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Discussion Starter · #1 ·
I've noticed that the A/C has been using a lot more gas then normal lately. What could this be? I was thinking it could be the serpentine belt.

Thanks
 

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A slipping belt will affect the performance of your accessories including the A/C, but I doubt it would affect your economy much. Excessive drag from a worn compressor could hurt it, but it probably wouldn't be a huge difference.

Just some food for thought though...

How hot is it where you live? We're having record heat down here in Texas. Temperature has a huge effect on fuel economy. I worked on a research project in college examining this. Even 10 degrees below or above the ideal operating range can have big effect on fuel economy.

Cold air is good for power production because it's more dense, but hurts fuel economy because the denser air requires more fuel for proper combustion. Colder, denser air also creates more aerodynamic drag. Extremely hot temperatures hurt fuel economy as well though. Most vehicles are programed to start adding extra fuel when the intake air temperatures get above a certain point. The richer mixture helps lower combustion temperatures to maintain proper engine temperature and prevent detonation. Having the A/C on makes this worse and not just from the drag of the compressor. All the heat the A/C is pulling out of the cabin is getting dumped right in front of your radiator, further increasing engine and intake air temperatures. In addition, if you do a lot of city driving the hotter air typically requires much more aggressive throttle inputs and RPM's during acceleration to get the vehicle up to speed.

The ideal temperature range varies by vehicle but is usually around 50-80 degrees, give or take 10 degrees each way. The further you get outside of the ideal range for your vehicle the steeper the economy drop off becomes. The difference between 90 degrees and 100 degrees can be over 20% in some vehicles. With the 110 degree heat here, my 2000 Vulcan has dropped from averaging 28ish MPG on the highway to barely 20 MPG. City driving is even worse.
 

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^^^^ +1

Ford PCMs are, IMO, also overly agressive about pulling ignition advance at high IAT temps, which also drops MPG by a significant amount. Since the high IATs hurt performance for several reasons, it requires a larger throttle opening to give the same acceleration which further hurts MPG.
 

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Discussion Starter · #4 ·
A slipping belt will affect the performance of your accessories including the A/C, but I doubt it would affect your economy much. Excessive drag from a worn compressor could hurt it, but it probably wouldn't be a huge difference.

Just some food for thought though...

How hot is it where you live? We're having record heat down here in Texas. Temperature has a huge effect on fuel economy. I worked on a research project in college examining this. Even 10 degrees below or above the ideal operating range can have big effect on fuel economy.

Cold air is good for power production because it's more dense, but hurts fuel economy because the denser air requires more fuel for proper combustion. Colder, denser air also creates more aerodynamic drag. Extremely hot temperatures hurt fuel economy as well though. Most vehicles are programed to start adding extra fuel when the intake air temperatures get above a certain point. The richer mixture helps lower combustion temperatures to maintain proper engine temperature and prevent detonation. Having the A/C on makes this worse and not just from the drag of the compressor. All the heat the A/C is pulling out of the cabin is getting dumped right in front of your radiator, further increasing engine and intake air temperatures. In addition, if you do a lot of city driving the hotter air typically requires much more aggressive throttle inputs and RPM's during acceleration to get the vehicle up to speed.

The ideal temperature range varies by vehicle but is usually around 50-80 degrees, give or take 10 degrees each way. The further you get outside of the ideal range for your vehicle the steeper the economy drop off becomes. The difference between 90 degrees and 100 degrees can be over 20% in some vehicles. With the 110 degree heat here, my 2000 Vulcan has dropped from averaging 28ish MPG on the highway to barely 20 MPG. City driving is even worse.
That make's sense. We have had record breaking heat here in Missouri (107ish). My fuel economy, I've noticed has been steadily declining. I averaged about 25MPG when I was in Louisiana, now I've moved up here and now am averaging 15-20 city and highway combined.
 

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mgg vs temp

I've noticed that the A/C has been using a lot more gas then normal lately. What could this be? I was thinking it could be the serpentine belt.

Thanks
Some data: For 5 years, I drove a loop of 3500 miles every 2 months. These driven in Lin Cont and easy to check mpg. And when we hit I-70 west of Columbus O, to Kansas City MO, the scenery does not change, and it is boring. For something to do, and for curiosity, I checked mileage time and time again. There were 2 things that kicked mileage. Wind and temp/sun.

When leaving Columbus early in the am got 26mpg and as the day wears on gets down to 24. Reset every 50 miles or so and can see it happen. Now this is in a already cooled interior, steady driving.

One more case. Driving I-64 St. Louis to Louisville, we stopped at Evansville, SE corner and had lunch, car sat about an hour in July sun. We hit I-164 which is flat and straight for about 10 miles. Set the MPG and ran 18 until the auto climate set back from max. Then after reset it went to 20 and so until back to 24 when the blower cut back and the system was steady state. So one could see 6mpg depending on how hard the A/C is working. Also consider the Taurus blower uses 20A at high speed and that is about 280 watts. By the time alternator makes that and you have efficiency losses, that takes a toll.

One more case. Driving just over 100 miles to a hospital, early am, still air, cold morning I got 29. In this case only one in car, no luggage, no stops. No A/C needed, only heat.

And one observation in the midwest. Wind can cut the mileage more than one would think. What is a real surprise is cross wind. At times I have seen a drop of 4 mpg in a crosswind. This is of course like 'yaw' in an airplane. It adds drag, quite a bit in aerodynamic shapes. In addition to drag, it adds more tire drag, and PS useage.

Oh well, not driving that any more, only one 1600 mile trip per year.

Happy cooling.

-chart-
 
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