Fuel Economy & A/C Usage
 

While on a trip from NC to KY I accidentally noticed once when I turned my A/C off while going down hill the fuel mileage on my scan guage jumped drastically. So when going down hill I continued to check to see if it was a pattern, and it was. I then checked to see if it made a huge difference when going up hill and although it did make a difference it was not near as drastic. I was driving through the mountains of eastern TN and western NC on interstate 40 so I would use the A/C when climbing and turn it to vent while going down the mountains. I was able to keep the car comfortable while saving gas. If you have a scan gauge it may be something you want to check for yourself to see the benefit. I got the best mileage ever on my 1997 Ford Escort Station Wagon a whopping 48.215 MPG. Not bad for a car that had an old highway EPA rating of 42 highway. It was 8AM-12:00PM so it was cool enough that I didn't need the A/C at all part of the time.

Probably DFCO is canceled if A/C is on.

When you turn the A/C off, if you can keep the air flow on recirculate instead of vent then it will stay cooler longer.

I think it varies by vehicle. In my V8 pickup truck I ran one full tank of gas with the a/c on max the whole time. The next tank I never ran the a/c and drove with the windows up (no matter how hot it got), so any loss of aerodynamics (Did I say loss of aerodynamics in terms of a 4wd pickup? Shame on me!) would not taint the result. The truck got exactly the same mileage on both tanks. I think the larger the engine, the less of an effect the a/c makes on economy. Overall the a/c takes a much smaller percentage of the engine's HP on V8's as it does on 4 bangers and V6's. Because of that I do not hesitate to turn on the a/c if I feel I need it. On my vehicle the effect of the a/c is negligible.

-Jay

AC kills my mileage.

DFCO stops about 1200 RPM instead of 850.

regards
gary

I'm surprised at this, because I believe (in theory) you would get better MPG using A/C on the downhills. It might look more drastic on your MPG gauge but I'd think for the total trip it would improve.

This is because essentially (long story made very short) you'd be holding your throttle closer to a constant position for more of the trip.

-Bob C.

I've had moments of weakness recently where I've used the AC. When I push the button I instantly lost 4 mpg.

When I finally break down and part with the cash I'll have to see what happens in my truck with a SG II, but like I said I ran 2 whole tanks in the summer, one w/ a/c , one w/o a/c. The truck got the same mileage down to like 5 or 6 decimal places.

It felt like I had down shifted when I turned the compressor on. I'm sure you are telling the truth. The AC is a very small factor in the overall load on your engine compared to my 2.2L and 2700 lb car.

I thought it would be better using A/C on the down hill also because it would be less total load on the engine, but as it works out it was the other way around.

BSFC and pumping losses explain why it's better to use A/C uphill. When it's all said and done, you need the same total amount of energy to be produced; and the engine produces that energy more efficiently when it's loaded more heavily.

Also, I wonder if some cars don't use DFCO when the A/C is engaged?

My car doesn't use DFCO if i'm in drive and the a/c is on.

If I manually downshift to second or take the transmission out of overdrive it'll do it but only to around 2000 rpm.

I've had the car almost stall before because i was engine braking in first while using the regular brakes and the engine speed fell so fast past the resume rpm it almost didn't catch.

AC and mpg
 

I have a 1995 Neon manual shift, that I cycle the air on and off by hand. When accelerating its always off. At cruise speeds 40 or 45 when ever possible and at normal 55 its on. I noticed a big drag on my acceleration with it on and sometimes I am late turning it off when accerating and the increase in power is highly noticable. I see a 20 % increase in savings doing this. With the vent closed it blows cold air for alot longer than it takes me to get to speed. I have found its better not to use the cruise control when using air conditioning( if there are alot of hills), maybe its just my Neon but its constantly reapplying power. I can still get close to 35 mpg highway doing this. My wife has an Aurora with all the bells and whistles, an instant mpg readout mode. With the bigger engine I thought the drag from the air conditioning unit would be very small but it was at least 10 percent and more noticable in cruise control mode.

AC and mpg
 

I have a 1995 Neon manual shift, that I cycle the air on and off by hand. When accelerating its always off. At cruise speeds 40 or 45 when ever possible and at normal 55 its on. I noticed a big drag on my acceleration with it on and sometimes I am late turning it off when accerating and the increase in power is highly noticable. I see a 20 % increase in savings doing this. With the vent closed it blows cold air for alot longer than it takes me to get to speed. I have found its better not to use the cruise control when using air conditioning( if there are alot of hills), maybe its just my Neon but its constantly reapplying power. I can still get close to 35 mpg highway doing this. My wife has an Aurora with all the bells and whistles, an instant mpg readout mode. With the bigger engine I thought the drag from the air conditioning unit would be very small but it was at least 10 percent and more noticable in cruise control mode.

I have heard that you get the best mileage if you turn the a/c off while accelerating and going UPhills.

I think you saw a greater increase in mileage while going down hill, with the a/c off, because you are using less/no power going downhill, so the a/c takes a larger chuck of the power provided.

I believe you will see better overall mileage if you turn it off while under acceleration and going up hills, Then turn it off while coasting down hill in neutral. If you start gaining too much speed put it in the appropriate gear to help you stop gaining speed, not slow you down. for example (in my car at least. 3.8L V6 Camaro) if I'm going moderate downhill in a 55 and start getting too much above 60 i'll put it in 5th gear. (about 2000-2100RPM) This gives me only a slight amount of drag so i stay at 61-62, then pop it back into neutral just before leveling out, and coast to 50 or so.

Welcome to the forum, soda_pop503. Post a thread about your car, I've often thought that it would be a good car for hypermiling and a good car for me. Reasonably tall gears, manual transmission, the much-ballyhooed GM 3.8, and a low aerodynamic drag coefficient...should all add up just right.

Use the AC in DFCO, its free. Downhill engine braking is DFCO.

regards
gary

yes, welcome soda pop.

i've(wife's car) got a 97 camaro w/ the 3.8. we've managed as much as 25mpg combined driving(city/hwy and her/me) in our automatic.

depending how many hwy miles you drive, i'll bet you could hypermile to near 30 mpg!