tyre size for GT

[lolo888]

Hi All,

I just bought a GT '05 (selespeed model).
It came with 18" mags. Just realised that the tyre width on the front is different to the back.
The tyre profiles from memory are:
front = 225/40/R18
rear = 235/40/R18

Could someone please confirm if this config is OK.
Does this affect the ride since this is a front wheel drive and the rear tyres are wider?
Does anyone have this kind of setup? Or should I get 225/40 all around?

Thanks in advance.

[shiny_car]

The potential issues may not be a problem because the differences in overall diameter and rolling circumference are minor, esp when tread-wear is factored in (might be more significant if the 235/40 are brand new with full tread, and the 225/40 are well worn).

Issues include:
*VDC: the VDC setup relies on every tyre/wheel having the same rolling circumference; if it detects one will 'slipping', it can 'activate' and systems can come into play to try and counter the effect (eg: cut power, apply brakes); with your setup, potentially, the front wheels are rotating faster than the rear (ie: smaller diameter tyres rotating faster to 'keep up' with the bigger ones), and the VDC electronics may interpret this as a problem; if you don't see the VDC warning light flashing/flickering (it will flicker when it activates and can be only momentary), then it's not detecting a problem...so there isn't one to worry about
*handling/grip: change in balance, not necessarily for the worse; more grip at the rear makes the car less likely to oversteer (and more likely to understeer); slightly higher rear profile will improve comfort in the rear but unlikely to be noticable
*roadworthy/insurance: I'm not certain on this one; the issue mostly refers to the wheels, which must not be different sizes/widths between front/rear (let alone left/right) unless spec'd by the factory; a difference in tyre sizes front/rear may not be an issue; if you're particularly concerned about this, I suggest getting advice from a qualified roadworthy centre (eg: ring your local RWC mechanic) or VicRoads

Overall, it will be 'safe' and not a major issue. Only if the VDC seems to be 'objecting' should you change back to 225/40 ASAP.

[lolo888]

thanks for that shiny_car, your reply is much appreciated.

I had a closer look and found out that the rear tyres are actually diffent measurements...
the rear passenger has 225/40/18 and the rear right has 235/40/18
will that be an issue? do i need to buy a new tyre (if I do.. which do i go for, 235s or 225s (to match the fronts))...

i have no idea why the rears have different widths... 

[shiny_car]

Dodgy! In VIC, the car would not pass 'roadworthy' and could not be sold in that 'condition'. I can't remember NSW regulations, but I presume roadworthy is not a condition of sale. 

I can only speculate that the 235/40 was 'available' and/or 'cheap' when one of the tyres was replaced. Or else it had two 235/40 on the rear, but one was replaced with a 225/40.

I would repalce it ASAP; not critical, but important, for reasons of handling and grip. And I would choose 225/40, because that is the standard size fo 18" wheels on the GT (in terms of what fits the wheel width properly, speedo).

[Sheldon McIntosh]

Yeah, not a good sign as to the PO's mindset regarding up-keep of a car. 

[lolo888]

i know what u mean... it doesn't give me a great deal of confidence...
but i'm biting the bullet and going ahead with the purchase =)
the pre-inspection i had done didn't comeback with any "red lights"... i also spoke to the mechanic which has taken care of the car most of it's life and he looked up it's past history and he said that all the previous work has been minor.


i'm still undecided as this which tyre to buy, the 235's or the 225's for the rear.
apart from the "looking nice coz it's wider" reasoning to getting 235's, is there any actual benefit as the car is front wheel drive..
are there any negatives?? eg, more fuel consumption coz there's more tyre on the road understeer etc???

[shiny_car]

The negatives will be minor:
*may be more expensive (not always, cos low-demand sizes of smaller tyres can end up expensive)
*tyre rub: not necessarily all the time, but possibly with suspension compression (eg: big bump), and at the front it will be more likely with higher steering angles
*speedo: given that factory speedo settings usually always overread (ie: might show 100km/h but you are travelling 97km/h), having a larger rolling circumference will make the speedo more accurate which is arguably a good thing; I like to correlate with a GPS/sat nav

It shouldn't noticably affect handling or ride comfort, but technically this is possible. It's not a big enough difference I'd say.

I wouldn't sweat over it. But I'd want all tyres to be the same size.

[scuzzyGTV]

Just to add my 2c, if it were me, I'd at least replace both rear tyres to avoid similar issues with wear between left/right side