Marked drop in highway economy?

[smashweights]

So I'm midway through a long road trip we have made half a dozen or so times over the last 2 years. Normally I get 21-24mpg highway per the dash readout. So far I'm getting 18mpg, which is just barely better than my mostly city commute to work mileage. City mileage has been the same. We have basically the same load, same time of year, same cruise speed, ambient temps, same fuel grade, etc. Only thing different I can see is I got new tires. I know you can get a little drop with new tires but this seems excessive. What else might be the issue here? The old tires were like 4/32" tread so it's not a huge difference in tires either. Inflated properly to 38-39psi.

 

[Malodave]

My Ecodiesel dropped 2 MPG with my current tank of Diesel. I blame it on the Winter Mix of Diesel with the anti gel agent added in.

Malodave

 

[Geddyflea]

Has there been a OTA update on our trucks? Starting about 2 weeks ago the mpg shown on the dash dropped on my truck about 2 to 3 mpg. This was after an alignment and tire rotation. I would think the milage would have gotten better.

 

[User_33319]

My mileage has dropped about 2 mpg and I am sure it is due to the winter fuel. The last tank of fuel was when I noticed the fuel mileage drop.

 

[Geddyflea]

My mileage has dropped about 2 mpg and I am sure it is due to the winter fuel. The last tank of fuel was when I noticed the fuel mileage drop.

It's possible but I noticed the drop on mine between fill-ups. One day I was getting 16.4 mpg and the next day its 14.2.

 

[CatServant]

It’s natural for a vehicle to use more fuel in colder weather to maintain the fuel-air mix. Denser air means more fuel required.

 

[BowDown]

It’s natural for a vehicle to use more fuel in colder weather to maintain the fuel-air mix. Denser air means more fuel required.

This.
Its not a winter fuel change, its that the DA changes as temps drop, the same reason you make more power with cold air. Cold air is denser and as CatServant said, to maintain the correct AFR, more fuel is added hence lower fuel mileage

 

[Dewey]

Mine dropped at least 2.5 mpg in the last week with a 40 degree temp drop. Nothing unusual there. Bumping tire pressures back up after dropping in cold temps helps a bit. Mine lost 5 psi in the last week.

 

[GerryS]

The last few days have been pretty windy here. If I was travelling east, I could have just deployed a sail...How's the wind been where you are?

Gerry

 

[smashweights]

Well maybe it was a fluke. Drove the second half of my trip 200 miles and averaging 22mpg so seemed the ECO mode kicked in a lot more today on similar terrain

 

[djevox]

Yep, filled up last night and noticed halfway through the day today that I lost a bunch of range compared to normal.

 

[Jako]

Saturday left NYC for Wilkes Barre Pa at 4 am to avoid traffic. My mpg is usually 20+- on trips such as this and now I was only at 15 / 16. I was very concerned so on the way back I gassed up to calculate mpg with some other gas in the tank and do the mpg for each way separate.
GOING Out 178.8 miles, Ram 15.8 mpg, hand 16.30
COMING Back 106.9 miles, Ram 25.2 mpg, hand 25.74

Did the math with the numbers taking into consideration the miles (178.9 106.9) and it worked out to 19.83 mpg.

Didn't notice wind in the Ram but believe it played a part as well as an increase in elevation heading into Pa.

I have noticed mpg changes with wind but never to this level.

Best mpg ever, now if I could always drive downhill with wind at my back.

Tires were also down in psi do to the 20 degree weather. I had adjusted the psi for cold weather but not for the 20s.

 

[Mountain Whiskey]

I blame my right foot.

 

[User_33319]

Besides the denser air requiring more fuel to keep air fuel ratio optimized here are other impacts from an online article on the subject:

• Engine and transmission friction increases in cold temperature due to cold engine oil and other drive-line fluids.
• It takes longer for your engine to reach its most fuel efficient temperature. This affects shorter trips more, since your truck spends more of your trip at less than optimal temperatures.
• Heated seats, window defrosters, and heater fans use additional power.
• Warming up your truck before you start your trip lowers your fuel economy - idling gets 0 miles per gallon.
• Colder air is denser, increasing aerodynamic drag on your truck, especially at highway speeds.
• Tire pressure decreases in colder temperatures, increasing rolling resistance.
• Winter grades of gasoline can have slightly less energy per gallon than summer fuels.
• Battery performance decreases in cold weather, making it harder for your alternator to keep your battery charged. This also affects the performance of the regenerative braking system on hybrids, plug-in hybrids, and electric vehicles.

It would be interesting to know by percentage how much each of these causes your fuel mileage to decrease and which one is the biggest contributor.

 

[Mountain Whiskey]

Do you honestly think you can measure the difference in mpg due to friction of cold oil for 5 minutes vs 2 or the friction of air density and other nonsense? Come on. Perhaps mathematically or if you happen to be driving in the vacuum of space but not on the roads of this planet. The meter on the gas pump is not that accurate.

Warming up the truck does not count since you are not driving, you are parked running a motor, not driving. As pointed out, it is 0 mpg since you are going 0 miles.

 

[Neil McCauley]

^x2

Seems like once oil, coolant and transmission is all warmed to 200*F and air is moving 50+ mph, the outdoor temps would affect gas mileage only barely more than the color of the truck. If internal friction and fuel-air mixes were this finicky we'd have much bigger problems than gas.

 

[Mountain Whiskey]

Yea, I smell Engineer scat.