Is it really the tires?

Discussion in 'Prius v Fuel Economy' started by Steve48, Jul 15, 2020.

  1. Steve48

    Steve48 Junior Member

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    I know there are similar threads, but I think I have a twist.

    I bought my 2014 Prius V new. For several years I got 42 MPG indicated running regular gas. Then a couple of years ago I took it to a new dealership for routine service.

    They sold me a set of tires (mistake on my part) and changed the oil. Here's the twist. When I went into the dealership my trip meter indicated 200 miles driven and 42 MPG average since the last reset. When I got it back, it was indicating 201 miles driven and 38 MPG average (!). What?? How can that happen?

    At first I suspected that they had done something that glitched the MPG estimation. Not so. I checked it manually as I had when the car was new, and the indicator worked as before- a little optimistic, but pretty good. The mileage decrease was real. I checked tire pressure and so forth with no answers.

    I took the car back to the dealership and complained. They checked it out and said that all was well. They claimed that they measured 42 MPG. However, this time they had reset the trip meter. It indicated 5 miles driven and 38 MPG.

    The next time I needed service, I returned to my old dealership and told my story. The said it was the tires that the other guys sold me that made the difference. That's a reasonable explanation, but it doesn't explain that jump change in the indicated MPG. I suspect that they made some kind of change to the car's firmware and aren't owning up to it.

    An alternate explanation is that they ran the engine a lot without going anywhere, and that's where the initial decrease in MPG came from, and the tires really are the cause of the sustained decrease. Does anyone here have thoughts on it? Has anyone heard of firmware changes to the Prius V system that have caused mileage decrease?
     
  2. David9962000

    David9962000 Member

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    What tires do you have on the car now? They might be too heavy. Unsprung weight is multiplied by 4.
     
  3. Steve48

    Steve48 Junior Member

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    They're the Coopers that the new dealership sold me. I understand that the tires may well be the explanation, but I'm hung up on the mysterious glitch in the mileage indication.
     
  4. The Electric Me

    The Electric Me Go Speed Go!

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    What tires you now have I would speculate is the WHOLE story.

    You often see a decrease in MPG with a new set of tires.
    If these are not Low Rolling Resistance tires, or tires designed to return higher MPG, compared to whatever you had on them before, you may simply experience a reduction.
    But it's probably more likely your MPG will increase as the tires break in.

    Check what tires you have, and how they rate as gas efficient tires.
    Then just give it some time. I wouldn't panic. A 4 mpg decrease after having new tires installed I think is more often typical of just having new tires installed.
     
  5. Steve48

    Steve48 Junior Member

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    It's been a couple of years, so if it's the tires, then it's not going to change. We'll see when it's time to replace them.

    No panic. It's a technical problem, nothing more. :)
     
  6. Montgomery

    Montgomery Senior Member

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    I just got Bridgestone Turanza Quiet Trak tires. Stock size for my car. After two tank fulls my mileage has dropped 4 mpg. I can live with it. The tires are definitely much quieter than the tires that came with the car originally.
     
  7. rjparker

    rjparker Senior Member

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    As described the mpg for the last 200 miles went from 42 to 38 when one mile was added. Long idling will do that. You can also dramatically change a 200 mile average mpg by getting up on the freeway and driving at 85 mpg. The relatively short trip miles will allow mpg to vary quickly when idling or speeding.

    Here is another mile and mpg related quirk. My car’s speed readout is always about 4% high. I can see the difference when passing radar speeding signs, with a gps and even by monitoring my obd2 port with an app. The car has the correct mph in its computer but displays high. I expect the overall miles on the car is overstated by 4% and it probably causes an erroneous mpg.

    From “Car Scanner App” while driving.
     
  8. Leadfoot J. McCoalroller

    Leadfoot J. McCoalroller Senior Member

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    Have you verified that the old tires and new tires were the same size? It's not unheard of to put on a very similar, but slightly different size. This can directly affect MPG, and can also have indirect effects because of the inaccuracy introduced into the speedometer/odometer/fuel use calculations done by the dashboard computer.

    Even for the same nominal size, there can be some variation- a new one that runs large for its size vs. an old one worn to limit.
     
