GXP wheel size question.

[Specter GXP]

235 50 18's

is this a strick size or can the GXP rim be fitted with another size 18 tire.
Or anyone with a GXP running a size other than stock on the GXP rim?

[00Beast]

IIRC you can run 245/45R18's, but you lose sidewall (therefore increase chance of damage from curbs and potholes, and your ride will suffer). This is all on the Bonneville GXP, I'm not sure about the clearance on the GP. I would assume it's similar, as they run 245's stock in the front.

[repinS]

Both the front Grand Prix GXP and the Bonneville GXP wheels are 18x8. To that tune, you could run up to 255/45/18, the stock tire size for the GP. The 255's will be just about the same overall diameter as the GXP 235/50/18. As for differing offset and fitment between the two, I can't say.

As Ed mentioned, 245/45/18 is another potential appropriate size.

[Specter GXP]

so a guy posted hes got 215/45/18 BFG KDW2

would these not fit? Sorry I am a tire noob. I am trying to learn but its mindgoogling to figure it out when your not sure whats what.

If they do fit would the tire look to small in the wheel well riding stock height?

[00Beast]

They would be tiny! Why would you want that narrow with that small of a sidewall? 215's?

[Specter GXP]

Its not a matter of why..lol..its a matter of would it be..but if they would look tiny yea nevermind. thanks.

I was looking on Mustang and Charger/magnum forums too as I guess they are the same size tires...still no luck. And can't go brand new for what I really want since I do have a budget.

So I really want something in the 235 range then.

here is an idea. someone said the front rims on the GP GXP wheels are the same almost.
the front GP GXP wheels is 255-45-18 so would these fit on the bonnie rims.

again sorry guys slap the blone wig on me..I have no idea.

[05gxp]

The stock 235/50R18 tires on the Bonneville GXP have the following dimensions:

27.4" O.D.
9.8" section width on an 8" wheel


While the 255/45R18 will indeed fit on the stock 8" wheel you will give up .5" in O.D. which will affect ride quality.

(Tire specs can vary from one tire and one mfg to another)

215/45R18 is out of the question as it is much, much shorter which will not only affect ride quality but pretty much every other aspect of the vehicle (effective gear ratio being a big one).

Remember that every tire, regardless of size or mfg has a recommended rim width range. While the O.D. measurement is important to ride etc the overall width (section width) is important for fitment.

Having said that my '05 GXP has 245/50R18's on it. I went up one size because I was given a free set of Nitto Invo's (only my 2nd set of freebies in 25 years in the tire biz) which are not made in 235/50R18. I personally like the added width but do know that while these are a bolt on there is virtually zero clearance between the top of the tire and the strut and they will rub when turned to full lock and compressed (pulling out of the driveway etc.).

Once I wear out the current Yokohama's I will probably go back to 235's if the tire I decide on at that point is made in this size.

Unfortunately, for our cars there are not many choices in 18" tires and the sizes which work are limited.

Here is my car with 245/50R18's installed (27.8" O.D.)....






As far as replacing the wheels make sure that whatever you use has a 5 lug on 115mm bolt circle.
Many shops may try to convince you that a 5 on 4.5" wheel works just fine but 4.5" = 114.3 mm which is clearly not 115mm. While it is a minimal amount it is still not correct.

[Specter GXP]

Well what I have is the Bonneville GXP rims on my Grand Prix GXP.

These are on my car now as my winter wheels.
The front stock rims on the Grand Prix GXP are 255-45-18 brand new
The rear stock rims on the Grand Prix GXP are 225-50-18 old and need to be replaced.


I have for the summer Bonneville GXP rims. Those are the stock 235 50 18's tires.

What I am thinking is I have brand new all season tires on the front right now on the Grand Prix rims. Two 255-45-18's.

Could I go to my tire place where I got those all seasons and pull those 255-45-18's off my Grand Prix GXP rims and mount them on the Bonneville GXP rims. Then buy two more 255-45-18's and mount them on the other two Boneville GXP rims.
SO I would have 255-45-18's all around on the Bonneville GXP rims on my Grand Prix GXP.

Or let me ask you. Even better. Could I get the 225-50-18's that go on the rear of the GXP and mount those on the same boneville GXP rims. SO they would be the correct fitment for the wheels. just on the summer rims.

thanks for all your help.

[05gxp]

I would be more inclined to run the 225 or 235/50R18's in the winter and the 255/45R18's in the summer.

Of course this depends on where you live and what type of weather conditions you will encounter.

If the GrandPrix came stock with 18" wheels why not just run the OEM size tire on the Bonneville wheels?

[00Beast]

I'd go w/ the 255's in the rear. That way you can rotate your tires all around, and keep it looking even.

The GP's came with staggered sizes, 255/45's in the front on 8" wide wheels and 225/50's on 7" wide wheels in the rear.

[Specter GXP]

I would be more inclined to run the 225 or 235/50R18's in the winter and the 255/45R18's in the summer.

Of course this depends on where you live and what type of weather conditions you will encounter.

If the GrandPrix came stock with 18" wheels why not just run the OEM size tire on the Bonneville wheels?

if that is possible I will do it.

I got about a month left to get this done so going to see what works.
The reason for the smaller staggered size in back is GM said that there might be a rubbing issue. Not sure but many guys buy same size tires and rims from the front and run all 4 the same size.

Thanks for all the help. you just saved me over 500 bucks.

[05gxp]

you just saved me over 500 bucks.

PM me for the address to send my commission!

[05gxp]

The GP's came with staggered sizes, 255/45's in the front on 8" wide wheels and 225/50's on 7" wide wheels in the rear.

That might be the stupidest thing I have ever heard GM doing....

[Specter GXP]

The GP's came with staggered sizes, 255/45's in the front on 8" wide wheels and 225/50's on 7" wide wheels in the rear.

That might be the stupidest thing I have ever heard GM doing....

Well not really. I did a lot of research on the car when i bought it. The reason is like Isaid before the 7" in the back is the biggest they could get away with without rubbing on the car. Though like I said many do put all the same size tires on the car and it works. Only a few said it rubs.

But the reason for the front rims being bigger. Trq. steer. They tried to offset that by giving a bigger contact patch for the front wheels to grab. Does it work? I dont know. But thats why the GXP fenders and diferent than the regualar GP's they needed that 1" more.

On paper GM gave the Grand Prix one hell of a going away present in the form of the GXP. It really made the FWD family car into an almost track worthy competition car.
In reality the people that bought the car really are not going to do what the car was built for. Its nice on paper, cool to have as a feature but practicle purpose nothing more than saying that it came with blah blah blah..
Thats why I went wit hthe Bonnie rims. Do I notice more steer when I give it gas of the line? Yea but that could be me or it could be tires or even yes the 1" less on the contact patch.

Again thanks guys :D

and ohh that commision..umm..pen ran out of ink..and the lights are flickering might lose power....oh ..nnoo...*blink*