better mpg with a/c on?

[MrShorty]

Been getting around 16 mpg in my '92 all winter long. However, In the 1/2 tank I burned Saturday, I got 19 mpg. In the 1/4 tank I burned yesterday, I'd estimate I'm shooting for the same kind of mileage. The only change is that both of those trips were the first in which we constantly used the A/C. My main question is: how does having the A/C on affect engine management? I know the computer is aware that the A/C is on or off, but I always thought that the PCM only used that information to adjust idle speed. Does this make anyone think of anything else I might look at?
At this point I'm wondering if it might be in the effect A/C has on engine temperature. All winter, my temp gauge has fluctuated, as if the new t-stat I put in last fall isn't sure about being open or closed. Backprobed the ECT, and the temperature swings from about 180-220 F with the A/C off. With the A/C on, the temperature is steady at about 210 F. I'm hypothesizing that, the unsteady temperature caused the computer to be unable to send a steady control algorithm to the engine.

 

[Alec]

My guess is that the A/C being on and compensated for is "band-aid"ing the problem. It is possible your replacement thermostat is bad.

Normally having A/C on will reduce mileage as it takes away power from the motor.

 

[Majisto]

Gas Mileage goes up in the summer since the air is warmer and not as dense, so the engine uses less fuel to keep the A/F mixture at a certain level.

 

[TPLYNCH]

Also, do you have the Oxygenated Gas (winter blend) in Utah?? That crap will suck a couple of MPG's...

 

[Alec]

Both true. It may not be the A/C itself, just the season.

 

[Robb]

Did you have the windows down when you weren't using the a/c?

I have heard from a couple of mileage gurus that keeping the windows up with a/c on will help with mileage because of the reduction in drag.

 

[EliteConcept]

using the A/C with the windows UP. theres a new concept

 

[brendan]

Yeah, it's definitely all about the season. This winter (in International Falls) I got about 14-15mpg on the highway. Now that it's summer I get 19-20mpg Talk about a drastic change!

 

[wilson_nick]

i get about 19-21 mpg year around, the only time my mpg goes down is if i drive w/ the rear window open, because of the extra drag i get horrible mpg. I typically run w/the windows down and the tunes up- not a huge ac user.

 

[Hokie]

Originally posted by Robb
Did you have the windows down when you weren't using the a/c?

I have heard from a couple of mileage gurus that keeping the windows up with a/c on will help with mileage because of the reduction in drag.

My windows claim a consistent 1-1.5 MPG on the highway, so I use the AC on the interstate

 

[Nitrous SSC]

Just to confirm Utah does use a crapier fuel in the winter time. I too notice the difference between winter and summer in my MPG too. It tripped me out at first because I was getting more mpg in the summer using AC than in the winter till I figured out why.

 

[X~FACTOR]

You cannot make your calculations using 1/2 a tank, or worse, 1/4 of a tank. Try it on for a couple of full tank on all highway or on all city driving and then compare the numbers.

 

[Nitrous SSC]

You cannot make your calculations using 1/2 a tank, or worse, 1/4 of a tank. Try it on for a couple of full tank on all highway or on all city driving and then compare the numbers.

True...

But there are different fuels up here that does effect mpg different from winter/summer.