2012 Toyota Prius v - Reduced mileage

In its early years, my Prius V got an overall average of about 42.5 MPG. As it approaches the latter part of its ninth year, with just over 90,000 miles, the car is averaging about 36 MPG in driving conditions that remain the same except that they’re just in summer, not including the winter months when the car is less efficient. So I’ve lost more than 15% in fuel efficiency. What should I do? I would like to keep the car another 5 years or so and buy an all-electric vehicle at that point.

Great question for the Prius club on Facebook. However, here are the first things I would check before assuming it is an issue with the traction battery.

  • How closely do you monitor your tire pressure?
  • Are the tires you have now the EXACT same tires the car came with when you got better mileage. Meaning same make and model (and size of course)
  • Have you performed the usual maintenance? Engine air filter, fluids, and any engine things in the manual?
  • Have you inspected the brakes closely to ensure you are not dragging one or two calipers?
    None of this cost money to check. If you do all that, then maybe it’s time to have a mechanic look at it.
    Last comment. You are checking your mileage at the pump, right? Miles driven divided by gallons input from fillup- to fillup? Not using the in-dash display?
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Check if the thermostat is stuck open/broke.



Good advise above on checking if your reduced MPG is caused/coincides with replacing tires having more rolling resistance, air pressure, brakes, thermostat.

When did you replace your 12V (auxiliary) battery last time?
If more than 6 years ago, it’s worth a shot to replace it before it fails.
Surprisingly, it bears a penalty on MPG not to replace it when it is on its last leg - you will find that on Prius specific forums and in my case it was true when replacing 7-years old battery.

What about your torque (high voltage) battery?
Do you notice it gets discharged faster and instead of hanging around 40…60% charge your Prius tries to charge it to max once you start driving?
If so, it may be a sign that some of battery sections are “out of balance” and car tries to “train them”.
Unfortunately older generation of Prii suffers from amnesia and will try doing it after every start, resulting in lesser MPG.
If you are DIY guy, you can get worn cells replaced before the hybrid system declares the battery failure condition and it will not only prevent the trouble, but will prolong the other cells life too.
Check Prius specific forums and repair videos on Youtube.

Unfortunately, what you’re experiencing is more-or-less normal. As the hybrid battery ages it becomes less efficient and your mileage drops.

I doubt if any attempt to fix it like replacing cells in your hybrid battery will cost less thanthe difference between 36 mpg and 42.

It is definitely NOT normal… or let’s say, it is not normal for Toyota.

I know some makes like Ford/Linkoln made a really strange decision to unconditionally reduce hybrid system assistance percentage with age, but it is not Toyota’s take.

If OP cares to research the topic, NYC taxi drivers successfully run their Prii well past 300K miles on original batteries, so 90K miles and 8 years of age are nowhere close to where efficiency gets reduced.

Here is a picture from my 2005 Prius with 127K miles as an example:

Disclaimer: this was after the bettery failed 3 of cells (out of 28) at the 117K miles, due to prior owner’ wife using it very sparingly (3-4 trips a year), I replaced ~10 cells, including ones with questionable performance.

I know some people tend to consider hybrids to run on some kind of magic and not to be touched, but realistically they are not much different from the “regular” cars, and both can be repaired and require regular maintenance to operate properly.

It is much less debate here that after certain age 12V battery on the “regular” cars has to be replaced (or at least TESTED for capacity and internal resistance), but given that 12V batteries in hybrids are not in the path of “cranking” load for some reason owners think they last until they completely die… not true, they bring ill effects once they are past 6-7 years of age and same as on the “regular” cars they have to be replaced.

My first bet is that OP’s 12V battery needs replacement and he will get 2-3 MPG boost just from that.

If tires installed are not low rolling resistance type - that would easily explain another 2-3 MPG.

@StephenBurrington, please let us know when/if you get your issues addressed and how!
It’s good to know for all people who happen to stumble upon this thread in search of the answer to similar issue.

Thanks to everyone’s who’s offered suggestions. I recently changed what my mechanic (at a garage on Hamilton Street in Cambridge) refers to as the starter battery, and have the car regularly maintained. One possible change since the car’s early years is a change in tire model, though the tire is a very similar type. I’ll consider this fully addressed for now but keep an eye out for further suggestions.

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Well, he doesn’t understand how your Prius works. The 12V battery doesn’t start your car, the big battery does.

if you check track results from TireRack reviews, tires of “similar type” routinely get up to 6-7% difference in MPG in comparative results.

If you happen to replace low-rolling-resistance tire with some high-performance one, that difference can probably get above 10%… I’m not claiming to be an expert here, just making observations.

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Two or three bad battery cells in the hybrid battery pack will noticeably knock down mileage. The batteries are connected in series and the pack is only as strong as its weakest cell. Corroded connections between batteries cause efficiency loss as well. If you pay a qualified shop to check your battery for both voltage and load, replace low performing cells, and then recondition the battery so everything matches, it will take a very long time to pay back the investment. Another option is a drop-in complete refurbished battery. This is also not cheap if you want a freshly refurbished battery with a decent warranty.

If you just check voltage with a cell phone app and replace the low voltage cells with junk yard cells, you might get your mileage back for just a few dollars and an afternoon’s work. That would be my first choice if it were my car. Be sure to polish all the electrical contact points on all the batteries. BUT if you do this quick and dirty battery cell swap based on a voltage-only test, you might do little or no good, and you might electrocute yourself if you don’t know what you are doing.

It does not look like @StephenBurrington is going to work on the high-voltage battery himself, so any kind of refurbishing operation will not be cheap.

Prius itself provides quite a detailed diagnostics on individual battery banks (consisting of two cells each). Here is my 2005 Prius recent battery check printed out from TechStream software:

It is easy to find instructions over Prius forums how to make regular cell phone with Torque Pro software to get similar data pulled.

It shows both voltages and internal resistance, so it is more than enough to determine which of 14 banks are in good condition and which are not. In my particular case, they are quite even on both voltage and resistance, so system operates as designed and it routinely gets 50+ MPG even afetr getting new tires not marked for low rolling resistance.