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Inexplicable Gas Mileage Drop

Discussion in 'Gen 2 Prius Fuel Economy' started by PriusForThePeople, Aug 14, 2010.

  1. PriusForThePeople

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    Hi all!

    I've had my 2007 Driftwood Pearl Package 2 Touring a little more than a year now. I had been getting 42-46 MPG every tank until I filled up my gas tank today.

    Let me preface this by saying, I got my front tires replaced just 3 days ago. I filled my gas tank immediately after and that tank averaged 45.7 MPG. With that said, I filled my tank today (10.6 gallons which seemed abnormally high, but then again the last pip was blinking). I proceeded to drive around 40 miles the rest of day and as of right now, the MFD is displaying 33.7 MPG!!! :mad:

    I think I'm going to drive about 140 miles tomorrow (mostly highway speeds), and I'll update this thread then. But as for right now, any ideas on why this is happening?

    All comments and input is welcome. Thanks!
     
  2. JimboPalmer

    JimboPalmer Tsar of all the Rushers

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    New tires always get worse mileage than old tires. That may be half the reason.
     
  3. GBC_Texas_Prius

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    The obvious answer, check tire pressures. Did you get low rolling resistance tires? That wouldn't account for all of the drop, but some.

    Not related to your mileage drop, but when I have tires changed, I not only check pressures, but also lug nut torque. My dealer checks out ok, but I've caught Firestone type places torqueing one lug nut to 200 ft lbs and the others somewhere in between trying to get rotor replacement business. It's the perfect crime, what can you do about it other than call and bitch? So sorry, we need to check our equipment, glad no damage was done.
     
  4. PriusForThePeople

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    This may be, but it seems odd to me that this would happen immediately after recording one of my highest tanks to date.

    Yes if I recall, he did say they were low rolling resistance, however, like I said, this is my second tank since I've gotten the tires changed and the previous tank was of near record proportions (for me anyway).

    Tire pressure:

    FL: 35 PSI
    FR: Could not, for the life of me, get the cap off my just my fingers.
    RL: 29 PSI
    RR: 24 PSI

    I'll inflate the rear wheels tomorrow and hope that that helps.

    I did get my tires changed at Firestone. How do I check lug nut torque and what is considered "okay", if I can ask?

    Thank you both for your responses!
     
  5. dogfriend

    dogfriend Human - Animal Hybrid

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    76 ft-lbf is the correct torque for the wheel nuts.

    Low inflation pressure is at least part of the issue. New tires will also give lower mpg until they are broken in (a few hundred miles).
     
  6. cwerdna

    cwerdna Senior Member

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    Please let us the know the make, model and size of tires both before and after.

    I can't speak to the tourings, but for the 15" wheels on the non-tourings, 35 psi is too low, by most accounts. Your 24 is REALLY low. What's the max sidewall pressure listed on these new tires?
    Tire Tech Information - Tire Rolling Resistance Part 3: Changes to Expect When Switching from Worn-Out to New Tires is a good article that someone else here PC dug up.
     
  7. PriusForThePeople

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    Thanks for the number! I've driven approximately 450 miles on the new tires. :confused:

    Currently the rear tires are Bridgestone Turanza EL400 (max PSI: 51) and the front tires are Bridgestone Ecopia EP100 (max PSI: 51). Before the fronts were changed I think I recall them both being Bridgestone Turanza EL400's. I'll try to inflate them and report back after I've been out and about today.

    Thanks for the article... It was very interesting reading, as were the articles listed under "Most Popular Links."

    Thank you both for your responses!
     
  8. PriusForThePeople

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    Just thought I'd update everyone on the situation. The 140 mile trip is happening tomorrow. But today I was convinced to resolve this issue.

    The first thing I did was inflate all the tires to 35 PSI. I then proceeded to take it on a trip that was 40 miles roundtrip consisting of 55-65 speed limits and a few stop lights here and there. About 4 miles in, after watching the MFD readout holding strong around 34 MPG, I angrily pressed the reset button on the display.

    For the remainder of the trip I got it up to 47 MPG.

    Yet through this trip, I found something quite perturbing. When I would accelerate from a complete stop at a stoplight, no matter how lightly, the first couple seconds would read out around 5 or 6 MPG and steadily increase from there. I use the word increase loosely.

    While looking at the Energy Monitor screen I found that the car seems to be charging the battery CONSTANTLY. Usually when you accelerate, it uses the battery power and the ICE, however it seemed to be using ONLY the ICE to move the car while simultaneously charging the battery, hence less than ideal gas mileage.

    Now I don't know if this is actually what's happening or if it's just what the computer is telling me.

    Since I arrived home after the 40 mile trip, I've taken it for a 2 mile trip and a 10 mile trip and managed to kill the MPG readout to around 43 MPG.

    Just a bit of quick math here: I went 45 miles before pressing reset on the MFD and was averaging around 34 MPG. So with simple division, I would have, by those numbers, burned through 1.3 gallons of gas. After hitting reset I've gone 57 miles at and average of 43 MPG, burning, by these numbers, 1.3 gallons of gas.

