[AWAHOO]

Would this terrific heat affect gas mileage? I purchased my Limited on 2/26/10. Since that time, I have had occasion to travel from Petersburg to Emporia, VA at least 25-30 times. I fill up when leaving Petersburg and my computer would show 35-38 mpg on the trip to Emporia. Once at home, when driving around town, the mpg would drop very little, as I drive conservatively, trying to keep the "eco" light on.

My last two trips from Petersburg to Emporia, the mpg has not exceeded 31 and when I drive around town the mpg drops drastically. Today I started up showing 30.9 and drove 1.5 miles and the computer showed 28.6.

Could hot weather be affecting mpg this much, or is it possible that the onboard computer has gotten out of whack and the fuel to air mixture is causing the mpg to drop?

Any suggestions? Will the service mgr. think I am crazy to ask about this?

 

[RXDOC]

I have seen, and mentioned on another thread on MPG, my mileage has dropped 2-4mpg with the heat and the constant use of a/c.
And yes, the SA will think you are crazy for bringing your vehicle in for a small drop in MPG.

 

[Blue07]

QUOTE (AWAHOO @ Jul 24 2010, 07:13 PM) index.php?act=findpost&pid=344456

Would this terrific heat affect gas mileage? I purchased my Limited on 2/26/10. Since that time, I have had occasion to travel from Petersburg to Emporia, VA at least 25-30 times. I fill up when leaving Petersburg and my computer would show 35-38 mpg on the trip to Emporia. Once at home, when driving around town, the mpg would drop very little, as I drive conservatively, trying to keep the "eco" light on.

My last two trips from Petersburg to Emporia, the mpg has not exceeded 31 and when I drive around town the mpg drops drastically. Today I started up showing 30.9 and drove 1.5 miles and the computer showed 28.6.

Could hot weather be affecting mpg this much, or is it possible that the onboard computer has gotten out of whack and the fuel to air mixture is causing the mpg to drop?

Any suggestions? Will the service mgr. think I am crazy to ask about this?

Like RDX stated, running the A/C will hurt mileage. Aside from that, hot air generally helps mileage. Hot air is less dense than cold air and therefore requires less force to displace. (The cars slips through it easier.) For every 10*F increase in temp, air density decreases by about 2%.

However it is possible that very high humidity could increase air density and therefore hurt mileage. Hot air can retain a lot of water vapor, sometimes making it feel as thick as pea soup. That thicker air could hurt mileage that same way that winter's cold air hurt's mileage. It's like trying to run in water. It just takes a lot more energy.

For the record, the heat alone will not affect highway mileage. While it is true that hot air is less dense and therefore contains less oxygen than cold air, that only affects the engine at wide open throttle. When floored, it cannot inhale as much total air and therefore cannot burn as much fuel. As a result it will produce less torque and horsepower. However at part throttle, all the engine's sensors will detect the lower air density and adjust the fuel mixture to maintain the optimal stoichiometric ratio. So when the car cruises down the road, the engine will still burn the same amount of fuel to produce the same amount of power.

 

[AWAHOO]

Maybe some do not consider mine, a drastic reduction in mpg, but I do. If I was getting 37 mpg (with A/C on) before, and now cannot get but 30.9 under the exact same conditions and speed, then I consider that drastic. Is it possible, as I said previously, that something has gotten out of whack with the on board computer/sensor?

 

[midas69]

QUOTE (Blue07 @ Jul 24 2010, 06:59 PM) index.php?act=findpost&pid=344463

However at part throttle, all the engine's sensors will detect the lower air density and adjust the fuel mixture to maintain the optimal stoichiometric ratio. So when the car cruises down the road, the engine will still burn the same amount of fuel to produce the same amount of power.

By adjusting the fuel mixture, doesn't that mean more fuel? I don't know, but it just sounds right.

However, one thing not mentioned is the actual gasoline itself. Many parts of the country have different formulated gas in the summer mandated by local government. This tends to happen in areas of high pollution. It's formulated to cause lower emissions but the cost is worse fuel economy.

 

[Blue07]

QUOTE (midas69 @ Jul 24 2010, 09:20 PM) index.php?act=findpost&pid=344480

By adjusting the fuel mixture, doesn't that mean more fuel? I don't know, but it just sounds right.

The computer will adjust the fuel up or down based on air mass. Air mass is function of air volume and air density. Air density is a function of temperature, air pressure, and humidity. As these variables change, the sensors help the computer maintain the optimal air/fuel ratio.

