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It's time for me to get new tires, and I realized that I my car currently does not have stock wheels and tires on it. I am the second owner, so I'm guessing the first owner changed it. Although they are Subaru branded wheels, so I'm not sure why they changed to a different size....

Anyway, I've noticed for a while that my speedometer is a little fast. Radar speedometers usually put me 2-4 mph under what my dash says. I've never considered it a big deal, but I figured that now that I need to get new tires maybe I can attempt to rectify the issue (preferably without getting new wheels).

I'm looking at this tire size calculator. It lists my stock tire, followed by the size currently on the car, followed by the size I was looking at changing to. By all metrics it looks like the new size would throw things off more, except for the "Speedometer Reading" field. For that one, it looks like this new size would be better.

Can anyone explain to me why the circumference is 1.5% worse, but the speedometer reading is 1.5% better? Would switching be a good idea? Are there other considerations I should be making?

  • In the link the stock rims are 16" (R16) but the current ones are 17" (R17). It shouldn't be possible to mount an R17 tire on a 16" rim. Could you clarify? – Zaid Mar 12 '18 at 19:30
  • Yes; the stock wheels are 16", but the wheels on the car are 17". Apparently the previous owner put on 17" wheels, although I don't know why. It makes it so I can't put on the exact stock tire size without chancing the wheels. – Robert Mar 12 '18 at 19:49
  • what sizes are you talking about? There are literally thousands of tire calculators on the web. If you have doubts about one try another. – agentp Mar 12 '18 at 21:59
  • The stock 17" wheel option for the 2013 Subaru Outback was paired with 225/60-R17 size tires. Unless your wheels are excessively wide or narrow, that will give you the correct wheel geometry. – finleyarcher Mar 13 '18 at 18:52
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The underlying equation that links vehicle speed with wheel revolutions is:

Linear Speed = Radius x Angular Speed

The speedometer is calibrated against angular speed (e.g. RPM, revs/unit time). So any change in wheel radius will translate to directly proportional change in the reading on the speedometer.

A wheel with larger radius will achieve the same linear speed with lower wheel RPMs, so what is truly 60 mph should read slower on the speedo. The opposite is true for a smaller-radiused wheel - the speedo will register a higher-than-actual reading.


Let's crunch some numbers

  • Stock Diameter = 27.85"
  • Current Diameter = 27.63"
  • Proposed Diameter = 28.52"

Here are the two comparisons:

  • Stock vs Current Wheel

    % Error in Speedo reading:

    = Stock / Current - 1
    = 27.85 / 27.63 - 1
    = 0.8%
    

    The positive value indicates that it is estimating the value to be too high, which is what the "Speed Variance" field also suggests.

    % Change in diameter:

    = Current / Stock - 1
    = 27.63 / 27.85 - 1
    = -0.79%
    

    Negative implies the diameter is reduced, which it is.

  • Stock vs Proposed

    % Error in Speedo reading:

    = Stock / Proposed - 1
    = 27.85 / 28.52 - 1
    = -2.35%
    

    The negative value here implies that the speedo reads a value that is lower than actual. The value here is also in fair agreement with the "Speed Variance" field.

    % Change in diameter:

    = Proposed / Stock - 1
    = 28.52 / 27.85 - 1
    = 2.41%
    

    Positive value implies diameter has increased, again in fair agreement with the "Diameter Variance" field.

  • Ok, so what you're saying is that the "Speedo Reading" values are actually wrong? They should be 60.48 mph and 58.6 mph, respectively – Robert Mar 12 '18 at 20:53
  • Yeah, something's not right with the "Speedo Reading" values reported on the webpage, especially for the latter case. – Zaid Mar 12 '18 at 20:59
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If you want to change tyres to improve the speedo accuracy then do so.

The circumference, when you say "worse" do you mean larger or smaller. If you go larger then the distance travelled for 1 revolution of the wheel is greater and a greater inaccuracy on the speedo. If you go smaller then the distance per rev is smaller which gets closer or matches the "standard" gearing for the speedo so it gets more accurate....

  • When I say "worse" I mean further from the stock circumference. The current tires are 0.8% smaller than stock, and the ones I am looking at are 2.3% larger. – Robert Mar 12 '18 at 18:22
  • My question is will this change actually improve my speedo? Or is this a bug in the calculator? The result's speedo field doesn't make sense to me, so I'm looking for clarification. – Robert Mar 12 '18 at 18:24
  • The speedo field seems clear enough - a reading and a difference compared to the first choice... – Solar Mike Mar 12 '18 at 18:38
  • Right, that make sense. But the values in that field don't align with the calculations. For the first size, it says a 0.8% variance, or 2.63 mph. For the second size, it says 2.33% variance, or 0.69 mph. How can a larger percentage variance be a smaller absolute difference? – Robert Mar 12 '18 at 18:44
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It depends what package you have the car came stock with steel 215/70R16 for the 2.5i package, the limited and premium come with alloy 225/60R17. The owner probably did not change it. A lot of people run 16 for winter and then 17 for summer. The speedo is only off by 2 exactly “ I know I own one” so changing tires to 16 might fix speedo but will not ride as nice

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