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Hi Guys, perhaps someone can help, I have a 2000 TDDI, its fitted with standard tyres 185 65 r14, I have been offered a set of 175 65 r14, any thoughts?


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Dropping down to the 175s will mean the overall diameter of the tyre will decrease by 2.18%, but that's within the generally accpeted limit of +/- 2.5%

This means your speedo will over-read; i.e. the speedo will show a higher speed (1.1%) than the car is really travelling at and you'll use more fuel to travel a given distance. Examples :-

  • At an indicated 30MPH you'll be doing 29.7MPH
  • At an indicated 60MPH you'll be doing 59.3MPH

You should also inform your insurance company if you decide to use the 175/65R14 tyres because if you don't and are involved in an RTC (Road Traffic Collision) the insurance company will have a very good reason to declare your insurance invalid; then you get done for driving without insurance, no money for the damage to your car and you'll have to settle the other party's damage claims yourself.

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Fair point there Simon! I think perhaps both myself and Rob may have been referring to the profile depth rather than the tread width in the 175.

In which case, my only concern would be, would a 10mm thinner tyre still sit the same on the same wheel? as long as this isnt a problem them it should be fine, but of course, you have lost some surface area, so handling would be reduced.

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Yep, the 175's will be fine and will fit on the rims no problem.

Handling shouldnt really be affected as long as your not into cornering like Colin McRae, which i doubt :)

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Well, if your not going to push the car round corners, then it should be fine, you shouldnt really notice anything, but to be honest you shouldnt have anything to worry about.

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65% of 175mm is less than 65% of 185mm so the tyre diameters will differ thus affecting the speedo readout

Again, 175 is the width not diameter...

Sent from my HUAWEI Ascend P2 using Ford OC mobile app

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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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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.

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