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Anyone have any idea what might be happening? Temp set to 70, getting 31.5 on gas and overall 35.5 mpge. Going 75 and 40 degrees out? This something I should get checked out or is this normal? https://imgur.com/a/S1dnb4o
Are you running on the OE Michelin EnergySaver A/S tires or have you put winter tires on the Volt? Check the cold tire pressure. It should be at least 36 PSI. Recommend setting ~40 PSI cold pressure.

Check that one of your brake calipers is not stuck or rubbing the brake pad against the rotor. Feel each wheel for signs of heat (but don't burn your hand.)

Are you towing a small trailer (some do)?

Try slowing down to 70 MPH and see how much your MPG improves.

Is there a headwind?

Are you crossing a mountain range?

What is the engine coolant temperature once the Volt warms up, it should be 185F - 205F. (Maybe your engine coolant thermostat is stuck open.)
 

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Low tire pressure, lower temps, rain/snow, rapid starts and stops, high road speed, going up hills, all can lower MPG whether the car is electric or not.
 

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I'll guess headwind. Can't see it, most people don't factor that in.
 

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2016 Volt Premier, delivered Oct/15
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The car is sensitive to headwinds,speed etc. I get about 40 mpg going 75 with the car loaded, on the drive to Florida, including the mountain sections (over a 1400 mile drive).

Make sure to use only Top Tier brands of gas ( https://toptiergas.com/ )
 

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Your graph shows that your efficiency changed and became worse in mid-September. Did anything change at that time, like new wheels/tires or any repairs, maintenance or modifications done? Car behaving oddly in other ways since then? Different route or driving conditions since then?
 

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Since you didn't mention any Check Engine Lights:

First the graph I'm looking at is shows only a 6% peak to peak variation for 3 seasons. That not much P2P for a year.

Lots of things affect MPG. Most are not unique to the Volt.
Winter blend or high ethanol fuel vs pure summer gasoline can be >10% in range depending on the engine tuning and design. Ethanol content harms range, volatile (easy start) HCs do as well.
Tire and road temp Cold tires and road have higher rolling resistance. Put a rubber spatula in your freezer and see.
Water, snow, and rain grooves hurt.
Driving style hurts (that's the biggest factor)
Weight hurts
Tire selection, wear, pressure, and alignment.
Relative airspeed and direction. Crosswinds hurts a lot since car are only optimized to straight airflow.
Density Altitude changes. This is the weight of a volume of air, not an altitude. High DA harms engine efficiency, low DA harms aero efficiency.

But the 38 mpg summer is what puzzles me. That's what your problem is. Not 36 in the coldest month of the year. That's not very good IMO. Closer to 80 mph in mild terrain 50-2400' ASL.

Good luck, but without any car data, it's difficult to tell all the factors that are harming you.
 

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I usually average about 40 if I'm going 75, but headwinds, altitude changes, tire pressure, and weather can significantly worsen that. If there are sustained winds or a one-way altitude change, you might notice much improved MPG on the way back. If you don't have a check engine light, the engine is operating within the designed parameters.
 

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Really low temps can also affect it, as I've learned these last few weeks.

I generally found that "low-ish" (28-40 degree) temps have a moderate effect on both my electric range and fuel efficiency (with all things equal, no heat use) when compared to optimal temps, I go from about 45 mpg down to 38-39 mpg. When temps went down to 10-15 degrees this week, my gas fuel efficiency went down to about 29 mpg.
 

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Just finished a 300 mile trip to Ct and back start, finish was on battery but highways pretty much 90% gas ..... i wasant hanging about , it was the mid 30’s out and she came back with a 49MPG average.... am I just a slow old fart now !! 😂
 

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that is normal. Remember that the force required to counteract air resistance is proportional to the square of the velocity, but the power required to counteract air resistance is proportional to the cube of the velocity.

The fact that air is more dense at lower temperature compared to higher temperature. A winter temperature of 40 vs spring temperature of 75 means heavier air by about 7.4% and that's the added direct penalty to power consumption.

Wet roads also add more tire resistance as tires become sticky, if there's a layer of water that needs to be parted, that could add up to, so additional force and power required, and it also picks up the water and hurl it centrifugally, and that is also a factor of the square of the velocity.
 

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Your graph shows that your efficiency changed and became worse in mid-September. Did anything change at that time
Like your gas changing to winter fuel mix? Most gas stations in the northern part of the US change over to winter blend in the Sept/Oct time frame, and back again in March/April. I saw a pronounced difference, even when running 91/93, in my old high-efficiency ICE before I got my Volt.
 
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