GenVibe - Community for Pontiac Vibe Enthusiasts

In the Cols Dispatch Saturday 5-14-11"Running the AC causes the engine to work harder and, in turn, uses about 20% more fuel according to the EPA."My experience in driving RT from Columbus to Wilmington, NC in summer with AC on and in late fall with no AC use is that there is not very much difference in the mpg. "Driving at 75 mph uses 13% more fuel than 65 mph, and 25% more than at 55 mph."I tried slowing down by 10 mph and did not see that much of a difference in mpg.

Can't comment about the AC part, but I see about a 15% jump in fuel economy by driving 55. You have to drive the whole tank that way though!

I never use the A/C so I can't really comment on that part, but throughout the entire last tank of gas, I drove 55 instead of 60 and tried to keep the RPMs under 2500 when accelerating and I ended up getting 29.4 mpg with 50/50 driving. That is 2.9 mpg higher than my last fillup when I was still driving 60 and not watching the RPMs.

Just like Carrie witnessed recently. My best two MPG readings of 37.2 last summer, and 36.2 just last month (full tank & 100% highway driving) were driven at or near the speed limit. The 37.2 was actually just below the posted 65 speed limit. In both cases NO A/C was on. If you don't mind EVERYBODY passing you by, then you can get some impressive numbers from the 2.4L engine

I've done a bit of experimenting with this in my car over the past couple of years. I've logged every tank of gas since new (just turned 72,000km today) - so I've got some data.For the A/C use, I find at least a 7% drop in efficiency - but that's not with 100% use either - so it's likely higher. I suspect the effect is greater in town than at speed though. The increased load causes the PCM to increase idle speed - both of which increase consumption.As for speed of driving - with the Vibe those numbers seem quite accurate. I usually try to keep my speed around our local speed limit (100km/h or 62 mph). If I increase that by 10km/h, it uses about 10% more fuel. Cruising at 130km/h (80ish mph) uses about 25% more than at 100. I think the tall shape of these cars is a real contributing factor to this, as my previous Sunfire wasn't affected quite as much.I've spent so long now sticking to 100km/h, I hardly even need to look at the speedometer to tell what I'm doing. I like the fact that I have absolutely no worries about "speed traps" either by doing so. When on a tight budget (trying to pay this car off!), saving money on gas and no worries about tickets seems like a win-win to me!I also try to limit my acceleration when possible. Short access ramps around here don't always allow for that though unfortunately - or else my averages may be able to improve further. My last tank I averaged 7.2L/100km or 32.7MPG U.S. with about 80% highway driving. No A/C needed yet here. My car is a 2.4 w/5 speed auto.John

Quote, originally posted by tpollauf »Just like Carrie witnessed recently. My best two MPG readings of 37.2 last summer, and 36.2 just last month (full tank & 100% highway driving) were driven at or near the speed limit. The 37.2 was actually just below the posted 65 speed limit. In both cases NO A/C was on. If you don't mind EVERYBODY passing you by, then you can get some impressive numbers from the 2.4L engine That's impressive for the 2.4L

Don't forget that when you have the two defrost modes operating you are also running the A/C.

...I measured a -12.5% drop in MPG with A/C "on" compared to A/C "off" over a year & half of driving same route to/from work.

Driving my daughter's Vibe I get between 27-35 based entirely how I stop and start. Slow starts and long stops with not much change in speed on the in-between made a significant difference in economy. I've only run two tanks through it, so much is yet to be learned.

Quote, originally posted by tpollauf »If you don't mind EVERYBODY passing you by, then you can get some impressive numbers from the 2.4L engine LOL! It's true!Quote, originally posted by j42.snyder »I've spent so long now sticking to 100km/h, I hardly even need to look at the speedometer to tell what I'm doing. I like the fact that I have absolutely no worries about "speed traps" either by doing so. When on a tight budget (trying to pay this car off!), saving money on gas and no worries about tickets seems like a win-win to me!Same here !Quote, originally posted by jake75 »That's impressive for the 2.4LI have been very impressed with the 2.4L. My best mpg since last May (when I started keeping track) has been 33.9 with 100% highway driving. Not bad at all in my opinion for the 2.4L auto .

I read that on the highway you are going to get better gas mileage using AC than riding with the windows open. The drag is worse than the extra motor work.

I've read other tests that claim the A/C vs. windows open isn't true. I believe those were done by Car & Driver, and Consumer Reports. Both found that A/C use always increased fuel consumption, and open windows had very little effect. Roof racks with cross bars were the worst offenders.I achieved the best mileage ever on the highway with the front windows open (6.0L/100km or 39.2 MPG U.S.) - where I've never come close to that using A/C. That was using ethanol-free gas as well - difficult to come by these days - at least in regular grade.I modified my HVAC controls some time ago so that I can control when A/C is used. If anybody needs to know how for the '09 +'10 cars, I think I posted it in the How To section. That way I can keep some heat on the glass without running the A/C - but still choose to if conditions require it.John

Yeah, the Car and Driver tests were a bit shocking - but they did test using a BMW sedan, so YMMV.AC on saw a drop of 15.3% at 35 mph, versus windows down drop of only 1.5%. Driving 55 mph the difference was still 9.3% to 3.3%. Even at 75 mph, the difference was only 6.8% to 2.0%.Why did windows down get better at 75 mph v. 55 mph? The higher engine speed makes running the accessories easier.http://www.caranddriver.com/fe...age_2

This reminds me of the truck test with the tailgate up vs down. They say the tailgate up is better (slightly) because the air swirls over the top and "pushes" the truck forward. Not sure how true it is though. All in all, on a highway at speeds Id rather the AC on because the wind can be brutal...

I'm a windows down person! Being the only (as far as I can see) Vibe owner here who doesn't have AC, the mileage is still real good!

Quote, originally posted by star_deceiver »I'm a windows down person! Being the only (as far as I can see) Vibe owner here who doesn't have AC, the mileage is still real good! Why don't you have AC?

Quote, originally posted by MotoAce51 »Why don't you have AC?I own a loss leader (wikipedia search it), it has absolutly no options! The A/C, power windows and door locks package was an extra $3100... besides, I'm not prissy, and I'd only use it 4 months of the year!