MPG Anomaly!

Somebody else drove it. Happens all the time to me when my wife drives the Prius. Other than that I have no idea why. The colder temps here have dropped my FE by about 2 mpg but it was a steady decline as the temps dropped. I blocked off the lower grill and much to my surprise I gained about about a 1/2 of a mpg.

 

Winter gas could be in stock now at your local gas station...this is not a myth. This combined with lower temps will certainly result in mpg loss. I'm running at about 62 mpg right now and I am usually in the 64-65 range during warmer summer months with normal gas.

 

Lower SOC on the traction battery during one of the first starts of the day could do it in my opinion.
SOC has such an effect on mileage it's kind of a wild card in computing mileage.
Just a little informal playing with P&G initially results in great mileage until the SOC drops, then the mileage advantage of P&G sort of goes away, to some extent.
Oh and heater use of course as well, I see your in Maine and I am just guessing there

 

Yes, I agree, winter gas formulations, at least in the past, will lower your MPG because there is less BTU per gallon.

Here in Oregon 10% Ethanol is madated year round. In the past Oregon only mandated 10% Ethanol gas during winter months.

Back in the 90's I had a 1994 Honda Civic 5 speed (VX) model, which was designed for pure MPG's.

With normal gas I would get in the low to mid 50's mpg. With Ethanol gas it would drop to the high 40's, consistently.


alfon

 

Don't think it's gas this time.

 

My guess is winter gas. It makes a difference.

Tom

 

Spoils,

I don't think you have to much to be concerned about. Here it has been in the low 30's at night and I work 2nd shift and I have used the Heater more, not Defrost but heater, and I have noticed about a 2 MPG hit as a result. I can tell you that had I hung on to my Civic Hybrid the results were far worse. About 8MPG hit in these temps, so the Prius is definitely shining through compared to the Civic. And as many posters mentioned they like to start to play with our Gas at this time, adding more Ethanol to the mix = a little rediced mileage. But all in all I see this 2010 as being far less affected by the colder weather.

 

I missed the single tank detail in the OP. In that case it will be temperature. Cold weather is a mileage killer.

Tom

 

The Altima also got significantly worse MPG. If you experience a 10% hit on MPG for some reason then you will see a larger drop (number) with the Prius (60mpg) than the Altima (30mpg). If you had been driving a 16mpg gas hog you likely wouldn't even notice a number drop but the effect would still be worse MPG.

*The numbers above are just for e.g.

 

/\/\If F8L hadn't said it, I would have./\/\

 

changes in mileage in middle of tank is usually temps and using the defroster. the heater would have to be extreme use to make a diff, but mile for mile, defrost is a bigger hit than AC.

AC eventually uses more since it runs more. i usually blast defrost at first and flip it on and off as needed.

 

Why would defrost cause a bigger hit in mileage than the heater would? Don't both cause the engine to run more in order to generate heat in the coolant that is used for both the defroster and the heater? Isn't the reason for the decreased mileage the increase run time of the ICE?

 

In defrost you are not only taking heat out of the engine but the AC compressor is also running.

Wayne

 

AC compressor runs in the summer as well and doesn't cut mileage significantly. Yes I understand any energy used comes from the fuel tank, just the AC apparently isn't much of a hit compared to the use of heat, wherever it's put.

 

I've lost about 10 mpg with my last fillup (~60 to ~50), and suspect it's primarily due to winter gas and colder temps. In my other vehicles, I generally take a 5-10% hit but this is closer to 15%. I'll see what happens next tank (fill up every 4 days or so) to try to rule out other factors.

A quick follow-up:
MPG has been steadily climbing the last couple days and is back up to ~55 indicated. It seems that the engine computer had to adjust to the new fuel mixture of winter gas -- that's my best guess, anyway -- since all other things were relatively equal. Next tank (fill up today followed by next Monday) will tell more about the true drop.