Decrease in gas mileage on 2010 prius

I have notice decreased gas mileage on my 2010 Prius and I am wondering if this is just normal age related wearing out of either the ICE or hybrid system or if there is something I can do about it. Here are some facts. Thanks for any help.

I drive the exact same route to and from work every day. It is approximately 50 miles (mostly hwy) each way. I have been doing this since the car had 4K miles on it and it now has about 70K. (2.5 years)

All mileage figures quoted are from the onboard display, which I reset every trip.

For about the first 36000 miles I reliably got warm-weather mileage of about 57 mpg to work, and 50 returning (probably due to elevation difference or wind I assume?)

Since then the warm-weather mileage dropped to about 48 to work and 42 returning. Approximately a 8 mpg drop in fuel economy. It's like I'm now getting Winter mileage all Summer now.

To my knowledge, nothing has changed that could account for this.

I replaced the tires at around 50K miles with the OEM brand, but there was no change in mpg.

Same 0w20 synthetic oil every change, good tire pressure, air cleaner replaced, engine top end cleaned recently. No improvement in mileage.

What could be causing this?

 

Bob, thanks for the reply. I am not sure why it would be useful to compare my mpg to the above benchmark since in my case I have the benefit of established driving history over the exact same trip every weekday for 2.5 years.

I am definitely interested in getting the metrics you mentioned. Should I go to the dealer and ask for this scan or is this something I can do at home with the purchase of some data interface device perhaps for a PC? I have such a device for reading OBD-II info but that is all.

Are there any self tests using the dash indicator like on the Gen 2 cars for battery condition?

 

I doubt it, but I will look into it. The reason I don't think it is the 12v is because the care starts easily and after idling for about only a minute the engine shits off which makes me think the battery is full charged (Just guessing here, but wouldn't it keep running if it was needing charge?), which makes me think it must have held its charge overnight pretty well. Can I simply test it with a volt meter?

 

How long do the high voltage batteries last on these? I assume that as they age they can hold less and less charge and so the ICE will be required to run a greater percentage of the time. I wish Toyota provided more data on the electrical drive system than just the little picture on the dash. I think many technically minder owners would appreciate basic parameters like instantaneous current flow, voltage, battery charge cycles, total energy use per trip, etc, with real numbers rather than little stacks of cars.

 

Yes, but what exactly is the HV battery warranted to do for this period? Are the conditions of that warranty spelled out somewhere? I would like to read up on it. I am guessing since all batteries weaken over time it may only be warranted against failure. To cover weakened batteries they would need to specify some level of minimum energy capacity below which it is considered unacceptable. Have you heard of any such thing?

 

This is great information guys. Thanks! I had no idea the HV battery started the car and was responsible for charging the 12v. I will go and measure the 12v voltage now and report back.

 

I just measured the 12v battery. After sitting overnight the voltage was 12.6.
After pressing the start button twice (accessory mode) it was 12.3, and after pressing start with foot on brake (run mode) it dropped t0 11.9 for a split second and then went up to 14.3.

 

Thanks. I already read that warranty document when I bought my car. It has no specifics about what qualifies as a good or bad battery. I suspect you are right about (c) though. If that is the case it is fairly useless coverage and only protects against catastrophic failures.

 

Thanks. That is the sort of info I was trying to find out. I was wondering if my drop in mileage was due to wear and the first thing that came to mind was the HV battery. I have no basis for this at all other than my experience with other devices that use rechargeable batteries. The batteries always seem to be the weakest link. I have a friend who is a tech at my local Toyota. I am going to ask him if he can hook up some expensive test equipment and tell me about the condition of my battery. Surely they must have a battery health metric in there somewhere. I did mention the mileage drop to him last time I was in and that's when he changed the air filter. Made no difference though.

Any suggestion of exactly what I should ask him to look for? Or should I just say mileage has dropped and leave it up to them to check? I just got the feeling they didn't take it very seriously last time. I really love the car and would like to get my old fuel economy back if possible.

 

Thanks. I guess my plan, then, is to go to the dealer and have them pull the codes at the next oil change.

I have already checked tire pressure, changed air filter, and had a treatment done to clean throttle body/top end.

I am ready to start searching deeper through the forums for answers. My primary reason is starting this thread was to first find out if this gradual drop in fuel economy was normal and something everyone was experiencing. In my initial searches so far I didn't find any other threads describing dropping mpg over time, and I would imagine that if this was indeed happening people would be up in arms about it and it would be all over the net.

Thanks for all the input so far. Any pointers to important threads you guys can think off would be appreciated (also any important questions to ask at the dealer's to help guide them to the problem).