    So by those numbers, I've burned through 2.6 gallons in 102 miles, so 39.2 MPG.

    But I would think that If I was getting such terrible gas mileage that it would have a negative effect on the gas gauge, but I still have 10 pips after 102 miles. And yes, I know the first one takes a lot longer to go away than the others, but I've had it go away in as little as 80 and as much as 125, but have never had such a terrible gas mileage readout.

    Am I just obsessing? Could it be the battery? Should I take it to the dealer and have them run a diagnostic?


    P.S. Sorry for the novel.
     
  9. cwerdna

    cwerdna Senior Member

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    I can't speak for the touring's tire pressures (size they're a different size than non-touring) but IMHO, 35 psi is still too low. Max sidewall pressure on non-touring OEM tires (Goodyear Integrities) was 44 psi. I'd bump mine to at least 38 front and 36 rear.

    Your 4 miles in was with likely a cold engine far part/much of the trip, hence the low mileage. When you reset it, the engine was warm for much of the rest.

    The 2 mile trip likely helped bring down your mileage, due to again to cold engine needing warmup. I can't speak to that 10 mile trip since I don't know what it consists of. How much city vs. highway? How much stopping and average speed?

    Forgot all the random math you're doing. I think you really need to do the manual calculations after having gone thru a couple tanks.

    BTW, per http://www.tirerack.com/tires/TireS...true&minSpeedRating=S&minLoadRating=S&tab=All, OEM tires were Bridgestone Turanza EL400-02 195/55R16. The Ecopia EP100s should be fine from a rolling resistance and fuel economy POV. Funny enough, I stumbled across http://www.tirerack.com/tires/tests/testDisplay.jsp?ttid=121& from another site. I've only skimmed it so far.
     
  10. PriusForThePeople

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    The 10 mile trip was mainly 45-55 MPH, stopping for a light once, average speed maybe 45 MPH.

    I guess the proof will be in how many miles I get out of this tank... I mean if it's normal, which for me is about 380-420, I'll feel a lot better. It's just boggling my mind and I can't help but think something bigger is at play, but I'm just a worrier.
     
  11. richard schumacher

    richard schumacher shortbus driver

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    To make better MPGs easier you can run the front tires all the way up to the maximum given on the sidewall maximum, and the rear tires 2 PSI less. This will make the ride and handling harsher, so it's your call.

    Check tire pressures at least monthly, and immediately after any service. Techs love to fiddle with tires.

    Grossly overfilled oil is another possibility. Always check the oil level yourself before driving away from every oil change.
     
  12. andyprius

    andyprius Senior Member

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    If nothing else works try rebooting
     
  13. PriusForThePeople

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    Thought I should update this thread.

    Took the car for about 200 miles today. Managed to get the MFD to 45.2 MPG, which is fantastic, for me anyway. It's possible it just needed a long trip.

    But those first 45 miles killed me. My gas gauge diminished to 4 pips. Despite all this, everything seems to be functioning properly.

    Crossing my fingers for a normal tank (even slightly less than normal would be fine with me). Will update when I fill back up. Thanks everyone!
     
  14. PriusForThePeople

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    This intrigues me. How would I go about rebooting my car?
     
  15. andyprius

    andyprius Senior Member

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    Click on andyprius, I have two threads on rebooting.
     
  16. qbee42

    qbee42 My other car is a boat

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    In England, does rebooting involve replacing the trunk of your car?

    Tom
     
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  17. lizmariecat

    lizmariecat New Member

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    okay, I feel better now--I just got 4 new tires on my 2005 Prius and I had a drop from average 52-53.5 mpg to 47 mph...I was losing heart. I just got this as a used vehicle in September and have 6 year loan so I was a little bummed. I was just so happy to be able to get a Prius I literally didn't pay attention to the really poor tires on the car--my fault, of course.:eek:
     
  18. lizmariecat

    lizmariecat New Member

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    well, I alos got my first oil change and new tires-all last weekend. Has brought the mileage way down.:rolleyes:
     
  19. uart

    uart Senior Member

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    Ok so what tires (brand/model) did you get?
     
  20. TXPriusOwner

    TXPriusOwner New Member

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    I too experienced a HUGE drop in mileage this weekend on a 2007 Prius. I've been running 45-48 mpg for 3 yrs (in town) and suddenly this weekend it dropped to 33. I checked my tires. PLEASE DO NOT TALK TO ME ABOUT TIRE PRESSURE. That is NOT the issue on my car. I was on a trip from Texas to Kansas and started like normal 48 mpg, then it dropped to 43 mpg (highway, which I used to get 51 mpg on trips at 70 mph). Then a few hours later (after checking tire pressure again) it dropped to 33 mpg. I've been reading posts, but everyone simply talks about tire pressure. My tire pressure on this trip has been the same from beginning to end. What else can can such a HUGE drop in mpg? We're talking about a 18 mpg drop in 1 day!
     
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