mixed up tires on 18 inch OEM wheels

[phil-simon]

Hi guys

I bough a set of the OEM 18 inch wheels and tires that came with the 2003-2005 Z4 3.0i Sport. Paid $420 for the whole set. The rear tires have about 80% left, the front ones about 50%. So far the good news. Bad news: The tires sizes are a bit mixed up. According to Edmunds the original specs where: 8x8.0 and 18x8.5 with 225/40WR18 and 255/35WR18 Bridgestone Potenza 050A. The ones I bought have 265/35 R18 on each of the rear wheels and different sizes (!) on the front ones: one has 245/40 R18 and the other one 235/40 ZR 18. I realized that when I picked them up so I got a discount and I think its still a good deal. Now my questions are
- rear wheels: are 265/35 ok when the stock tires where 255/35?
- front wheels: I obviously have to buy at least one new tire to either have both tires at 245/40 or at 235/40. Does that even work when the original tires size was 225/40?

Thanks
Philipp

[Hawkeye]

Well, obviously those tires did not come with those rims. I know you can go a little wider than the rim in some cases, but I would probably stay as close to specs as possible.

Hopefully someone that knows more about tires than me can chime in as well.

[3002 tii]

The problem is that your tires are all now different heights. The stock sizes of 225/40 and 235/55 put you at a sidewall of approx 3.5" tall, but the 235/40 is 3.67", 245/40 is 3.82", and the 265/35 is 3.62"

Because the 235/40 and 265/35 are closest in height, I'd ditch the 245/40 and buy another 235/40 up front. If you are using oem wheels on oem suspension, I doubt you will rub so I think overall you should be fine.

[GomboRombo]

I was looking into a 265 rear for my stock 8.5 rims. The guy at America's Tires told me my rim was not wide enough for a 265 and was not recommended. But I guess if it looks good on yours, I may go to another tire shop for a second opinion.

[3002 tii]

Originally Posted by GomboRombo

I was looking into a 265 rear for my stock 8.5 rims. The guy at America's Tires told me my rim was not wide enough for a 265 and was not recommended. But I guess if it looks good on yours, I may go to another tire shop for a second opinion.

It's not suggested but definitely better buying 1 new tire beats buying 4 new tires.

[phil-simon]

maybe a stupid question but is the height of the original 255/35 rears also 3.62'' like the 265/35 I now have? If the wheels get bigger in diameter overall that means that the actual de-facto gear ratio changes - will the car be more sluggish? Has anyone observed this with switching from 17 - 18 inch in general?

[3002 tii]

Originally Posted by phil-simon

maybe a stupid question but is the height of the original 255/35 rears also 3.62'' like the 265/35 I now have? If the wheels get bigger in diameter overall that means that the actual de-facto gear ratio changes - will the car be more sluggish? Has anyone observed this with switching from 17 - 18 inch in general?

Originally Posted by Dubbedown

The problem is that your tires are all now different heights. The stock sizes of 225/40 and 235/55 put you at a sidewall of approx 3.5" tall, but the 235/40 is 3.67", 245/40 is 3.82", and the 265/35 is 3.62"

I already answered that.

It's a simple calc, 255 (width) x .35 (sidewall ratio) = 89.25mm, converted to inches multiply by 0.39 = 3.48"

As you increase wheel size, your overall diameter remains the same if you use the recommended tire sizes since your sidewall changes accordingly. The only thing you noticed going from 17->18 is the effect of more unsprung mass. I'm sure your 18s with rubber weigh more than your 17s with rubber.