Reduced Gas Mileage

A mileage change like that could be anything: new tires, reformulated gas, resurfaced roads, a change in driving habits. You really can't tell. It's too small of a change to automatically indicate trouble.

Tom

 

Ditto that. Watch it for a few months.

 

Maybe you could have your hybrid battery checked out to ensure all or the majority of cells are good?

I agree that 46 to 41 is a fairly small change and many things can affect this change but if you have kept all of the variables fairly constant then maybe the exra milage on your car has indeed had an effect. I assume you are in the 70k-90k miles range?

 

could be new tires, winter gas, several things. I agree with everyone above. ditto on the air pressure check

 

I'd suspect alignment first. You should have it checked yearly. Tires next. Engine performance next.

Get your alignment checked. Get the before/after measurements and you'll easily be able to tell if anything was out of spec. If it was, there's your culprit.

Check your tire pressure. You should be doing this at least every month.

Is your engine oil clean/fresh? Changing at recommended intervals? Is it overfilled?

Run some Techron or other reputable injector cleaner through. I'd do that every oil change.

Check your engine air cleaner.

 

Up until today, i had been wanting to ask this same question. I got my 2007 prius on January 2. It had about 28K miles and was at around 43 or 44 mpg when i first started driving it. Gradually over the almost 4 months i've had the car, the mpg has gone down. For a long time it was over 43, staying pretty stable. Then, i wasn't used to the car so i wasn't pushing it as much as i have since i've gotten used to it. And i was trying to keep the mpg up, using techniques that seemed, intuitively, to work. But then it went under 43. It was in the 42 range for a long time, weeks, gradually getting closer to 41. Pretty stable, i think. Then it went under 42 and it remained in that range for more weeks. Eventually, weeks ago, it got to 41.1. I was amazed at the way it stayed at 41.1, week after week. Very stable. That seemed like a good thing, since my driving was always the same.

Then, all of a sudden, yesterday, i looked at the display and it was 37.8. The day before, i drove home from work like always and it was 41.1. I was shocked to see it that low, not just because it was relatively low but because of how it went down several mpg's overnight, and i hadn't driven any differently. I'd made about a 10 or 15 mile freeway trip in fast traffic yesterday, right before i noticed this. This is abnormal for this car.

I drove around residential streets slowly today, coasting along, and the mpg came up to 38 after about 10 minutes of very slow driving, mostly coasting at 99mpg. But if i gave the car gas at all, the mpg went down easily. It wasn't doing this before. It took a lot to get it to change, which i will say, i thought was odd.

Counter to my expectations, when i would go on long highway drives (100 mile round trip) with fast moving traffic, my mpg would not go up. It would even go down sometimes.

My normal driving is hurried. I drive a long commute to and from work in Los Angeles traffic. Going to work isn't bad, i have a car pool lane, it's great really, but coming home, I have a long slow gridlock drive on the 405 from mid city to the Valley which involves a long fairly steep gradual incline of about 10 miles in gridlock, several days a week. I also drive on my job at least a couple of days a week, i put on about 450 a month that i get mileage reimbursement for, not including my commute to the office.

In the almost 4 months i've had the car, i've put on, so far, over 5000 miles, i'm at 34K now, as of a couple of days ago.

I've been worrying about the battery, but from another thread, i got the idea that my batteries are normal. The car seemed to always be at 6 blue bars when i first got it. After about 6 or 8 weeks, i got a car wash for the first time. The car wash people apparently left my car running for a couple of hours and when i got it back it had one or two purple bars. I had never seen anything but blue bars before and was worried.

Since then, i often see purple bars. When i sit in my car to have lunch, if i leave the ignition on and the radio on, in less than 5 minutes, it goes from 6 blue bars to 2 purple bars. this bothered me. Why should it go down so fast? But on another thread i got the impression this is not abnormal.

Sometimes when i'm driving, accelerating, such as when i'm going up the 405 incline in rush hour, my battery goes down, whereas i would've thought it would go up. It goes down a lot, maybe 3 or more bars, it gets low, while i'm accelerating. This is counterintuitive to me, based on other cars i've had. Is this normal, to lose battery level quite a bit while driving and accelerating? I would've thought the ICE would charge the battery. Apparently the car is running off the battery at those times. I often have the climate control and radio on, or the bluetooth handsfree for the phone.

This car has had all of its routine service. I put new tires on it in January, Goodyear TripleTread, i can't remember the size numbers now, it was what was recommended in threads on prius chat. I had the tires inflated at about 41 front 39 rear. I haven't checked them in a couple of months. I just checked, the car hasn't been driven in several hours, it was 39 front, 37 rear. The weather is on the warm side, in the 70s in the garage at 11pm. The wheels were aligned when i got the tires, and then a few weeks later, i had a service at the dealer, 30K service, and according to the receipt, i think the tires were rotated, although i told them they had just been installed. I have no steering symptoms that would suggest the car is not in alignment.

I basically drive the same roads, or the same type of roads. The weather has gotten warmer recently.

I have two questions. One is, is there any explanation for why the mpg would gradually go down over time? I am a hurried driver, i have a lot of driving to do and time pressures. But my driving 'style' or whatever has not changed since the first month or so i had the prius. The second question is, why would the mpg suddenly go down several mpg's overnight? (also, is the battery behavior i described normal?)

 

Please read my thread regarding the same thing. Search, "After Service Issue". It may shed some light on the problem, but I haven't been able to come up with a cure.

