[Need4Camaro]

I'm not super concerned about fuel mileage and what not but I couldn't help but notice this.

My commutes are about 7 miles one way, 80% street, 20% highway. A full tank will last me about 2 weeks usually.

Normally I get around 18 - 21 MPG on these trips because its all stop and go and places where heavy acceleration is needed so the reason I bring this up is to state, I don't really get alot of opportunity to see my maximum fuel mileage capabilities. Normally the only time I can do that is when I'm driving to Houston, or Atlanta.

When I first moved to Austin TX mid summer of 18 I was getting around 28 MPG all highway with the A/C running at about 80 MPH on the freeway.

I've done about 5 round trips to and from Atlanta since that period and all of them except my most recent one replicated the same results, between 27 and 30 MPG depending on if Im using the A/C or not.

The last time I went to Atlanta and back to Austin BEFORE my fuel mileage dropped was in November for Thanksgiving and I scored 28 MPG on that trip as well.

Well this time in mid December, I went to Atlanta for the Christmas Holidays and while I was on the road my car developed the infamous torque shudder. I had it serviced in Atlanta because I was going to be there for a month. They did the basic stuff, triple fluid flush, additive. Well, the beginning of January, I head back to Austin - same driving routine, set cruise at 75 and 80 when I get to Texas, similar road conditions, albeit maybe slightly colder than it was in November (It was in the 30's when I came to Atlanta in November.) ... This time I could not get above 24 MPG, while averaging around 22 - 23 for most of the trip, all highway.

Now a number of things could have changed (looking for input on this) -- such as winter blend of fuel (not sure when they roll out with this, but could this have happened after Thanksgiving after my previous trip?), or possibly the colder weather caused for a different fuel map to be used, but was also concerned that the trans flush may have in some way decreased the efficiency of the drivetrain.

Any thoughts?

[cignul9]

It's likely two things: First, the winter fuel blend. Second, the fronts pushing from west to east likely made your return trip from ATL to AUS into the wind. The second one is... less likely. It's probably the winter fuel blend.

[Emoto]

Same brand of gas all the time? I'm thinking fuel blend.

Have your tire pressures changed? (rolling resistance affects mpg)

[nancygrl]

I'd say winter-blend gas and colder temps account for crappier fuel mileage. It does in my SS here in NC

[ctrlz]

probably multi-factorial. I would not panic until you can recheck when things get warmer.
I drive similar to you, 10 mi daily commute about 80% highway and average 21 mpg. On longer 80 mph highway trips I have never gotten above 24 mpg. This doesn't surprise me.
What surprises me most is that you get a 3 mpg difference between using A/C and not using it! Most studies have shown zero difference to the old "windows-down drag vs compressor drag" argument. Modern automotive A/C is very efficient.

[mdhopt36]

Colder air = dense air = more fuel mixed in with it to maintain proper mixture.

Winter blend more volatile so ignition is most likely retarded, and that won't help either.

Car needs to warm up which also uses more fuel.
*********

Mine went from quite stellar performance, to around what you're getting.
Just what i've noticed, but not the reason why I got a Camaro.
My travel, roughly 75% highway, 25% street around here, not including trips.

[Stex]

Probably all the above. However if it is a GDI engine, you might consider this:

https://www.bgprod.com/services/engi...i-performance/

Or just a can of BG 44K can be real helpful in any engine. I use this regularly in my non-GDI engines. No I am not connected to BG or sell their products.

[Bayles76]

I agree with everyone here. My car even throw a Check Engine Light sometimes when it gets below 20's in the morning when I start it up. The Air/Fuel ratio is so wacky it makes the car shake for about 10-15 seconds like it has a big cam in it then it calms down to a normal idle and everything is fine.

Also, my MPG was a little out of wack once the cold weather got here. Check your tire pressure! Since the air temp dropped here a couple weeks ago my tires like about 5lbs a piece at parked temperature. Road-heated temps didn't get me above 32lbs. I put 35 in front and rear at cold tire temps and have noticed my MPG go up accordingly.

Hope this helps!

[drperry]

You shouldn't be throwing a check engine light, Bayles.... Not on a stock SS, anyways...

To the OP, as others said, it's a combination of winter fuel, and cold air.

Thankfully the car warms up quick..... Temp guage moves in 5 minutes or less, lol

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[ctrlz]

Originally Posted by drperry

You shouldn't be throwing a check engine light, Bayles.... Not on a stock SS, anyways...

I agree. If it throws a CEL it will log a code. Even if the light goes away there should be a "pending" code. I would guess a misfire code, but I agree with Dr. Perry that I would not even expect that at 20 degrees. Maybe -20, but not +20. I would check for any stored codes with a scanner.

[cellsafemode]

dont knock the effects of wind.

Over the course of a long trip, wind resistance can definitely show up in mileage. you're talking a force pushing back on the car of mid 100lbs for the average car cross-section when traveling at 60mph. Increasing the speed (or increasing the wind against or with your travel direction) will certainly raise or lower that. Though usually wind isn't constant at our low altitudes.

[drperry]

Originally Posted by ctrlz

I agree. If it throws a CEL it will log a code. Even if the light goes away there should be a "pending" code. I would guess a misfire code, but I agree with Dr. Perry that I would not even expect that at 20 degrees. Maybe -20, but not +20. I would check for any stored codes with a scanner.

There should never be a CEL from outside temperatures.... Until its cold enough the engine can't maintain operating temperature while driving...

She definitely handles -20*F and colder...
I haven't hadba CEL from starting in the cold... Including -40.

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[Need4Camaro]

Originally Posted by drperry

There should never be a CEL from outside temperatures.... Until its cold enough the engine can't maintain operating temperature while driving...

She definitely handles -20*F and colder...
I haven't hadba CEL from starting in the cold... Including -40.

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what kind of anti-freeze do you use at -40?

[MackSteelPrivateEye]

How many miles on your car?