Tyres

[hanswuk]

Your most pressing problem would be the fact that it'll knock your speedo reading out and make it inaccurate.

Would it though? The wheel and tyre diameters would be unchanged - only the width would be different and I don't see how that would affect the speedo calibration.

235/40 are not absolute values, but the relation of width to wall-height of a tyre.

Therefore, 245/40 has a different diameter. You'd need 245/35 (on 20" alloys) to have (more or less) correct speedo readings.

On reflection I think TomC is correct. Therefore it won't really be wise to swap tyre sizes at all - it's nerve racking enough keeping to speed limits with a correctly functioning speedo!

 

[RCZ-Performance]

Changing tyre sizes is a common practice. Knowing tyre dimensions and load ratings is a must when you do this though. Making sure you do not exceed the wheels capability is a must as well, in other words a 275 might fit on a 8.5" wide wheel but don't do it!

For those that are not educated on how tyre sizes work (235/40/19 used for explanation):
235 = width in mm (in most cases the tread contact to road)
40 = percent of width used for sidewall (the lower the number the less sidewall, result firmer ride)
19 = width of wheel in inches

A dimension that needs to be taken into consideration is the overall diameter:

235/40/19 = 26.4"
245/35/19 = 25.8"
245/40/19 = 26.7"

It is the overall diameter that will make changes to the speedometer in the car (also odometer), so plan wisely and remember having different tyre sizes is not a defence for speeding tickets The bigger the diameter, your speedometer will show less than what you are really going.

Load Rating is a must as this is the safety factor that the car manufacturer has required for the tyres and something that insurance companies will look at in the event they need too. The factory 235/45/19 Conti's on the RCZ are rated at 96Y, do not go below this!

Not all tyre makers make the sizes you are looking for either. If the size you want is a size found on new cars especially models with a large amount being made and are popular, then these sizes are readily available and from many manufacturers. But certain size tyres not being put on new cars can be harder to find and more expensive.

Hope this helps.

 

[Abzynthe]

This may come in handy to tell you the speedo out %age when changing tyre.

My new wheels will be 245/35/20. On that calculator, its says that @70mph, I will be doing 71.23mph

Tyre Size: 245/35 R20
Diameter: 680mm / 26.77"
Circumference: 2136mm / 84.09"
Difference: 1.76%
Speedo @70mph: 71.23
Fits Wheel Rim: 8x20 to 9.5x20

http://www.alloywheels.com/Tyre_Calculator

 

[Anonymous]

Really helpful posts RCZperformance and Abzynthe! :thumbup:

 

[Anonymous]

Many thanks for these comprehensive answers. As I indicated, I am not au fait with the technicalities of tyres and, in my ignorance, I was taking it that the '40' referred to a fixed measurement rather than a %.

I am now wiser and will stick with the recommended sizes - thank you all. :thumbup: