MPG Anomaly!

[FONT=&quot]I have my Prius for about 4,000 miles now and have consistently hit between 58-60 mpg on the display per tank. My all time (no reset on trip B )is showing 58.4 Curiously, my current tank with about 240 miles started a nosedive to 56.7 – 56.9 which is hardly a winks difference to some, but it matters to me. I did not change my route, temps are about the same 55-66F and no change to the vehicle has been made. I was about to check tire pressure this evening when the MFD began to inch up again to 57-57.7 but still not to the 60 it was before. Whatever happened has put a dent in the average for the tank. Has anyone else experienced this? And what other possibilities can cause this? (In addition to route, driving style, temps and PSI) I ruled out variation in gas quality because the change minus / plus FE all occurred during the current tank.
Any Ideas?[/FONT]

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

Thanks everyone, since my original post the FE has gone up and down erratically. After reading all the posts I started looking more closely at the outside temps. Last few days they have been dipping to 42-50 and I used the heater (front defogger). I have experience with the Altima hybrid and the winter FE hit, but for some reason the Prius seems more sensitive with temps. The Altima did not suffer till temps were in the 30'sF. I am concerned that the temps alone have affected FE drastically even before the winter formula gas hit. Thanks again, will keep monitoring the issue.

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.

Thanks Bob, I have to chill out a little with this MPG thing. I'm holding on too tightly.

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

Tom

Thanks, will keep an eye on it. Lets see what happens in the 30's f

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.

Yep, I get it. Just did'nt expect to start seeing mpg hit until the 30's temps and winter fuel. Not disappointed in the overall FE performance though.

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.