Sudden fuel mileage decrease

[Lapslah]

The other day I made a 600 mile round-trip from Illinois to Iowa in my '15 Legacy 2.5 Premium. This trip is 100% open-road, 4-lane highways, straight and level. I have always averaged close to 36 mpg (by the display - we all know reality is slightly less) in this car under similar conditions.

Filled up on my way out of town, and all was close enough to normal - averaging about 33. After about 100 miles I was driving along with the cruise set around 70 and noticed the mpg bar graph suddenly down and average mpg dropping pretty quickly. I switched over to current mpg display and it says 22 mpg. Again - open road, flat and level (I even switched the nav display to show me elevation to confirm), cruise on, no other changes.

After a while of this I pulled off at the next rest stop and checked that my hood was closed all the way and checked my tire pressure - all good.

Continued on to pick up my son from college in Iowa, and the mileage improved slightly, but only up to about 25. Car sat parked on campus for about an hour, then filled up again with fresh gas (was down to about half) and back on the road, hoping it was just a wicked headwind that would now be at my back.

On the way home I averaged about 32 mpg for the first 150 miles, then again the mileage dropped into the mid-20s for the rest of the trip.

What gives?

[Pilot1226]

Is it possible they switched to the winter blend of fuel? That gets lower gas mileage, plus the impact of idling a car and the car not being at normal temperature.

 

I get around 18 city in my 2011 2.5 when the winter blend hits.

[Lapslah]

Don't think that could be it. Here in the Chicago area the change to winter blend occurred 3 months ago, mid-September.

[Jdog81]

tire pressures dropped lower possibly or maybe a fault with the air flap in the front of the car

[gathermewool]

Is this the first time you've driven this 600-mile trip? If so, then not much can be gleaned from the little bit of info we have so far. Unless you also noticed lost performance or can provide us some other information regarding the drive (wind, traffic, etc.) that might have contributed to the lower mileage, we'll have to wait until you burn through your next tank or two, after driving your usual route.

 

Wind has a huge impact on MPG.

 

Traffic has another huge impact; however, I've found that moderate traffic usually helps my gas mileage, since I have no issues leaving space and coasting (not constantly gas-brake-gas-brake, like many people seem to like to do), so my average MPH is lower. I'm sure the close proximity and volume of cars causes a bit of a drafting effect, which likely helps reduce wind resistance.

 

Comparatively, all else being the same, I get worse gas mileage when I set the cruise high and drive with very little or no traffic.

[Da Roo]

Maybe you need to change your oil or air filter. Just throwing out ideas.

 

Just an FYI, anything above 65mph results in poor mpg. The car will suck up about 15% more on avg.

 

FWIW, my in town mpg is normally around 25-27, so reducing that by 15% falls in your 22 range.

 

Sent from my LG-H871 using Tapatalk

[ammcinnis]

Wind has a huge impact on [highway] MPG.

 

Worth repeating. Highway fuel consumption is roughly proportional to the square of airspeed (i.e. road speed +/- wind speed). Even a crosswind costs you extra fuel.

 

Out here in the Great Plains, the wind has been howling (35-40 mph) for the past few weeks, and the direction varies significantly from day to day.

[pshadden]

It's just the Iowa City curse (I assume UI since you're from Illinois and I don't know of anyone from there who went to any other Iowa college by choice)

[YeuEmMaiMai]

did you notice a loss of heat? a sudden mileage drop like that indicates leak or car is cold.

[GTEASER]

Winter fuel, colder denser air needs more fuel to maintain air/fuel stoichiometry, colder air in tires reduces air pressure in tires creating more rolling resistance, colder denser air to push the car through, typically windier in winter, longer engine warm up times require more fuel to get to operating temp, etc. It's just winter and mileage goes down for everyone.

[waldguy]

Same car, same trip factors:

 

High factor (30% +/- swings):

COLD - dense air, energy to warm moving parts

WIND - what ammcinnis said, any wind resistance (eg car top carrier)

[miscalculations or faulty guages]

 

Medium Factor (10% +/- swings):

ELECTRICAL LOAD - eg A/C on Max

TIRE PRESSURE

WEIGHT

FUEL GRADES

 

Marginal Factor (2% +/-)

OIL WEAR OR TYPE

AIR FILTER

[ammcinnis]

... colder denser air needs more fuel to maintain air/fuel stoichiometry ...

 

I have to take issue with this point. Consider:

 

• Our engines use a mass airflow sensor (MAF), which measures actual intake air mass directly, independent of ambient air pressure and temperature. (For this discussion, I'm purposely ignoring the minor effect of humidity on air density.)
  
• It is air density in the cylinders, not ambient air density, that determines the fueling requirement.
  
