MPG going down...HELP!!!

Hmmm, sounds a little strange.
I'm sure that one of the poseters here will be able to make some better suggestions, but: Are you using the A/C or other battery dependant devises more then before? Drop in external temperatures?

 

Is this your first winter in the Prius? The MPGs of all cars drop with the temperature; it's just more obvious in a Prius. Check tire pressures at least monthly.

 

That sounds normal... for any vehicle. It's the seasonal cycle, caused by factors totally out of your control. Temperature & Fuel changes are the major influences which many people simply aren't aware of until having a Multi-Display. Having a Prius brings a whole new meaning to the coming of Spring.

Here's a summary of the 5 years of driving data with my Prius:




Details and more graphs are available on my website.

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How long ago did you replace the tires? I noticed about a 15-20% drop in mileage for about 1000 miles when I replaced my tires recently. Thankfully everything for me returned to normal though afterwords.

Temps can also have significant effect. With temps in the 70-80F here in the hilly SF Bay Area I average around 55 MPG in mixed freeway driving but that drops into the mid to upper 40 MPG range when temps are much below 60F.

Cheers,
-- Ian

 

My guess is John has hit on the key difference--the coming of cooler weather. But if you've been through the seasons with this car before (and with 24,000 miles it seems likely) then perhaps not.

With regards to the battery--you don't have anything covering the battery ventilation duct on the rear passenger side do you? You don't have dogs or lots of dust, etc. in the car that could clog the battery's ventilation blower, right? If the battery gets too warm then it won't charge or discharge normally.

It does seem odd that the battery is not taking more charge, but the SOC indicator has some ability to reset itself if I understand correctly. My '08 likes to stay in the bar just below green. And on steep or long downhills it frequently goes up a bar into the green. (Elevation changes here are usually only 100 to 200 feet although steep and frequent.) I occasionally draw it down two below and once or twice three below green in some ICE-off flat stretches of over a mile. I'm a little puzzled as to why it would not hit green during regen in WV mountains. I would expect it to max out with two green bars on the steep, long stretches. Green isn't really good though so don't aim for it. It represents some excess charge. High blue is the actual target zone of the control system as I understand it.

New tires are probably costing you mileage as the originals are very low rolling resistance. They aren't the best for handling, but they are fuel efficient. Typical observation is that tires take several thousand miles to break in and that mileage will improve as they wear.

You might want to check oil level unless you did the changes yourself and have already verified level. Unfortunately, Toyota's own specs will result is a slight overfill. Excessive overfill can reduce mileage or cause other more serious problems.

Does the ICE shut off normally or do you find that it is stubbornly running at times it used to be off? (If it is always trying to charge the battery that might be the case.) Also, alignment problems could result in more steering input and more drag/rolling resistance, and therefore the engine running more often while moving.

Any chance of seasonal fuel blend changes for the region?

Check tire pressure. Lower temp = lower pressure. Time = lower pressure too. (Defroster definitely indicates low temp.)

I doubt any one of these would fully explain the difference you are seeing, but several might.

 

Forgot to ask about the tires... Once you pass the break-in penalty, they won't ever be quite the same anyway. The impression is that those from Japan aren't quite the same as the ones available locally, even if you do get the same name & size. And if you did get a different type, change is inevitable. I upgraded to ones with greater traction, despite awareness of a permanent MPG drop.

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Don't panic. If they put in too much oil that can reduce MPGs.
Tomorrow morning before you drive the car check the tire pressures and the oil level.

 

In Washington they begin adding 10% ethanol to the gas in the fall. I noticed a drop in mileage the last couple of tanks. It is exaggerated in the Prius vs standard cars.