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The mpg difference 15s vs. 17s

Discussion in 'Gen 2 Prius Fuel Economy' started by F8L, Sep 25, 2011.

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  1. F8L

    F8L Protecting Habitat & AG Lands

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    Aye. I'm sure the CT200h crowd would find the data very useful. Same with the Prius v.

    Those wheels sound sweet! That's approx 7.5 lbs lighter than oem.
     
    #61 F8L, Nov 29, 2011
  2. sidecar

    sidecar Member

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    and around 1/3 the weight of OEM 17" 's
     
    #62 sidecar, Nov 29, 2011
  3. sidecar

    sidecar Member

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    #63 sidecar, Dec 15, 2011
  4. F8L

    F8L Protecting Habitat & AG Lands

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    I was playing around with a wheel offset calculator today after thinking about how far my 17" wheels stick out compared to my OEM 15" wheels. The only real reason I hate the 15s is that they are tucked so far into the wheel well. That got me thinking about how much more aerodynamic this setup is. 32kcolors brought up a point about the GenIII 17" wheels being more aerodynamic and while I agreed they probably were I didn't consider offset differences.

    The GenIII and GenII 15" wheels has an offset of 45mm. The OEM 17" wheel offset is 50mm. The 17" wheel extends the wheel outward 8mm more than the 15" wheel. My 17" wheels have an offset of 42mm and they extend 16mm outward. Compared to my OEM setup I have 16mm more tire sticking out closer to the edge of the wheel well. Is this enough to make a difference in aerodynamics? The wheel doesn't stick out beyond the fender but the front wheels sit pretty flush. Hmmm maybe I need to try a set of OEM 17s for awhile.

    Wheel Offset Calculator

    Rolling resistance is still higher even at low speeds so I doubt this offset idea makes a big difference except at high speeds.
     
    #64 F8L, Feb 14, 2012
  5. sidecar

    sidecar Member

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    I dont know and haven't seen any experimental data, but the trend in design is to make this tolerance fit tighter rather than push the wheel inside the wheel arch.

    My feeling is there is less turbulent draggy air when you can get right to the edge of the fender. Another way to do this is with the offset you have is spacers or adapters.

    I still think your loss of MPG one of the most mysterious, if not the best documented. Im watching A$ US$ parity right now as events have turned favourable again, if I can import for under A$1k+transport I dont pay any taxes on it, so Im bruising to pull the trigger on wheels again :)
     
    #65 sidecar, Feb 14, 2012
  6. F8L

    F8L Protecting Habitat & AG Lands

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    That makes sense. Hmm, I'm not sure what else is to blame then. Maybe it is just all weight, frontal area and tire friction.
     
    #66 F8L, Feb 14, 2012
  7. sidecar

    sidecar Member

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    well there are few factors in play
    aerodynamics if the width is affected, where we can remedy with spats
    and road friction, see below

    the mystery to me is this,
    1) with the options in play we can make wheel weights even out
    2) even where a wheel is larger the diameter of the tyre is the same, indeed the tread could be identical.
    3) with less sidewall there is less sidewall deflection, a big contributor to RR.

    I just dont see how rolling resistance is so adversely affected
     
    #67 sidecar, Feb 14, 2012
  8. qbee42

    qbee42 My other car is a boat

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    Rolling resistance is adversely affected because of market pressure. The main market for large diameter wheels is for performance cars, not Prius. As a result, most tires in this category are optimized for something other than rolling resistance. Obviously that doesn't have to be the case, but at present it is.

    Tom
     
    #68 qbee42, Feb 14, 2012
  9. sidecar

    sidecar Member

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    no Im not talking about 22" wheels and tyres that would siut some kid at the end of the block, Im talking about sensible and more moderate choices (which is where 17" fits) including low RR tyres not at all dissimilar in specification from the 15" dia originals
     
    #69 sidecar, Feb 14, 2012
  10. qbee42

    qbee42 My other car is a boat

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    My comment still stands. Marketing forces are at work. Not all LRR tires are equal, and at present most 17s are not as free rolling as the better 15s. Perhaps that will change as more demand develops.

    Tom
     
    #70 qbee42, Feb 14, 2012
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