Help! with gas-milage problem

When did you last change the transaxle fluid?

Who changes your oil? How much oil shows on the stick?

How does your 12V test out? When was it last replaced?

Does SOC behave any differently since the fall off in mpg?

When was alignment last checked?

Have you checked the MAF sensor?

How does the air filter look?

 

And what are the tire pressures now?

 

We bought a 2005 Prius brand new from the dealer. The first few days we were averaging 52 mpg. After that we never got more than 47mpg in town, but we would average 50 to 52 mpg on a road trip. I was wondering why the in town dropped out of the blue but let it go since overall I liked the car. But over the years it's been getting worse. Especially in the cold and hot months. This never was an issue the first few years of the cars life. Now that it's 5 years old these extreme months seem to take it's toll on the mpg. One time we came back from a week long trip and found the battery totally dead at the airport. Had to get it jumped, took it to the dealer and they said everything checked out okay. Never had that problem again....weird.

So here I am, February 2010, temps in the mid 30's to mid 40's and I cannot get the mpg in town over 35mpg and highway up to 39/40 mpg at best if I really try. I know all the coasting tricks and they are no longer working. I checked the oil and it's not overfilled. The emergency brake is fully released. The tires are 37 front/35 rear and balanced/rotated every 3,500 miles. All fluids are filled to the correct levels and the air filter is very clean.

The car just doesn't feel like it has any power whatsoever in the colder months. It almost feels as if I am dragging an anchor behind the car. If I am on a totally flat level surface I can at times pull away from the stop on battery power only but at 17 mph the gas engine kicks in, even if the battery is fully charged. Back in the day I used to consistently be able to get up to 27mpg before the gas engine would engage.

I called Toyota about the 60k service and was surprised when the service guy told me the transaxle fluid wasn't really a necessity and that it was such a minute amount of fluid. Most of the other stuff on the 60k list was pretty simple stuff and I checked and did it myself. I don't mind bringing it in and spending a few hundred to have Toyota do the full 60k but I don't want to throw $$ out the window if it's really not going to do anything for my mpg issue. From what I recall the first major service isn't until around 110k.

Can this possibly just be because my battery is 5 years old and the cold temps are getting to it ? Or perhaps I need to run even a higher psi in my tires ? I believe the tires say 44 psi max.

Sorry fort he winded post.

 

If you notice that the engine is staying on at most stoplights instead of turning off then you may want to have your 12v battery tested. This is assuming you car has reached operating temperature and you're not running the heater or A/C at full blast.

If the 12v battery is failing then the Prius will be constantly trying to charge it via the inverter and hybrid battery such that the engine stays on longer than it normally would and this will in turn reduce MPG.

 

Nope, once the gas engine is heated up it always shuts off at stop lights.
I think I am going to try to up the psi in the tires a bit to see if it helps.

 

Anyone know if a transaxle fluid change would help this at all. I've read some posts that say YES and some say NO.

 

Are you always trying to stay in electric only when trying to accelerate or is this a comparative test you're running?

Are you still on the original Goodyear Integrities or have you changed them?

To copy from my q & a:
- How long are your trips?
- How much of it is city vs. highway? Roughly what's the average speed in overall and and of each segment? Is there a lot of stop and go driving?
- What's the terrain like of your drives? (e.g. flat, gentle hills, steep hills, etc.)
- Did anything significant change on your car (e.g. accident, hit a curb or big pothole throwing off alignment, oil change/other maintenance/repairs, changed tires or wheels, etc.) or your commute?

 

Fresh fluid (e.g. transaxle) is better than fluid that is spent. Not a panacea, but a measurable improvement is possible.

You should definitely check the 12V. It is near the end of its expected life, and if failing, would drag the mpg down noticeably.

SOC is state of charge of the HV pack. I was asking if it loses bars or fails to charge like it used to. It would be rare for the HV pack to fail, but it can happen.

Yes, the MAF can get dirty. There is a special cleaner for it. If dirty, the air-fuel mixture can be off, and that is big.

 

New tires> It take several thousand miles to break in a new set of tires. Just another thing to consider.

 

Just a test I am running. Normally I just "drive" the car. But I was trying to see if it still had the power to pull it up to 27mph on pure battery like it use to. It doesn't.

The Good years are long gone. I've got to get down to the car later today to check on the brand of tire I purchased. I know it was from Discount Tire and it was the only one they had in stock at the time that would fit the Prius. Has a 60,000 warranty. We keep the PSI a little higher than factory standards since everyone knows those don't work. But I may need to put more PSI in them even still.
These are my 3rd set of tires. Put them on at 60,000. THe factory originals only lasted 25,000. Second set 35,000.

My trips vary. If it's my wife driving, then it's almost 100% in the center city driving, mostly flat, some slight hills, lots of stop and go, not many curve - just a city grid and she only goes 2 miles to work. So that may explain the horrible mileage she gets during the winter time. But then I can take the car to work for 3 or 4 days which is 100% Interstate travel, 10 miles each way and the mileage wont improve but maybe 1 mpg at best. This week I filled it up and have taken it all week to work and I am currently at 41 mpg. And that's with me babying it the whole way, 55 mph in the slow lane.

