different size tires

[rfa]

Tried a search and came up dry. I recently purchased a 2006 sts 3.6 and noticed that the vehicle called for two different size tires, {235/50r/17, 255/45r/17} is it necessary when buying new tires that you purchase the same size or can you go with the 235/50r/17, it is very confusing. Is there a reason for this. Please explain, I do not understand, never ran in to this. Thank You

[STE_6000]

if it showing 2 different sizes, it because of the staggered wheel. ur rear wheels are larger then the front. to be sure, just look at the tires on the rim themselves and u'll see.

[pjbizjak]

It's true,. the rears are bigger than the fronts and this was Cadillac's way of matching BMW, Lexus and the like. When buying new tires you can go with just one size, the smaller or bigger, then you can rotate them in any configuration as long as they're not 'directional' tires. It's no big deal, so when you're ready to replace the tires just go with what's on there now.

[rfa]

sorry i must have presented my problem the wrong way. i am concerned about buying the same size all around or staying with a staggered pair the smaller size being the prevalent size thank you for your reply.

[STE_6000]

u cant buy the same tire for staggered, u must replace them how they are. tires are designed to fit circumference and diameter and width of a wheel. the fronts are 7" or 7.5" width and rear is 8" the tire size on the back will be to large, width wise to fit on the front.

[JGarten]

Originally Posted by STE_6000

u cant buy the same tire for staggered, u must replace them how they are. tires are designed to fit circumference and diameter and width of a wheel. the fronts are 7" or 7.5" width and rear is 8" the tire size on the back will be to large, width wise to fit on the front.

Not true! I bought my 2005 V8 1SF used with 235/50ZR17 Goodyears on all four, staggered rims (later discovered that both, wide rims were mounted on right side to boot) and ran them that way for a year before I replaced all four with 235/50ZR17 Michelins. No problems of any kind (and I drive it pretty hard). Now weird tire wear, alignment issues, traction control, nothing. I did have them put the wide rims on the rear for grins.

Yes, the 255/45ZR17 has 20mm wider tread, but the tire diameters/circumference are the same on either rim i.e., 235/50ZR17 = 255/45ZR17 with a little extra tread width. The "rear" rims are only 1/2" wider than the "front" rims (1/4" each left/right cross section) which only slightly deflects the sidewall, but not enought to affect the tire. If you check out tire on Tirerack.com, you'll find that tires are rated for minimum and maximum rim widths. The only down side to using the narrower tires on the wider rims is that rim may be slightly more susceptible to curb rash.

I would think that the extra tread on the rear end would only be benificial with VERY spirited drving under ideal conditions with a big honking STS-V motor. My "puny" V8 doesn't need 255s, 235s do rather nicely and I can rotate them!

[STE_6000]

i would double check the rims. but i dont trust that statement. im sorry but the tires are designed for different thickness. now u have have tires that will give you more tire wall ,then yes the side can be changed, but then your gonna set off ur speedo. best off talking to professional inless ur going to change ur own tires urself.

[JGarten]

Useful site: http://www.1010tires.com/TireSizeCalculator.asp

Stock Tire - 255/45R17
vs. 235/50R17

Section Width: 10.03 in 255 mm
Section Width: 9.25 in 235 mm

Rim Diameter: 17 in 431.8 mm
Rim Diameter: 17 in 431.8 mm

Rim Width Range: 8 - 9.5 in
Rim Width Range: 6.5 - 8.5 in

Overall Diameter: 26.03 in 661.16 mm
Overall Diameter: 26.25 in 666.75 mm

Sidewall Height: 4.51 in 114.55 mm
Sidewall Height: 4.62 in 117.34 mm

Radius: 13.01 in 330.45 mm
Radius: 13.12 in 333.24 mm

Circumference: 81.77 in 2076.9 mm
Circumference: 82.46 in 2094.4 mm

Revs per Mile: 799.0
Revs per Mile: 792.3

Speedometer: 60 mph 100 km/h
Speedometer: 59.4 mph 99.1 km/h

Speedometer Difference: n/a
Speedometer Difference: 0.845% too slow

Diameter Difference: n/a
Diameter Difference: 0.84%

...so not *exactly* the same, but all in all pretty close (-0.6 MPH, +6.7 RPM). I believe that their recommendation is not to exceed OEM by 3%. As long as the manufacturer rates the tire/rim combination, you're golden (they wouldn't dare rate an unsafe combination). And I do have my tires professionally mounted (don't we all?).

[STE_6000]

so that just shows the difference. problem with tires that are so small width wise it can become unseated easy and cause a blow out. and by blow out i mean where the tire becomes unseated where the wheel's lip is. over side u might be ok, but u'll cause awkward pressure on the tire wall from the tire being larger then the wheel. i would just highly recommend if ur sticking with ur stock wheels to go with the correct stock tire sizes. to fit the wheel correctly and to not offset ur speedo.

[z06bigbird]

Buy three extra rims (front size), and sell two of your orig ones. Use third for a real spare. Then buy 4 matching tires. Use one orig for spare if you like, or buy a 5th tire.

This will allow you to rotate tires every 5,000 miles or so. Everyone lives happily ever after.

[JGarten]

Originally Posted by z06bigbird

Buy three extra rims (front size), and sell two of your orig ones. Use third for a real spare. Then buy 4 matching tires. Use one orig for spare if you like, or buy a 5th tire.

This will allow you to rotate tires every 5,000 miles or so. Everyone lives happily ever after.

Good advice! Maybe he can just find someone to trade? Someone out there must be nuts enough to want 255/45ZR17s on all four corners!

[STE_6000]

Originally Posted by JGarten

Good advice! Maybe he can just find someone to trade? Someone out there must be nuts enough to want 255/45ZR17s on all four corners!

ya who wants the small wheels? heh tho the size widths not that much.

[EChas3]

1/2 Inch wheel width - no big deal. 1 Inch... Maybe...

The 17" staggered wheels pose a good question. I'd probably go with the larger tire and put the bigger wheels on the back. I'd rotate occasionally but put 2/3 of the miles on as described.

I may be wrong but I'm not uncertain!

[STE_6000]

Originally Posted by EChas3

1/2 Inch wheel width - no big deal. 1 Inch... Maybe...

The 17" staggered wheels pose a good question. I'd probably go with the larger tire and put the bigger wheels on the back. I'd rotate occasionally but put 2/3 of the miles on as described.

I may be wrong but I'm not uncertain!

I wouldnt chance it. i like drive quick at times and feeling the car take corner well, anything that was specific for that rim is a chance not worth taken. He is better sticking to stock specs. When he rotates the wheels he will till have to move the wider wheels to the front which could throw the system off. seeing it is setup to have wider wheels in the rear by factory defaults.

[next2pool]

As others have said (maybe in a different way) the issue will be running the 255's on a narrower wheel than what is recommended. I know recommended is just that, but there's a reason for those wheel width specifications. The contour of the tire is affected by the mounting width and therefore won't be in a normal design range. Will you have problems?--probably not. Will you notice it?--only if you are sensitive to handling and traction issues. It just won't be optimized.