Question about fuel economy

Andrew, who really gives a shit? I'd rather be cool AND comfortable no matter WHAT the cost.

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Using the air conditioner will consume more fuel. I don't know what you consider "much more." My guess is that it uses a little more fuel. We used to have a Ford with an AC that shut off under low vac conditions - boy was that annoying!

Remember the days when vacuum operated windshield wipers would shut down while climbing hills?

I have heard of vac operated wipers but that was before my time. The oldest car I've ever had was a 65 Nova. Just lucky, I guess. :-)

In theory using the A/C should consume more fuel. In practice, I've never been able to detect the difference. I keep fuel log books for all my vehicles. I've looked at fuel consumption by month numerous times and find that often my lowest average fuel consumption is in the summer, when A/C usage is the highest. My assumption is that changes in fuel composition, driving patterns, warm up time, etc., etc., totally mask any additional fuel consumption related to A/C usage.

I think all new cars have the ability to disable the A/C during period of maximum power requirements. There is no longer a need to use a vacuum switch for this function. The engine computer can decide to disengage the A/C compressor only at wide open throttle or some as the result of other combinations of conditions based on the various sensor inputs. This is a lot more sophisticated than using a simple vacuum switch.

Ed

If you do a little web searching on your own you'll find car A/C systems are a lot more sophisticated than you think. All my cars get their best average MPG in the summer, despite A/C use.

If you do a little web searching on your own you'll find car A/C systems are a lot more sophisticated than you think. All my cars get their best average MPG in the summer, despite A/C use.

******** If this is the case (and I would guess you have measured it carefully enough to be relatively sure of this), are you of the belief that running the AC gives you this economy boost? Or is it more likely that summer gasoline blends, or perhaps average operating conditions (temperature, humidity, etc), cause the effect?

The extra fuel required to operate the A/C is trivial and is easily masked by the factors you mentioned and many others. Here area few i can thingk of:

• Car are more fuel efficient when warmed up, so anything that reduces warm up time will improve fuel economy.

• Rear axle and transmission fluids are less viscous when warm, resulting in lower gear train losses in the summer.

• Rubber is more flexible when hot, so the tires use less energy to "flex" when rolling.

• Summer and Winter fuel blends are different, so that can make a difference.

• You use your headlights more in winter, which like the A/C, requires additional power.

• Summer driving probably involves less bad weather driving (driving on snow and wet roads decreases fuel economy).

• Warm air is less dense, slightly lowering air resistance.

• You are more likely to take longer highway trips in warm weather, biasing your fuel economy more towards the highway mileage.

• Becasue driving conditions are better, commuting probably involves less stop and go driving in the summer.

Etc. Etc. Etc.

Plus, don't forget most cans run the A/C in the winter when you turn on the defroster if the outside temeprature is above 40F. So even if you are not using the A/C...you are.

Ed

A lot of good points, Ed. Why anyone would hesitate ONE SECOND before activating the AC, in the car OR the house, is beyond me. Life is too short to suffer heat...or cold.

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Yes, yes, and maybe a few. I do it manually, switching the AC off to accelerate and back on at cruising speed. Don't open the windows wide instead, though, because that causes as much drag as running the AC with the windows closed, at highway speeds.

You left out the most important factor. If you turn the A/C off, you open the windows, greatly increasing drag. In modern cars, the air drag from open windows is greater than the draw from the A/c. This is of course more so at higher speeds. And it probably doesn't matter much on some non-aerodynamic boat like say, a 1964 Cadillac. But on a Camry (or most other cars) of the 1990s or newer, it is another story!

Regards,

DAve

Nope, the A/C consumes some engine power.

=A0Or is it more likely that summer gasoline blends,

I can detect increased fuel economy as air temperature rises.

I wouldnt waste my time manually switching off the A/C unless there were another reason than economy to do it.

Driving a car with the windows open will likely cost you a little in fuel.

I have noticed that my car is getting worse mileage now than it did in the winter. And the fuel blend is the only thing I can think of for that, at the moment.

I've done it for more then 20 years. Our '95 4 cylinder Camry sedan gets from a low of 27 mpg in Winter to a high of 34 in Summer, in driving that is about 3/4 highway, 1/4 city. The A/C consumes significant fuel when the compressor is engaged while you are accelerating. Simply avoiding that situation removes most of the hit on fuel economy from the A/C, while still keeping you cool.

You are probably also driving a bit more aggressively, assuming that you get snow and ice in Winter. You may also have a dirty air filter or some other problem. Check your tire pressures as well.

I just serviced the car and it has new filters, oil change, etc. No issue with tire pressures. We seldom have ice here.

I usually drive at the speed limit, seldom over. My observations are being made under highway driving conditions. The difference in mileage is not much, but seems to be repeatable.

I drove my Solara to Dallas and back this weekend, and monitored the indicated mileage carefully. I gassed up with Exxon regular before I left, so there was no fuel difference to concern me.

Going west, the mileage was consistently indicated to be in the range of

30 mpg. Coming east, it was indicated to be 34.

This is pretty much flat country driving both ways...no apparent difference in grade. Neither was there much difference in ambient temperature, etc.

Averaging ca 32 mpg is not bad, IMO>

What car, what year?

I replaced the engine coolant temp sensor on my '96 to stop the stop- sign/red light stalling problem. Before I replaced it, I remember my all-time low was 19 mpg city in the summer, which I thought was kind of low. Now I average 25-30 mpg (California reformulated crap).

Michael

What car, what year?

I replaced the engine coolant temp sensor on my '96 to stop the stop- sign/red light stalling problem. Before I replaced it, I remember my all-time low was 19 mpg city in the summer, which I thought was kind of low. Now I average 25-30 mpg (California reformulated crap).

Michael

******** 2008 Solara V6. Just a year old. Recently, however, that statement was reversed and I got better mileage than ever, 34 mpg over a 200 mile drive. I am keeping an eye on it.

I am not complaining in any case.