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Can I 175?


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#16 turbohater


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Posted 16 March 2014 - 02:01 AM

The 65 figure is the % of the tyre width,so a 185 tyre will have a slightly bigger diameter and cicumference than a 175 tyre.The circumference will differ by approx 2",not much but as rob says it will affect the speedo slightly.

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#17 Captain Haddock

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Posted 16 March 2014 - 08:26 AM

The terse explanation is :-

  "Width" is not just the tread width and it does affect the speedo and your MPG.


The full explanation now follows; which will be probably TLDR for some people, but 'in for penny, in for a pound'.


An explanation of the most common numbers you will find on Car/LGV/HGV tyre sidewalls.

Note: I can't speak for MotorCycle/Earthmover/Aircraft/Floatation/Racing tyres as I never trained to service those.


Let's use Coleco82's tyre original size 185/65R14 (they probably also have 82H or 82V marked on the sidewall too) as an example :-

  • 185 = Section Width (in mm); this is not the width of the tread as the measurement also includes part of the sidewall since Section Width is defined as being the dimension across the widest part of the tyre (the width of the treaded area is narrower than this).
  • 65 = Aspect Ratio (%age); this the ratio of the height of the sidewall to the Section Width.
  • R = Radial; unless you look at a vehicle/trailer/caravan that's late sixties or earlier I doubt anyone will encounter a Crossply tyre.  Before anyone asks, the answer is 'Yes, I have fitted Crossply tyres and on many occasions'.
  • 14 = Wheel Rim Diameter (in inches); this the diameter of the rim the tyre is designed to fit upon.
    Yes, it does seem crazy to use mixed units of measurement, but the rim diameter measurement is a carry over from the early days of motoring (Crossply tyre sizes are marked in inches, e.g. 6.40" x 16").
  • 82 = Load Index; this is a look-up code that indicates how much weight a single tyre is designed to support. You will commonly find load indexes of 82 (475kg) or 84 (500kg) used on most modern cars, with larger vehicles (e.g. Vans/Estates/MPVs) using 84 (or above) load index tyres.  High performance cars also often have a higher load index.
  • H = Speed Rating; the continuous maximum speed the tyre is designed for when supporting its designed load (Load Index).


So from the rather lengthy explanation above you can see that for a correctly inflated tyre and a constant Aspect Ratio (65%) by changing the Section Width the rotational diameter (as measured around the centre of the tread) will increase for larger section widths and decrease for smaller section widths.

  • 65% of 185mm = 120.25mm and the tyre's rotational diameter will be 596mm
  • 65% of 175mm = 113.75mm and the tyre's rotational diameter will be 583mm

So a 185/65R14 tyre needs to rotate 1677.85 times to cover 1km whereas a 175/65R14 need to rotate 1715.26 times to cover 1km; which is a 2.22% increase in rotation for the 175/65R14 versus the 185/65R14 :-

1715.26 - 1677.85 = 37.41

(100 * 37.41) / 1677.85 = 2.22%


Since a wheel with a smaller tyre has to rotate more times to cover the same distance the speedo will over-read as it will have been originally calibrated for rotation rate of 185/65R14 tyres.


Here endeth a cure for insomnia :D.

#18 Coleco82



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Posted 16 March 2014 - 11:01 AM

Nice one Captain Haddock

#19 mixmasterlooney


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Posted 16 March 2014 - 11:32 AM

Very well explained. It cannot be put any better than that

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