need tire size help
Cadillac
 

Cadillac Forums | Help Us Help You | Advertise | Cadillac Parts | Cadillac News | Cadillac Classifieds / (Old System)

Cadillac Technical Archive | Cadillac Dealers | Cadillac Reviews | Cadillac Dealer Reviews | Cadillac Vendors

CadillacForums.com is the premier Cadillac Forum on the internet. Registered Users do not see the above ads.
+ Reply to Thread
Results 1 to 15 of 15
Like Tree1Likes
  • 1 Post By Submariner409
Cadillac STS Forum - 2005 through 2012 Discussion, need tire size help in Past Cadillac Vehicle Discussion; First off let me say that this site is great! I just bought a 2008 STS and love it!! It ...
  1. #1
    BWhets is offline Cadillac Owners Member
    Automobile(s): 2008 STS
    Join Date
    Feb 2014
    Age
    36
    Posts
    3

    need tire size help

    First off let me say that this site is great! I just bought a 2008 STS and love it!! It doesnt have a ton of options but is a super clean car and only has 37k miles. My question is... I have a set of Hipnotic Zen 20" wheels that im in the process of painting to match the body. In the meantime I need to buy new rubber for them but am at a loss on what size tire to put on. Ive seen many pics of 245/35/20 but I dont care for the look in the rear. Does anyone have a pic of a car with a 35 front and 40 rear? Or possibly a 40 on both front and rear. Is this even possible to do? Id rather not lower the car but if need be i wouldnt be opposed to looking into it. Im sure the info is burried somewhere in the site but when I search I never seem to get any results back. Any help is appreciated and if you are going to bash me for the 20" rims just remember.. to each their own. Thanks

  2. Remove Advertisements
    CadillacForums.com
    Advertisements
     

  3. #2
    Ludacrisvp's Avatar
    Ludacrisvp is offline Cadillac Owners Connoisseur
    Automobile(s): 08 STS4 V8 F55 1SG Euro tails & 01 Monte SS Pace Car
    Join Date
    Feb 2009
    Location
    St Michael, MN
    Age
    27
    Posts
    3,211
    The tire size you can use depends on what it originally had. Post what the sizes are in the sticker on the driver door frame ... This will allow you to determine the correct replacement upsize tire for the 20" wheels. Also how wide are the 20" wheels and what offset?

  4. #3
    BWhets is offline Cadillac Owners Member
    Automobile(s): 2008 STS
    Join Date
    Feb 2014
    Age
    36
    Posts
    3

    Re: need tire size help

    Tire size is 235/50/17. Wheels are 8.5" with a +38 offset. The plus sizing take me to a 35 series tire which I am trying to avoid if possible. I dont mind the look but would rather have a little less look and a little more tire. I know the 40 is bigger than recomended but in my opinion not too far out of the question. Thanks

  5. #4
    Ludacrisvp's Avatar
    Ludacrisvp is offline Cadillac Owners Connoisseur
    Automobile(s): 08 STS4 V8 F55 1SG Euro tails & 01 Monte SS Pace Car
    Join Date
    Feb 2009
    Location
    St Michael, MN
    Age
    27
    Posts
    3,211

    Re: need tire size help

    Using your current setup:
    Front:
    225/35-20
    When speedometer reads 60mph (96.6km/h) actual speed will be 59.8mph (96.3km/h): 0.3% slower.
    Rear:
    265/30-20
    When speedometer reads 60mph (96.6km/h) actual speed will be 60.0mph (96.6km/h).

    Code:
    Wheels:			Wheel 1			Wheel 2
    Wheel size		20x8.5 ET 38		20x8.5 ET 38
    BackSpace		158 mm (6.2'')		158 mm (6.2'')
    Typical Weight		~ 35 lbs / 16 kg	~ 35 lbs / 16 kg
    Tires:			Tire 1			Tire 2
    Tire size		225/35-20		265/30-20
    Section width		225 mm (8.9'')		265 mm (10.4'')
    Sidewall		79 mm (3.1'')		80 mm (3.1'')
    Tire diameter		666 mm (26.2'')		668 mm (26.3'')
    						2 mm (0.3%) taller.
    Rim sizes		20x6.5 to 20x8.5	20x8 to 20x10
    Circumference		2091 mm (82.3'')	2098 mm (82.6'')
    Revs per mile		769.6			767.3
    Typical Weight		~ 21 lbs / 10 kg	~ 24 lbs / 11 kg
    Wheel+Tire Weight	~ 56 lbs / 26 kg	~ 59 lbs / 27 kg

    If you had the car programmed to use the AWD model stagger setup (front: 235/50-18 rear:255/45-18) you could do:
    But this would likely look odd.

    Front:
    235/40-20
    When speedometer reads 60mph (96.6km/h) actual speed will be 60.3mph (97.1km/h): 0.5% faster.

