Why did my mpg drop suddenly today?

My first thought is Air Conditioning. Yesterday and today are the first days this year that I've used it. If not set conservatively, A/C can drag your mileage down. Any chance?

 

If you have not pressed the Consumption screen "Reset" for thousands of miles, what you've been seeing for FE has been a running average of the FE for all those thousands of miles.

Now you will see some wild "swings" until your running average grows again.

Just a simple math issue really.

Best wishes,

Frank

 

Since you pressed the reset button it will only show your average with the few miles you have driven since hitting the reset instead of the many miles driven before. This will cause your "average" to flucuate alot more than before.

 

Good question. I took my Civic Hybrid in because the battery went to zero while I was driving it and the mileage went down also for the last 6 months. They said everything was fine and shoved me out the door. Dealers in general do not care unless the vehicle is visibly broke. Tire pressure, oil, AC, tires, electronics, AC on low fan and recirculate.

 

Don't worry about it. Your battery is probably fine. Every once in awhile, the Battery ECU needs to recalibrate itself. You will notice unusualy activity during that time. Yes your mileage will drop. I expect your mileage to go back to normal. If not, let us know.

 

In addition to "AC on low fan and recirculate.", set the temp to the highest comfortable temperature.

Have you recently had your Prius serviced by the dealer? As mentioned above by ETP, dealers tend to overfill the oil and reduce the tire pressure. Both of these can negatively affect mpg.

 

Speaking of dealers, I've also heard that if the 12v battery is disconnected for some kind of electrical servicing, the ECU resets and has to relearn your driving habits over a couple of weeks.

 

I would not worry, occasional MPG loss without any obvious reason is normal in my experience. It happens to me too. I would start to worry only if it drops in a long term. Battery recalibration is needed from time to time; The hybrid system controller drains it just to the allowed limit and then lets it recharge almost fully. This can hinder your MPG. BTW, congrats to your splendid MPG! I only get between 50-55 a tank in my gen II, I must improve my hypermiling skills I guess...

When you are drawing air from outside, it needs to be cooled/warmed to match your desired inside temperature. When you use recirculation, the air inside the cabin already has the desired temperature (if the AC had been running for a while), so only little energy is needed to keep it from getting warmer or cooler, depending on outside temperature. Therefore, the AC is drawing much less power when the air is recirculated. But you still need to refresh the air from time to time, or you would get very uncomfortable with oxygen levels going down and CO2 levels rising :mod:.

Yes. When it is nice outside, you could even turn the fan off and open the driver's window and the opposite rear window just a little. This is sufficient to create enough airflow without hindering the aerodynamics much, and is probably a tiny better for your MPG .

 

I do not use my AC at all when I am alone. I just crack the windows a little. But when my wife is in the car and it is 90-110 outside you do not want that hot air coming into the cabin while running the AC. That would be like opening a window in your house and running the AC,

 

The traction battery starts to fail when it won't hold a charge. The symptons are rapid swings between fully charged & 2 bars and a decrease in fuel economy because the ICE runs more frequently to charge the battery.

I keep the windows up because I can't stand the noise. Running the AC with the temp high (~79) and the fan speed low doesn't seem to affect FE much.

 

Jim, this sounds very much like Dominic's description. I'd hate for him to panic over it. Perhaps he should continue to drive while keeping a sharp eye on the SOC to see if the HV battery returns to normal behavior, or continues to behave oddly.

 

I've also noticed a drop recently. I blocked the upper grill last week and noticed a drop of 4 mpg on the last tank. Of course, one tank does not a trend indicate, so I'm not really concerned.
I did notice this weekend that my mileage suffered from start-stop driving on a shopping trip to the Bangor area. OAT was about 84F and the spousal unit requested multiple stops at a number of retail establishments. A mile here, 2 miles there.
Once we got on the road back Downeast and was able to travel 55-60, things got back up to 52.
Scan Gauge ordered and EBH to be installed soon.

 

Now that you mention it I have noticed a correlation between FE and the proximity of the spousal unit. When she is not in the car, FE is close to 60. When she is in the passenger seat, it drops to 55MPG and the interior temperature drops to 72. When she is in the drivers seat, well, I don't want to talk about it.

 

Hello Dominic:

Sorry that I didn't understand that you always reset your FE after filling your tank.

As far as AC, I usually leave it on all the time unless I am running with the windows open. The AC compressor will only run as fast as needed, given the requested cabing temperature versus the external temperature. So when there is little temperature difference with the outside, the AC draw little power from the car.

Along with this, I let the climate system set the recirculate function as needed.

Please let us know if your FE issue goes away with your next tank, or if it persists.

Best wishes,

 

Really? I thought the AC is single speed (is there such a thing as a variable speed compressor?) and the engine coolant is used to warm up the air if higher temperature is required?