MPG drop? Cooler weather or something else?

POS VETT · October 20, 2015

Since the weather temperature has been going down in your area, have you checked tire pressures?

The change in the "viscosity" of air assumed to increase aerodynamic resistance is incredibly small. None of these trucks, I truly believe, are driven at speed remotely, remotely, remotely close to the speed of sound. Fuel consumption rate is not readily affected by "thicker air", trust me on this one.

tnchevy · October 20, 2015

I check my tire pressure on all my vehicles/motorcycles every 2 weeks and I "up" the pressure this time of year to compensate for 30 degree mornings and 75 degree evenings. All of my vehicles still lose 1 to 2 mpg during the cold months. I think most is the vehicle pumping on more fuel to get up to operating temps plus very thick oil, diff fluids, and trans fluids that have to warm up before they lean out.

BigBadSierra · October 20, 2015

Cold air has a higher air mass and you get a better burn from the fuel. So if the mass air flow sensor detects denser air it will increase the amount of fuel sent to each cylinder. This results in more power but will burn more fuel.

If it didn't do this the engine would operate lean and would run poorly and over time damage the engine.

Now how much fuel it gets comes down to the mapping GM put in. Which from what I've seen in the past always seems to be on the slightly rich side.

This is the gray area that the some hand tuners can play with to save your fuel.

robvas · October 20, 2015

Cold air has a higher air mass and you get a better burn from the fuel. So if the mass air flow sensor detects denser air it will increase the amount of fuel sent to each cylinder. This results in more power but will burn more fuel.

If it didn't do this the engine would operate lean and would run poorly and over time damage the engine.

Now how much fuel it gets comes down to the mapping GM put in. Which from what I've seen in the past always seems to be on the slightly rich side.

This is the gray area that the some hand tuners can play with to save your fuel.

This has nothing to do with it. Cold intake air makes your engine more efficient.

The EPA ratings for a vehicle are at 72 degrees F. There are a lot of little reasons why your car has worst mpg when it's cold than when it's hot.

Let's go over the little reasons first:

You're running your heater, electric defoggers, fan/AC for defrost, heated seats, rear window defrost, your alternator/battery have to work a little harder...

You have winter blend gas. This alone is only 1/2% less efficient

Lower PSI in your tires. Make sure you re-inflate when it's colder

Cold air sucks heat away from your engine block causing cylinders to lose heat

Cold air sucks heat away from your exhaust causing you to lose exhaust velocity

This might not apply to you, but you might start your car 5-10 minutes before you leave to 'warm up'

Those add up, but here are the two big reasons:

It takes longer for your engine oil, rear diff fluid, and transmission fluid to warm up. So it's less efficient to turn those parts with the cold, thick oil in them.

It takes longer for your engine to get to operating temperature, so your engine runs in 'open loop' so it's far more inefficient

Short trips are going to kill you when it's cold out. But your car/truck really won't be any less efficient on longer trips on the highway, you're overcoming the cold start losses.

robvas · October 20, 2015

The change in the "viscosity" of air assumed to increase aerodynamic resistance is incredibly small. None of these trucks, I truly believe, are driven at speed remotely, remotely, remotely close to the speed of sound. Fuel consumption rate is not readily affected by "thicker air", trust me on this one.

makes you wonder what some people hear/believe doesn't it?

BigBadSierra · October 20, 2015

makes you wonder what some people hear/believe doesn't it?

Sorry Air Mass has everything to do with it in a fuel injected engine.

If your mass air sensor fails you will burn through gas like mad.

Now that being said other things like you brought up cause it to fall and it all adds up.

But basics More air = more power will result in more fuel burned.

TxTruckMan · October 20, 2015

Sorry Air Mass has everything to do with it in a fuel injected engine.

If your mass air sensor fails you will burn through gas like mad.

Now that being said other things like you brought up cause it to fall and it all adds up.

But basics More air = more power will result in more fuel burned.

Yes, but more power means more speed too, right? So this really isn't a player. You're saying that you will accelerate faster when the air is denser - true, but the driver will compensate by letting off the accelerator. It's a wash.

BigBadSierra · October 21, 2015

Yes, but more power means more speed too, right? So this really isn't a player. You're saying that you will accelerate faster when the air is denser - true, but the driver will compensate by letting off the accelerator. It's a wash.

The computers will put more gas in per cylinder regardless of how hard you press the gas.

It has to match up the fuel amount to the air mass or the engine will start to run lean.

Even idling it will use more fuel than it did with warmer less dense air.

BeastCobbM5 · October 21, 2015

This is very odd. Since the cooler weather has started and I'm not constantly running my A/C I find I am gaining about 2mpg. Not sure when the "Winter Blend" starts down here but definitely see the difference. The past few weeks I've been averaging 21mpg.