Wheels And Tyres

[jac114]

O.k here we go.... on the mk1 gs300 the std wheels and tyres

are 225/55/16 i want to take mine up to 17 or 18" when searching for a new set (tyre package deal) what should i be looking for..i get confused with the middle part ie how can you get ***/40/17 and ***/40/18 surely having 40 on a 17 and 40 on an 18" And still be in keeping with the overall dia ... If i was going for a 17" what size tyre/ wheel package should i be looking for... likewise on the 18"????? the last thing i want to do is get it wrong and throw out the speedo..

cheers

jac

[andyhart21]

The first bit refers to the width of the tyre in milimetres.

The second bit is the aspect ratio of the tyre wall to the tyre width, NOT a measurement in millimetres.

The third bit is the rim diameter in inches just to confuse matters.

Hope this helps :)

[JCC]

You need to get the rolling circumference (the distance travelled in one revolution of the wheel) about the same for the different combinations.

You can work out the circumference as follows

(diameter of wheel & tyre) * 3.142

the diameter of the wheel is the size eg 18", to this you need to add twice the height of the tyre. The aspect ratio is the 55 bit or 45 or whatever and is a percentage of the width (the 225 or 235 bit). However the width is in mm. So a 225/55 tyre has walls 123.75mm high and a 235/45 tyre has walls105.75mm high. The difference is about 18mm or 3/4" so you would get a 1" bigger wheel to keep the speedo about the same.

Roughly speaking a 225/55/16 is about the same as a 235/45/17 combination.