Oil was last changed 4,000 miles ago and I am going to change it again next week. I looked at the dipstick yesterday and it's not overfilled and is still golden and looks brand new. I also always double check with the mechanic to be sure the proper weight oil is put in.

My mom was in town last night and I checked her Prius. She was getting 45 mpg and she just drove into town, all highway miles. Her Prius is notorious for getting above 50 mpg at all time on any terrain. But she said ever since she got new tires about 6 months ago she can't get it out of the 45 mpg range. And she had the dealer put on the exact tires that came with the car originally. He PSI is dead on as well.

Maybe it's just the unusually cold and long winter we are having in this region ? Most days we haven't got out of the 30's and that's unusual for us. Especially to go on for 3 or more months.

 

Please do get back to us on the exact tire model, sizes and pressures. Many tires out there have higher rolling resistance than the Integrities (and other reasonable tire choices) and will most definitely hurt your mileage, some quite badly.

As for mom's tires "PSI" being dead on, the factory specified pressures on the driver's side door sticker are too low. If you've have the 15" wheels, I'd bump them up to at least 38 psi front/35 or 36 rear.

Your wife's 2 mile drives in stop and go city traffic in cold weather will kill your mileage. I'd expect mileage in the low to mid-30 mpg range with that w/stock tires.

With "This week I filled it up and have taken it all week to work and I am currently at 41 mpg. And that's with me babying it the whole way, 55 mph in the slow lane" is this after you've (or the car itself) has reset the trip computer or does this include the previously poor mileage from your wife's short trips. Are you using the cruise control? If you're doing that on the highway, it'd be probably best to use CC.

To be more specific about your outside air temperatures, is all this driving happening in 30-39 F weather or even colder? And, yes cold temps hurt mileage.

More hints:
- In your wife's driving and city portions of yours, turn the HVAC all the way to hi then press Off for the fan speed. Don't use auto. When the car is past a certain stage, the ICE (internal combusion engine) can shut down when stopped/at low speeds BUT if coolent temp is <=145 F, the ICE will run to provide cabin heat, which hurts your mileage. It's ok to turn your fan speed above off in cases when the ICE must run anyway, like when you're >42 mph or when you need harder acceleration.

Also, this won't matter if you ICE is really warmed up (coolant temp >> 145 F, at say 180+ F) but you need something like ScanGauge to monitor that.
- If your outside air temps are in the 30s or up to 49 F, I suggest grille blocking to cut down on engine heat loss.
- Consider an engine block heater too. But, it's not nearly as cheap as buying some pipe insulation to block your grille first and checking your tires.
- If it turns out you have high rolling resistance tires, then next time, get some recommendations from this forum as for ones that yield little or no mileage hit.

 

whats a maf sensor?

 

Mass air flow sensor

 

Last week I took my 2005 Prius on a 1000 mile trip... and averaged 45mpg. That was up and down a couple of Cascade mountain range passes and back and forth across Washington State. I found that on my return trip I was averaging 48mpg, till I hit the freeway and the 70mph speed lowered the MPG back down to 45 for the remainder of the trip.

Also, I'm becoming more of a believer in how the outside temps affect the fuel consumption. I need to look more into blocking the lower third of the radiator. This week the temps have raised into the high 40's and even the 50's. The Prius gained a 5mpg increase in average fuel economy and is sitting right at 50mpg. That's not bad considering where ever I drive it always seems like I'm going uphill both ways.:usa2:

 

I'm having the same issue with drop in mpg on my 2007 43k miles. I'm glad to see I'm not the only one! Mine was consistent 47~49 mpg, then suddenly dropped to 38~39 mpg about 3 months ago. Some tanks are all highway, others city, some both -not much difference in mpg. Florida terrain is flat as a pool table. No noticable change in SOC.
- Air pressure is 35 psig all 4 tires, wear pattern is fine.
- Change oil and filter every 4000 at local Chevron station.
- 12V battery was replaced.
- Checked speedo and odometer and its dead-on accurate.
First trip to dealer they told me it was an increase in the %Ethanol in the gasoline and there was nothing they could do. I tried every gas available - Shell V-Power premium, Sunoco 99 - for a 1~3 mpg increase. Second trip to the dealer they did full inspect and sold me on fuel injector cleaner, throttle body cleaning, air filter, oil change for $300. I'm at 40 mpg right now.

 

42/40 PSI front/rear will make it easier to get better MPGs. Wait for the return of summer gasoline. Stop wasting money on anything higher than 87 octane (the "1-3 mpg" improvement was illusory happenstance).

 

Update: I checked the emergency brake no problem there. I increased tire pressure to 44 PSI. Filled up with 87 octane. 38.6 MPG after all this, my all time worst tank mpg ever. What could be causing this? Leak in gas tank? :mod: