Suddenly getting 18-20mpg in my 2008 Prius

Discussion in 'Gen 2 Prius Fuel Economy' started by Starcheckered, Aug 10, 2011.

  1. Starcheckered

    Starcheckered Junior Member

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    - Have you read This Thead Yet?
    Yes

    - What fuel economy are you getting and how are you determining fuel economy? (trip computer or manual calculations)
    I'm getting about 18 or less mpg. The bar graph and my average (reset 150 miles ago) show 40mpg average. However, I drop a gas bar every 15-18 miles and am only getting less than 200 miles to a full tank of gas meaning that can't be correct. 

    - What fuel economy are you expecting and why?
    For the last 9 months, except the first two or three tanks I ever drove while learning how to drive a Prius, I've gotten a very consistent average of 40 mpg or 400 per full tank to one bar (not flashing) so this is what I have come to expect. 

    - What are the approximate outside air temps?
    88-90 degrees Fahrenheit. 

    - How long are your trips?
    30-40 minutes, 15 miles, is my commute to work daily. 

    - 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?
    It's all suburb driving, average speed 40-50 mph and not very many lights or stop signs... Maybe 4 traffic lights the whole way. Not stop and go. It is 90% flat road. 

    - What region/state are you in? (if you haven't set your location in your profile)
    I live in Baltimore (county) Maryland. 

    - What's the terrain like of your drives? (e.g. flat, gentle hills, steep hills, etc.)
    Mostly flat, a few very small hills. 

    - Is your oil overfilled? (i.e. above the full mark on the dipstick)
    No. I had it changed about 1,000 miles ago. 

    How old is your 12v battery? What is the voltage reading of your 12v battery after sitting over night? (Method Here)
    .... No idea. 

    Have you had your alignment checked? Any pulling or abnormal tire wear?
    Hasn't been checked, but no pulling and just had one tire replaced (drove over a screw) and all are in excellent shape. 

    - Are you using the factory tires and wheels? If not, please indicate tire make, model and size (e.g. Goodyear Assurance Fuel Max 185/65R15).
    Using factory tires. 

    - What are your tire pressures?
    35 front, 33 back. Checked today. 

    - Make, model, year, engine and transmission of previous car? (e.g. 08 Honda Civic Si 2.0L 4 cylinder, manual transmission) What did you actually get on the same trips/commute? (Please give us actual numbers, not EPA ratings.)
    I used to drive a 2004 Saturn ION. I didn't drive this route but drove one similar, and got 16-20 mpg. 

    - How are you trying to drive (e.g. trying to stay in electric only?) and how hard are you braking?
    Braking slowly and gently, accelerating and gliding. 

    - Are you "warming up" the ICE (internal combustion engine) by letting it idle after powering on?
    No. 

    - Are you driving using D or B mode?
    D. 

    - HVAC settings? Are you using the heater, AC, auto mode, etc.? If using auto, what temp is it set to?
    Using AC, about 70 degrees. It's been much cooler this week than previously (when I had better gas mileage!)

    - If reporting a mileage drop, 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?

    Oil change 1,000 miles ago. Not sure if the problem was happening before then, but I had it changed at the same place I always have it changed. 

    I also had a tire replaced a week ago (drove over a screw) with a new factory tire and all tire pressure is normal. Problem was happening before the tire though. 

    Commute has been the same for 4+ months, bought the car December 5, 2010. 

    ************************************

    Long story short, I'm getting about 18-20 mpg and have been for a few weeks. I have no idea why. Nothing at all has changed, not commute not where I get my oil changed, not even where I get my gas. What's going on? I'm extremely frustrated. :mad:

    Thanks in advance guys. And sorry for the lack of HT
    L, I typed this on my iPhone.
     
  2. cwerdna

    cwerdna Senior Member

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    Thanks for answering!

    How are you determining "18 or less mpg"? Forget how miles you are getting out of each pip/bar. Your "guess gauge" could be out of calibration (http://priuschat.com/forums/gen-ii-prius-main-forum/45425-gas-gauge-didnt-show-empty.html#post595534, I wouldn't try this just yet). Assuming you've filled your tank, after you travel x miles, how many gallons did you put in (call this y)? Please give us some numbers.

    Any idea what viscosity oil they used?

    Do check your 12 volt battery condition via the instructions after the car has sat overnight.

    The tire pressures you're indicated are checked while cold (before any driving, after car has sat many hours or overnight), correct? 35 front/33 rear is too low for the stock tires anyway but running at that won't yield you such poor mileage.
     
  3. Rokeby

    Rokeby Member

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    Starcheckered,

    :welcome: to PriusChat, hon.:cheer2:

    At 20 MPG, something is not right. Even if you're driving your Prius
    like it's a non-hybrid, you should be seeing low/mid 40 MPGs.

    As to oil level, you should check, it should be just below the upper
    mark, never above it, which you might see if a full 4 qts was put in.

    Tire pressures are OK but 42f/40r would be better for MPgs and a
    slightly rougher ride quality.

    These are small potatoe MPG issues.

    Two things that can lead to substantial MPG drops are a weak 12V
    battery (service life is typically ~4 years) and a dirty/fouled MAP,
    combustion air mass sensor. There have been cases of bugs/dirt
    getting past a torn or incorrectly mounted air filter and totally
    buggering up the sensor.

    From you post, it looks like you've already been using the search
    function, good for you. :rockon: A little more poking around using terms
    like '12 battery' and 'dirty MAP sensor' will get the whole story, and
    more, on these subjects.
     
  4. Starcheckered

    Starcheckered Junior Member

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    Okay so, I just checked my battery and oil. (Already? Yes. Possessions I love more than my car = ... nothing.)

    I checked my battery using the URL listed in the questionnaire I posted earlier.

    And it read:
    12.2 off in maintenance mode 

    11.8 when on

    Charged up to 14.2 then drops to 13.9 in about 2 seconds 

    Battery at 5 bars when turned on not in maintenance mode.

    This seems to be in the normal range except 11.8 is a little low, the guide said 12.0 is the lowest it should go. Should I have this looked at? I hadn't driven the car in about 2 hours.

    Oil level was normal, about 2/3 below the line.

    Here's the thing about my mpg, it MAY be a wonky gas gauge:
    I was on one bar (not blinking) and filled up to full. So full the pump won't go anymore. Prices here are about 3.79 a gallon. I filled up $22 worth which equals out to about 5.9 gallons. So... That seems a bit off. Usually I drive about 100 miles before the first gas bar drops off then the others drop off around 40-50 mpg. This time I lost one almost immediately and each other after 20 or less miles. I filled up full, and have driven 159 miles and have 3 bars left. I will fill up completely tomorrow and see how many gallons I get to see if that's the issue.

    Something odd:
    In February of this year I ran out of gas. Car manual says on one blinking bar you have 3 gallons left, and full tank is 11.9 gallons. I just went on blinking bar. Drove 30 ish miles. Bam. out of gas. What gives? Now I go to the nearest gas station when I hit one bar, never blinking, to ensure that never happens again. Makes me kinda miss the ole red "E"! Then I at least knew how far I could go!

    Also, my car door says 35/33 for tire pressure. So that would be right... They were not cold though. I'd been driving about 1 hour right before I checked. Cant check now cause I dont have a gauge, used my bosses. Should I inflate em a little?

    Thanks again so much.
     
  5. Starcheckered

    Starcheckered Junior Member

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    Oops. So I was supposed to check the battery after sitting overnight not just 2 hours. Will update you all again at the buttcrack of dawn - to me - 630am.
     
  6. bisco

    bisco cookie crumbler

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    i would buy a gauge and check them cold. unless they are almost flat, they are not the problem. and 2/3 below the line is not normal but it also won't affect mileage. fill it up to the top line and not over. as far as figuring your mileage, because the tank has a bladder, it doesn't always fill up properly. it's best to average it out over several tankfulls. but still, 20 is way low unless it's just one fillup you're talking about.
     
  7. cwerdna

    cwerdna Senior Member

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    Don't worry about the # of (HV) battery bars on the MFD. That's not of concern here. That has nothing to do w/the condition of the 12 volt.

    In the future, please track exact numbers, not guesses like "about 5.9 gallons". How many miles did you go on those "about 5.9 gallons?"

    Do this each time: Fill up completely. Reset trip odometer AND MFD. Drive until somewhat low on fuel. Fill again. Note how many gallons you put in. Then, calculate miles driven divided by gallons needed to take you to full. Reset trip odometer AND MFD. Repeat.

    Example:
    Drove 500 miles, put in 10 gallons afterward = 50 mpg

    Forget looking at how many miles it takes before the 1st or any bar goes away. It's not linear and the # of miles before the 1st goes away can vary A LOT.

    Where in the manual does it claim that you have 3 gallons left when you have one blinking bar left? I've NEVER seen that. If you make that assumption, you will run out of gas, as you learned.

    The bladder tank (which varies in capacity) coupled w/the guess gauge can make things a bit tricky. You will see considerable variance between MFD numbers and your manual calculation numbers (sometimes MFD numbers are higher than the manual and vice versa). However, track them both separately in a spreadsheet and average the values of each (separately) over time, you'll find they're very close.

    I personally would never assume that the tank has more than ~9.5 or so (off the top of my head) usable gallons, to be on the safe side. I've never put more than 9.975 gallons into my tank and I never sit there topping off repeatedly (may cause damage to the tank/canister) unless there's some obvious problem where it's not filling as much as it should.

    If your tires were at 35/33 after an hour of driving, then when cold, your tires are definitely underinflated. However, this still wouldn't account for getting 18-20 mpg. Assuming you're driving a non-touring, I'd say you should go to at least 40 psi front/38 rear, on COLD tires.

    The "recommended" pressures on the door sticker (35 front, 33 rear) are too low, unless you like to have uneven tire wear.

    Side note: You might find http://priuschat.com/forums/other-cars/94001-gas-gauge-says-full-but-thats-not-quite-true-npr.html somewhat insightful.
     
  8. cwerdna

    cwerdna Senior Member

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    Any update?
     
  9. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    Just a comment from some one in the (very hot) deep South, Yesterday I put 11.3 gallons in my tank before the nozzle clicked off. (New gas station) Today I drove 210 miles before the first pip went away.

    All of this is normal.

    In the winter (such as it is) I will only get 9 gallons in a tank, those north of me will get even less. I lose pips at 60 miles in winter.

    All of this is Normal.
     
  10. richard schumacher

    richard schumacher shortbus driver

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    Either he finally noticed that he was mistaken about getting 18 MPG, or he discovered that the gas station he just started using was screwing him with an inaccurate pump.

    And of course tires at 35/32 PSI when warm are a bad idea.
     
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