• Except at full throttle, the air density entering the cylinders is always substantially lower than in the intake tract upstream of the throttle plate. Restricting airflow is what the throttle plate is there for.
  
• Any residual fueling error is automatically trimmed out by the ECM ... operating in "closed-loop" mode, using data from the exhaust O2 sensor(s) ... more than 10 times per second.
  

Bottom line is that the engine fuel system design removes any dependence on ambient air density during normal operation.

 

That said, engine pumping losses will be slightly higher with increased ambient air density, and that does lower fuel economy by some small amount.

[GTEASER]

But a MAF measures air in grams per second (g/s).

 

Assuming a constant pressure, would you agree that cold air is more dense? In other words, that there are more air molecules (the weight of which are being measured in grams) in a given volume, say a liter, at a cold temperature than at a warmer temperature?

 

An engine can only ingest a set volume at a given rpm because of the physical constraints of the cylinder size. If that cylinder is ingesting a colder temperature of air, then there are more oxygen molecules in that volume than there would be at a warmer temperature, correct? Therefore, more fuel is needed to maintain the appropriate combustion ratio of air to fuel.

 

Conversely, the engine cannot ingest less air simply because the air is colder and more dense, it ingests a set volume regardless of density. So fuel must be adjusted.

 

This is why you can tune a dirt bike engine to run perfect at sea level, but need to rejet it to run properly at 6000 feet of elevation. It will run rich at 6000 feet. You need to put a smaller jet in it because the air density is less. The ECU in a car compensates for this automatically by taking into account the MAF reading in g/s, adding the correct fuel, and self-checking with the o2 sensor to ensure the correct result of a ~14.7 AFR.

[WILLINH]

Winter blend

[tornichoe]

Wind is usually a big factor. winter or summer. The 36 mpg you get is usually in conditions with absolutely no wind or when the wind is actually pushing you down the road.

If you did the 600 mi on a day with 35+ mph winds, its that much more the engine has to work against.

 

Did you roll down windows or was the moon roof open? those add to aerodynamic drag.

Check the under carriage to see if any of the clips fell off and the plastic down there is flapping away.

[Lapslah]

It's just the Iowa City curse (I assume UI since you're from Illinois and I don't know of anyone from there who went to any other Iowa college by choice)

 

That's only because some schools are so superior they were clearly out of reach for someone who would consider IU.

[balbrec2]

Colder weather drops my mpg by 3-4. All the previous reasons are valid.

Also I've noticed the engine just runs cooler in winter than summer. My oil temp

just reaches 200 in winter yet up to 230 on a hot summer day. Surely this must make the engine somewhat less efficient.

[ehsnils]

Winter blend

Is only responsible for about 2% of the difference, maybe even less.

 

People tends to blame that instead of realizing the added oil viscosity, warm-up time, heated seats and the idling added when you have to get rid of the mist from the windscreen that are much more important factors.

[WILLINH]

I don't buy that, when gas goes from winter blend to summer blend I notice a difference right away. Warm up time and idle time yes.Heated seats na and we have wind year round.

[Brisvegas]

Saw this today.

 

Nothing new really, but at least some figures are provided to show what we all knew anyway.

 

 

[YeuEmMaiMai]

in all of my years driving I have never seen a car like my 2014... my average dropped from 32mpg down to 23.4 due to winter. I drive the same route every day and I do not let the car sit and warm up... damned near 30% fuel economy drop.

 

2003 Legacy 2.5L H4 dropped from 24.X down to 22.x roughly 2 mpg

2002 Acura CL-S 3.2L V6 dropped from 25x down to 24x roughly 1 mpg

1998 Accord 3.0L V6 remained relatively the same at 25mpg...

1994 Accord 2.2L remained relatively unchanged at 28mpg....

[capttris]

I drive about 15,000 a year for work, mostly all highway, long 300-350 mile round trips in a day. I'm always watching MPG.

My thoughts:

1. Were you possibly going up a slight elevation most or part of the trip? You may not have even realized it if it was gradual.

2. I've found that below 30F degrees or so, the MPG's drop even faster

3. like others have said, higher speeds = lower MPG. I got 38 MPG over 150 miles when I was doing straight 58-60 MPH. But doing 70+ will drop me to 30-29 especially in the winter.

[capttris]

It's funny, I was reading another post about a stuck-open thermostat.

This is also a possibility. This could cause reduced MPG.

Something to look at.

[balbrec2]

When the ambient temp is 20F how long should it take for the

temp gauge to reach the middle of the range, driving on the expressway at 60mph?

I realize these engines are relatively cold blooded compared many other makes.

[YeuEmMaiMai]

my friend states he as full temp within 4 miles

 

me it took about 12 miles before I replaced my t-stat now it takes about 4-5 miles