225 to 235 .... Another "Tire size on a Soobie" question. (if therehasn't been enough already)

Discussion in 'General Motoring' started by nobody, May 13, 2006.

  1. nobody

    nobody Guest

    I recently replaced all 4 tires on my '00 Outback Limited. The original
    Potenzas were toast; but 94K wasn't a bad run for those as OEM!

    The OEM size by the manual is 225/60R-16 , as were the Potenzas. I upped
    that to 235/60's, Falken ZE512's.

    I had the chance to measure mounted OD on the 225 and 235 versions of
    this as well as the (worn) Potenzas.

    28.8" on the Potenzas (remember, they are down to "Lincoln's head" on
    tread wear)

    29.1" on a 225/60R-16 ZE512

    29.4" on the (now installed) 235/60R-16 ZE512

    The main reason for the larger size was gaining a little tire overlap
    (rim size being slightly smaller than tread width) for a softer ride.

    I felt it was a reasonable choice; as did the tire dealer. A few days
    later I went to the local Soobie dealer to pick up some new floormats
    and got my butt reamed by a mechanic for the tire upsize. He did say
    that was standard procedure (as far as he knew) to always disuade even
    minor size changes.

    He did like the tires tho ;}

    I could understand problems with major changes, but .3" ???

    I've been upsizing tire size on vehicles since 1966, I know the pitfalls
    and all of the usual fixes like new differentials if you go too far. I
    realize the AWD system does put more into the "issues" but IIRC the
    actual size of these 235 Falkens is in the range of 225's from other

    Did I screw up??
    nobody, May 13, 2006
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  2. nobody

    Body Roll Guest

    I don't think so. I think the mechanic is full of shit. As long as all
    4 tires are of the same model/size
    you should be fine, The possible problem is the clearance for the
    chains and an possible issue
    with the wider tire not clearing the wheel well liners at the extreme
    positions of the steering

    I think the mechanic's gf/wife refused to put out that morning and that
    he took it out on you.
    Body Roll, May 14, 2006
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  3. nobody

    mulder Guest

    When you change the tire size it effectively alters the gearing, and
    also changes the speed sensor readings to the ECU and ABS controller.
    Your speedometer will not be as accurate, ABS operation may be
    effected, and the ECU will get inaccurate speed info.
    That .3" difference in diameter translates to about a 1" difference in
    rolling circumference which could be significant.
    It is best when changing tire sizes to use a proper "plus" size that
    will keep the same diameter and rolling circumference to within a very
    small tolerance, usually 1% or less. There are tire size calculators
    available online that can assist with this. Tire Rack has one on their
    site. You can do a "plus zero" using the same wheel size, but you
    can't arbitrarily go up to the next wider size tire.
    mulder, May 14, 2006
  4. nobody

    Grolch Guest

    BUT, even tires with the same nominal dimension are different diameters
    between brands and models so rhetoric that this .3" difference is a problem
    is pure bullshit. I got rid of the OEM Geolandars on my Forester and
    replaced them with the same nominal size Goodyear Tripletreads and found my
    speedometer was out by over 8%, I went back to Fountain Tire and they
    installed the next size up to most closely match the original tires
    Grolch, May 14, 2006
  5. nobody

    Blair Baucom Guest

    Good point, as I am considering going up in size to correct my speedometer
    error. Tirerack shows the Forester stock Geolandar's tires as having a
    diameter of 26.2". When I read 70 mph I am really going 67 mph. So if my
    math is correct, I would need a tire of 27.4" for the correct reading.

    I am also considering the Goodyear Tripletred. It shows 26.1" vs. 26.2" for
    the Geolandar in the stock 215/60 HR16 size.

    Which size did you put on the Forester?

    Blair Baucom, May 14, 2006
  6. nobody

    Grolch Guest

    215 60R16
    Grolch, May 14, 2006
  7. nobody

    Blair Baucom Guest

    That is the only size I see used for the 16" wheel on the Forester from the
    start. I see for 2006 in Japan most Foresters come with 17" 215/55 R17 which
    still has the similar diameter of 26.3"
    Blair Baucom, May 15, 2006
  8. nobody

    Grolch Guest

    BTW, you'll love the TripleTreads
    Grolch, May 15, 2006
  9. nobody

    nobody Guest

    In this case, I *did* do my research. Putting on a researched/measured
    tire that just happens to be a 235 instead of a 225 turned out to be
    better than putting on a 225 from a manufacturer that makes 'um bigger.

    I hit a 10 mile 'Speedometer Test Area' today where it was reasonable to
    slow down to an indicated 60 mph on cruise control and fairly tight
    stopwatch action. Speedometer was 59.5 mph; odometer was 9.94 miles. I'd
    say that was a pretty damned good match. It's possible the Soobie's
    speedo was off before, but I haven't owned it long enough to tell about
    the old tires.

    Post FWIW:
    Speedometer error is more common than most people think. I never
    understood why my father never got a speeding ticket with his '83 Ford
    Rander pckup. My brother later found out when we pulled off the stock
    185/75-14's and put on 215/70-15's (about 1.7 " larger) and got almost a
    perfect 60mph over 10 miles after the swap. Doing the math said he was
    doing about 54 mph with an indicated 60. Now we know why.....
    nobody, May 30, 2006
  10. Take a look at the Miata Tire Size Calculator
    ( http://www.miata.net/garage/tirecalc.html )

    This will calculate the difference for you.
    GreenieLeBrun, May 30, 2006
  11. nobody

    John Murfet Guest

    The issue is not so much the radius of the tire as the cross section. If you
    put a tire on a rim that's too narrow for it you'll distort the case of the
    tire. This effects the handling of the vehicle and in extreme cases the
    sidewalls move so much it feels like flat tires.
    John Murfet, May 30, 2006
  12. nobody

    Body Roll Guest

    What would happen if you stick to the proper width but go down 1 size
    in the aspect ratio,
    say 195 50 15 instead of 195 55 15 ?
    Body Roll, May 31, 2006
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