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  9. Steve48

    Steve48 Junior Member

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    Yes, that was my alternate explanation. They just let the engine idle after changing the oil.

    Interesting thought. I would have hoped that this wouldn't happen at a Toyota dealership, but you never know.
     
    #9 Steve48, Jul 16, 2020
    Last edited by a moderator: Jul 16, 2020
  10. farmecologist

    farmecologist Senior Member

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    It is most likely the tires....I saw a substantial decrease with new tires on our 2010 liftback and our 2012 v (wagon). Wait until the tires have 'broken in' a few months...and then report back.

    Weight, rolling resistance, and the diameter/circumference of the new tires vs. the worn tires all play into it. And the higher the MPG your vehicle is rated at, the more you will notice it. This article really drives home how much of a difference it can make :

    https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=177

    Also note that there are quite a few other 'tire MPG decrease' and 'new tire' threads around here if you search around a bit. Some of them have pretty good data as far as people reporting back, etc.... Some of them are well worth your time.

    Hope this helps!
     
  11. bat4255

    bat4255 2016 Prius v #2 and 2008 Gen II #2

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    What are you running for tire pressure?

    The dealer want's to run 36 PSI but I instruct 38 PSI.

    for me yearly average.
    36 PSI = 39 mpg
    38 PSI = 41 mpg
    40 PSI = 42 mpg
    I know 40 PSI can give me about 3 mpg better, but the ride is better with 38, a good compromise for me.

    Side note:
    I still have the OEM tires with 50,000 miles and I have noticed about a 2 mpg increase from new.
    I believe well scrubbed-in (worn) tires produce better mpgs than new. I still have safe tread for another 8-10K.
     
  12. Steve48

    Steve48 Junior Member

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    The consensus seems to be that it really is the tires. I'll try a higher pressure and see if it makes a difference.
     
  13. The Electric Me

    The Electric Me Go Speed Go!

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    I was going to ask the same question.
    But the OP seemed so dedicated to wanting to believe it was some covertly installed firmware "glitch", that I didn't bother.

    All the OP say's is "Coopers".
    I'd be interested in EXACTLY what Cooper tires were installed. Cooper makes a lot of tires, designed to promote a lot of different attributes. Tread Wear, traction, Winter, Summer, Touring, All-Season...tires designed specifically to promote better fuel efficiency.

    I don't know but sometimes the simplest most direct answer...is the answer.
    I'm far more apt to ignore the immediate MPG reading change, and attribute any 4 MPG mileage drop to the change in tires.
     
  14. Steve48

    Steve48 Junior Member

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    Dedicated- no. I wanted to explore that, and where better than here? But as I indicated above, the consensus is that it really is the tires, and I accept that.

    Cooper CS3 Touring. 205/60R16 . The previous tires were the car's original equipment. Alas, I don't remember what they were.

    Agreed. Occam rules.
     
  15. Tim Jones

    Tim Jones Active Member

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    Did they by any chance perform the Inverter Firmware Update?
     
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  16. RRxing

    RRxing Senior Member

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    This is normal. Common to all cars. If it read low, you would be above the speed limit when it says you were at the limit.
     
  17. Steve48

    Steve48 Junior Member

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    If so, they didn't tell me, and I don't know what it is or what effect it would have.

    As a follow-up, I indicated in my last post that I accept the consensus that if really is the tires, and nothing has happened to change that. If anything, I've seen a slight improvement as time goes on. Other posters suggest that that might happen as the tires got older, even though I already had thousands of miles on the Coopers when I started this thread.
     
  18. Tim Jones

    Tim Jones Active Member

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    There are several updates that others have claimed that it hurt their mpg. Make sure you have them because it could matter when your Inverter blows up. These are inverter updates......
     
  19. Steve48

    Steve48 Junior Member

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    Thanks. I'll look into it.
     
  20. Tim Jones

    Tim Jones Active Member

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    I run 42 front and 38 back...... defenders.....40 to 42 mpg (with ac on) usually and my v has 254,000
    running 0 w 40 mobile 1
     
    #20 Tim Jones, Sep 19, 2020
    Last edited: Sep 19, 2020
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