Alloys and Tyre Sizes

  • Ok all, ive been looking into new alloys for the v6, now as you all know the std rims are 15'' on 205/55 rubber, so i've done a little comparison on rim size and tyre fitment to see the differences that would and could effect the car..
    As some of you may know or not the overall size of the wheel and tyre effects the rolling dia thus effecting the speed reading and could cause to have a clock that reads incorrectly, ie above the actual speed you are doing or below..
    An example of this would be if your wheel dia is out say -1% then at 40 mph it would read more on the clock than you are actually doing so in a restricted area say the 40mph you would not set off a speed camera, in reverse it would be +1% so the clock would read less than 40mph , less than you are actually doing hence going faster than you think you are, consequences of this ie mr plod / speed camera nicked..
    There is a variable that is implied on speed camers etc of an allowance for a margin of 10% error, so if doing 40mph in a normal car std wheels and tyres you can be doing no more than 44mph , this does not mean you won't get a ticket…

    hope you can all see where this is
    add into this variable the factor of incorrect tyre size and it could mean 3 point and a sixty quid fine easily..

    Here's a short list of rim sizes with tyre choices givin the margin in % that they are out...

    Tire Size Comparison

    Specification Sidewall Radius Diameter Circumference Revs/km
    205/55-15 113mm 303mm 606mm 1905mm 525 0.0%

    205/55-16 113mm 316mm 632mm 1985mm 504 4.2%

    205/50-16 102mm 306mm 611mm 1921mm 521 0.8%

    205/45-17 92mm 308mm 616mm 1936mm 516 1.6%

    205/40-17 82mm 298mm 596mm 1872mm 534 -1.8%

    225/40-18 90mm 319mm 637mm 2002mm 500 5.1%

    225/35-18 79mm 307mm 615mm 1931mm 518 1.4%

    225/30-18 68mm 296mm 592mm 1860mm 538 -2.4%

    As you can see making the right choice is improtant, this has been debated on several other forums i am a member of with my cally v6 and one conclusion always remains get it right, before it causes issues with your car and licence..
    I hope i have been of help to some of you over future tyre choice ie size as a fellow car enthusiast i hate to see plod win and get your licence ..

  • Very good post, some handy information in there.


  • Don't forget the effect that sidewall height has on handling and speed.
    And of course unsprung weight.
    Also your speedo reading.

    The lower the sidewall height the less warning you have before you lose grip but the more responsive it feels when entering a corner. Higher sidewalls aid it making quick acceleration due to the flex in the walls.

    The weight of your wheel and tyre combination is important as they are unsprung weight. The lighter the better, ROTA make some really light wheels for the MX-3.

    Speedo - Normally when you are changing the size of your wheels, the overall size of the wheel and tyre stays the same. The bigger the rims the lower profile the tyre has to be to stay the same size. If you increase the overall size your speedo will be inaccurate and you will need to get a seperate speedo to compensate.


    Seeed cameras have to allow for a 10% discrepency in your speedo.
    They also have to allow for a 10% discrepancy in the camera itself.

    No standard car has 100% accurate speedo, just compare your speedo with a sat nav speed reading at 70mph on a straight road, one of my cars was only doing 62mph when the speedo read 70. Sat nav using GPS is 100% correct on a straight road but not on bends of course.

  • Excellent piece of extra info there admin..thanks for the input…

  • @49127be8f7=ukmx3admin:

    Sat nav using GPS is 100% correct on a straight road but not on bends of course.

    Also you will have to be travelling on a flat piece of road becasue they dont take into consideration when the incline of the road changes so you could be travelling 70mph up a large hill and it only reads 60mph.


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