car tyres sizes

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  • car tyres sizes
  • Premier Icon SaxonRider
    Subscriber

    If I bought rims off ebay that currently had 195/70 r15 tyres on them, is it possible simply to put on 205/65 r15 tyres?

    Would they fit? And if they fit, would the overall diameter change, thereby throwing off the speedo? Looking at one tyre site made it seem as if it would, but another gave me the impression it would be alright.

    Premier Icon Woody
    Subscriber

    Do you know what the rim width is?

    You can see how it will affect speedo accuracy HERE

    nealglover
    Member

    Or try putting in the details on http://www.willtheyfit.com

    (ignore the ET figures. Just put in something like 44 for both)

    Ho hum
    Member

    Not a huge change in rolling radius to be honest.

    It’s the 5mm decrease in tyre height that affects the speedo accuracy.

    % wise it won’t amount to much.

    Premier Icon Northwind
    Subscriber

    First you’ve got to understand the numbers. First part is width, second part is the height as a precentage of the width.

    So 195/70 is 136.5 tall and 205/65 is 133.25. So you have a total rolling circumference difference of not much- 2034mm vs 2055mm, around 1%, giving you a speedo error also of around 1%. (and a gearing change too) But, what this also means is that when it’s about halfway worn out a 195/70 is roughly the same size as a 205/65 is when its new

    But you also have to know 2 other things… One is that your speedo is almost certainly already inaccurate by design, it’ll overread (by more than 1%)

    The other is that not all tyres are the size they claim to be. I don’t have numbers for car tyres but I did some measuring for motorbikes and the error rate was pretty considerable, as much as 5% off, so a bigger variance than you’ll get with your change of size. Car tyres may be better.

    Course, they’re wider so you might want to check your rim width too. 10mm is not an awful lot but you never know.

    Premier Icon downshep
    Subscriber

    Northwind speaks wisely. Speedos’ must not underestimate speed, so often overestimate. Fitting tyres with a lesser rolling circumference will diminish the error.

    Be aware that insurers will run a mile if you deviate from manufacturer spec and have a bump.

    wrightyson
    Member

    One other thing if you’re going wider is to check for any rub on internal arches when on full lock. But for that difference I can’t see it being an issue!!

    shaxi
    Member

    i hope that will do as you expect ! 😀

    AndrewBF
    Member

    The Wheel and Tyre Bible.

    (Almost) everything you’ll need to know on the subject:

    http://www.carbibles.com/tyre_bible_pg4.html

    The 17″ ‘equivalent’ tyre table down at the foot of the page is illuminating in showing how the various combinations can be the ‘same’.

    Premier Icon SaxonRider
    Subscriber

    Thanks for that. I remember encounter ‘car bibles’ some years ago, but forgot about the page. It’s an amazing resource.

    Premier Icon Northwind
    Subscriber

    That’s an excellent link- I just did the maths without deformation since both tyres probably deform fairly similiarly but it’s nice to do it right.

    downshep – Member

    Northwind speaks wisely. Speedos’ must not underestimate speed, so often overestimate. Fitting tyres with a lesser rolling circumference will diminish the error.

    Is it not the other way round? Smaller tyres travel less distance for the same revolutions so decreases the actual speed compared to the indicated? (my head is full of manflu so that might be bobbins)

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