Now that Awahoo eliminated A/C as a potential cause for his mileage decrease, he may have a problem. Going from 37 mpg to 31 mpg is a 16% decrease. That much cannot be explained by temperature.

 

[midas69]

QUOTE (Blue07 @ Jul 24 2010, 09:36 PM) index.php?act=findpost&pid=344491

The computer will adjust the fuel up or down based on air mass. Air mass is function of air volume and air density. Air density is a function of temperature, air pressure, and humidity. As these variables change, the sensors help the computer maintain the optimal air/fuel ratio.

But that was my point, if the air density goes down doesn't the richness of the fuel have to go up to maintain the proper mixture? Or does the mixture get leaner? I honestly don't know, but I want to say that less dense means less oxygen. Since oxygen is required for combustion, I would think more fuel would be required to make up for the lack of oxygen.

QUOTE

Now that Awahoo eliminated A/C as a potential cause for his mileage decrease, he may have a problem. Going from 37 mpg to 31 mpg is a 16% decrease. That much cannot be explained by temperature.

I've heard that the summer gas formula in a place like L.A. can lower gas mileage by as much as 10%. Still it's been hot as blazes around here lately and I'm only seeing maybe a 4 or 5% decrease. 16% seems pretty dramatic.

 

[Blue07]

QUOTE (midas69 @ Jul 24 2010, 11:05 PM) index.php?act=findpost&pid=344493

But that was my point, if the air density goes down doesn't the richness of the fuel have to go up to maintain the proper mixture? Or does the mixture get leaner? I honestly don't know, but I want to say that less dense means less oxygen. Since oxygen is required for combustion, I would think more fuel would be required to make up for the lack of oxygen.



I've heard that the summer gas formula in a place like L.A. can lower gas mileage by as much as 10%. Still it's been hot as blazes around here lately and I'm only seeing maybe a 4 or 5% decrease. 16% seems pretty dramatic.

I used the wrong words. This confusion is totally my fault. When one discusses air to fuel ratios, there are TWO ratios at hand. There's an air-mass to fuel ratio and an air-volume to fuel ratio. The air-mass to fuel ratio controls the oxygen to fuel mixture. That ratio will remain constant. (Well, relatively constant. The computer will vary it slightly based on ignition timing, load, rpm, etc.) However the air-volume to fuel ratio will change based on air density. When I wrote "The computer will adjust the fuel up or down based on air mass," I referred to air volume. I am so sorry.

To inhale a certain amount of air mass, the engine will inhale different amounts of air volume based on air density. As air density decreases, in order to inhale a certain amount of air mass, the engine must inhale a greater amount of air volume. So if it is hot outside (or at higher altitude), we need to drive with the gas pedal pressed down more. (That's what controls air volume.) But at first impression, one may think that because the gas pedal is pressed down more, the car will get worse mileage. I'm saying that is not true. Even though the throttle is opened more to increase air volume, the computer knows the air mass is less. As a result, it will decrease the amount of fuel delivered to the engine relative to air volume. That was the perspective of the adjustment to which I referred.

On the subject of corrections, I have another. When I wrote that Awahoo's 16% decrease was explained by temperature, I wanted to write humidity. Humidity will cause an increase in air density which increases air resistance.

 

[Flying Fossil]

Sounds to me like your mileage is pretty much where it should be.
If your not using pencil and paper to figure your mileage, You have no idea what your getting.
The trip computer is wonderful for showing your miles driven, but don't count on it to give you realistic gas mileage figures.
Enjoy the car.

 

[ssmuff]

Yes heat and ac will affect final mpg. More of a factor is the humidity of the air.

 

[midas69]

QUOTE (AWAHOO @ Jul 24 2010, 07:33 PM) index.php?act=findpost&pid=344472

Maybe some do not consider mine, a drastic reduction in mpg, but I do. If I was getting 37 mpg (with A/C on) before, and now cannot get but 30.9 under the exact same conditions and speed, then I consider that drastic. Is it possible, as I said previously, that something has gotten out of whack with the on board computer/sensor?

Forgot to address this. Just because the A/C was on in both instances doesn't mean anything. It's how often the compressor is actually engaged that affects mileage. If you have the A/C on at 75 it's not going to actually have the compressor engaged that often. But running at 90+ degrees will have the compressor on almost all the time. So it's not really the exact same conditions if the temperature is vastly different.