including Pontiac Vibe, as it is a Toyota Matrix) have a reputation for poor cooling to the rear calipers. That has been a problem for any Toyota/Lexus vehicle made from mid-1990s to past 2010ish. They do end up getting seized more often than most vehicles. Guide pins seize up from not enough cooling, and it's a snowball effect from there until the caliper completely seizes.
Edit: And as mentioned earlier - a sticky caliper is more than enough to cause such a drop. Toyota/Lexus vehicles (including Pontiac Vibe, as it is a Toyota Matrix) have a reputation for poor cooling to the rear calipers. That has been a problem for any Toyota/Lexus vehicle made from mid-1990s to past 2010ish. They do end up getting seized more often than most vehicles. Guide pins seize up from not enough cooling, and it's a snowball effect from there until the caliper completely seizes.
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Good call. I haven't checked my tires in a while. Also, I've had mixed opinions about my alternator. I've had it tested a few times and it puts out about 13.4-13.7 volts; some said it was fine, others said it was weak. (all auto part stores-free testing) I haven't had it tested with the AC on yet though.
My rear brakes squeal sometimes under light braking and I use my handbrake almost every time I park. The rear pads and rotors are fairly new. I'll keep that in mind, though I don't really know what to look for. But, again, none of this accounts for the 4-5 mpg drop from running the AC unless it coincidentally happened as soon as it got hot outside.
One more thing I just noticed. I drove around for a bit and checked my OBD. My long term fuel trim is at negative 7%. I've never seen that before. I recently addressed high positive fuel trim and it had been close to 0 for a few weeks. I wonder what that could be about. Sorry to throw so many things out there, but I really appreciate the feedback.
On the highway I usually see about 10% drop in fuel efficiency with the a/c going. Yours dropped approximately 20% which really doesn't surprise me with city driving. If the mileage exceeds the extra cost per gallon on E/0 I'd run E/0 all the time. My cars seem to get a little better mileage on E/0 but not enough to make up the approximate $ .20-.30 a gallon extra cost. E/0 has about 3% more energy than E/10 but some cars get more than 3% increase. I suspect that's because the engines are designed to run on gasoline. Even EPA testing is done using E/0. One car that I had and retired about 10 years ago showed no noticeable improvement in mileage between E/0 and E/10.
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sounds like they didn't service orifice tube/expansion valve. Any modern vehicle shouldn't see a 20% drop in fuel economy from the AC running unless it's in extreme conditions, such as Australian summer. But your test is far from scientific and within margin of error. If your compressor cycles with a duty cycle of about 40% after the interior has cooled off, I wouldn't worry about it.
my 2c.
my 2c.