 

Thank you for that info. I read the whole thread. Sorry to hear about this issue.

Thankfully, my car's episode was short. By the next day, it was trying to get back to normal, and has been normal ever since. There was no explanation for the sudden several MPG dip in terms of things like changes in driving habits, road conditions, fuel. I made a fairly short drive using typical conditions and driving habits. I assume this constancy is why my MPG has been stable around 41.4 for most of the time i've had the car. Then, suddenly it went down to 37.6 and for that day, it stayed there. The next day, it was coming up steadily on one short trip i made involving both freeway and surface streets. Since then, about a week and a half, it's been stable again at around 41.4. It goes up to 41.6 and it goes down to 41.1. This is just like it was before.

I'm interested in the possibility that a brake adjustment was involved, or some problem with the brake. Since i got the car, i've been surprised at how hard i've had to push the parking brake down to get the car to be secure. If i used a lighter touch, which i tried to do, a habit from my previous car, it didn't hold the car steady. When i'd take my foot off the driving brake, the car would shift a bit after i'd put the parking break on with a lighter pressure that i've been used to. And the driving brakes work so well, very sensitive, so this surprised me. I've gotten now so that i just press down hard because i want to be sure the car cant roll. One day when i'd put the brake on lightly, i went to get something out of the hatchback and when i leaned on the car, it rolled forward slightly.

I'm relieved and thankful that the gas mileage is back to what it was, but mystified about why it went down, and concerned.

 

The parking brake is supposedly auto-adjusting... Try pushing it down all the way, and release fully a few times... Then check again to see if its holds as well?

 

I have some of the same questions. I have been consistently increasing my average MPG to 49-51. After purchasing new tires the milage abruptly decreased to averaging about 43-44. Driving habits are very conservative and have not changed. I recently blew out the air filter. I was wondering if the summer gasoline with ethanol mixed in, which I believe is required by the 1st of May, could have made this much difference. I had been proud of my average and am now very discouraged!

 

New tires almost always drop your milage, at least for awhile. Even if you purchased the exact same brand/model/size of tire to replace your old ones with. There is much information on the subject. If you do a search in this forum for "tires" or "new tires" you'll get more information than you probably wanted.

 

Tires also need to "break in", and will slowly decrease in rolling resistance as they break in.

 

Tires will also become smaller in diameter as they wear down and this will increase RPM and the perceived milage counted on your odometer and MFD. This will give the illusion of better MPG above and beyond what you are truely getting due to reduced rolling resistance. It may be a minor effect but it is still an effect.

 

I've learned a few indiosyncracies with my '09. Maybe one or more of these may be your issue:
1.Gas: Some brands perform pretty crappy and other quite good. The best that works for me in my location/climate is Chevron and Shell.
2. Driving style: If in gingerly drive it I get better performance - that is it is easy to get it into stage 4 and keep it there. If I drive with a heavy foot, the opposite holds true.
3. Memory: I've learned the car has a memory. If I drive with a light to reasonable foot, I get into stage 4 easy almost all the time and if I use a heavy foot for a few minutes, I still get into state 4 easy. The opposite is also true. If I drive with a heavy foot then it is hard to get into stage 4, and if I do that for a while then drive very gingerly, I still have a very difficult time getting into state 4.
5. Temperature: Here in AZ the summer temps get quite high. I've learned if the outside temp pushes 95 or higher, it tends to drive off the engine more than the battery, or, stage 4 is hard to achieve. This is without use of the A/C.
6. A/C: When I use the A/C, the added draw on the battery makes the car use the engine more often to keep the battery charged. No brainer here, but I've also noticed that if the greater the differential between inside commanded temperature and outside temerature makes the variable duty-cycle compressor work longer and therefore draw the battery down. But, when the cabin is cooled to my command temperature, the battery draw drops and I'm able to get into stage 4 more often.

One of these scaenarios may be the cause of your performance drop.

 

After observing over time, i find both of these to be true, apparently, in my case. I was noticing how sometimes the gas mileage goes down, regardless of what i do, over a period of time, and then, it started going up over a period of time, same driving conditions, same routes and trips. I now think that my car does best on Shell gas. And i do think there is that memory effect where there is a tendency for the mpg level to stay stable and only change very gradually by tenths of an mpg increments. This is still speculation but i marvel at how it goes up or down seemingly with a mind of its own. "Hmm, i think i would like to increase my mpg now for a while," it seems to be thinking. And i'm like, oh thank you that's very nice of you. So much for my own sense of having any effect on the whole process. Sometimes i think it does better at higher speeds and not in surface street around town traffic, and other times it does the opposite of what i expect. It doesn't seem very systematic, the way it varies.

 

I have no solutions, but I'm reporting more of the same. Since June 2010, my mpg has dropped from 50+/- to 40+/_ mpg. In June, the 2008 Prius had 30,000 mile maintenance and new tires. Toyota blames it on summer blend gas, but why the big drop now, prevous summers haven't been a problem? I try to avoid gas with ethanol and have been successful using the water test, so that hasn't changed. The tire pressures are ok, the 12V battery checks ok, my driving habits haven't changed. I can't imagine the new tires would cause a 10 mpg reduction.

 

10mpg would be a stretch but 6+/- is not out of the question. What tires did you purchase?

 

I purchased Falken ZE-912 P195/55R16 87V. The original tires were Bridgestone P195/55R16 86V