    Rear:
    225/40-20
    When speedometer reads 60mph (96.6km/h) actual speed will be 60.1mph (96.8km/h): 0.2% faster.

    or
    front:
    265/35-20
    When speedometer reads 60mph (96.6km/h) actual speed will be 60.1mph (96.8km/h): 0.2% faster.

    Rear:
    255/35-20
    When speedometer reads 60mph (96.6km/h) actual speed will be 59.9mph (96.4km/h): 0.2% slower.

    here is a good place to go for specs.
    http://www.rimsntires.com/specspro.jsp

  6. #5
    Submariner409's Avatar
    Submariner409 is offline If it won't run, chrome it
    Automobile(s): 2002.5 F55 STS/65500mi, 2004 Ford F150 SuperCab4x4
    Join Date
    Feb 2006
    Location
    MD Eastern Shore - Kent Island
    Age
    74
    Posts
    44,160

    Re: need tire size help

    The car/engine/transmission/speedo is programmed to run on specific outside diameter tires and unfortunately, your opinion matters not one whit to the factory set calibrations.
    daddy_o likes this.

  7. #6
    BWhets is offline Cadillac Owners Member
    Automobile(s): 2008 STS
    Join Date
    Feb 2014
    Age
    36
    Posts
    3

    Re: need tire size help

    I have an understanding of the diameter throwing off the speedo and thanks for the info Ludacrisvp. I also know some people are die hard OEM ↑↑↑. Im not that guy and since im paying for the car my opinion is the only one that matters. What im asking is will it physically fit under my car without rubbing anywhere and how does it look. Has anyone got this setup (245/40/20) by chance?

  8. #7
    daddy_o's Avatar
    daddy_o is offline Cadillac Owners Fanatic
    Automobile(s): 2010 STS, AWD,3.6 auto, Black cherry.
    Join Date
    Oct 2011
    Location
    P. R. of MA.
    Age
    53
    Posts
    229

    Re: need tire size help

    The circumference numbers might add up, but, a wider, lower profile requires the FE5 set-up.
    The FE5 keeps the wheel more upright in corners, placing the weight equally on both sidewalls
    The FE1 would be too dangerous cornering at speed. Too much weight transfer.
    The outside wall of the outside tire would take all the weight.

  9. #8
    MoistCabbage's Avatar
    MoistCabbage is offline Cadillac Owners 10000+ Posts
    Automobile(s): Crimson 2001 Seville STS, 1990 Seville (RIP)
    Join Date
    Sep 2010
    Location
    East Boston MA.
    Age
    26
    Posts
    20,927
    Wider/lower profile tires do not require high performance suspension

  10. #9
    daddy_o's Avatar
    daddy_o is offline Cadillac Owners Fanatic
    Automobile(s): 2010 STS, AWD,3.6 auto, Black cherry.
    Join Date
    Oct 2011
    Location
    P. R. of MA.
    Age
    53
    Posts
    229

    Re: need tire size help

    Quote Originally Posted by MoistCabbage View Post
    Wider/lower profile tires do not require high performance suspension
    I'll qualify my statement by saying it is only my opinion.
    I have driven both set-ups, and feel that at speed, a softer suspension with stiffer side walls, would put the weight on only one sidewall.
    Dangerous.
    It is certainly not a requirement, but I recommend a stiffer suspension with lower profiles.

  11. #10
    MoistCabbage's Avatar
    MoistCabbage is offline Cadillac Owners 10000+ Posts
    Automobile(s): Crimson 2001 Seville STS, 1990 Seville (RIP)
    Join Date
    Sep 2010
    Location
    East Boston MA.
    Age
    26
    Posts
    20,927
    Here is a tracked E36 M3 on low profile performance tires. Take a look at the sidewall:



    It flexes and "rolls" just the same as it would on a softly sprung SUV, it just isn't as obvious.

    In any case, the tire isn't going to fail from a little weight on the sidewall:


  12. #11
    daddy_o's Avatar
    daddy_o is offline Cadillac Owners Fanatic
    Automobile(s): 2010 STS, AWD,3.6 auto, Black cherry.
    Join Date
    Oct 2011
    Location
    P. R. of MA.
    Age
    53
    Posts
    229

    Re: need tire size help

    Ok Moist Cabbage, the idea is to keep as much tire tread flat on the road, then control is better.
    A stiff suspension is hard normally on the driver because the idea is to keep the wheels as vertical as possible, more rubber contacting the road, the better the control.
    A softer suspension allows the wheels to tip a bit, hence, the need for a taller profile, softer sidewall.
    The tire can still flatten out, allowing full contact of the tread. Again, better control.
    Vertical wheels can make for hard ride, great speed, and great control.
    Tipping wheels make for a softer ride, moderate speed, and great control.
    Any race car driver will tell you full tread contact is important at any speed.
    I say putting low profile tires on a soft suspension takes away from good road contact.

  13. #12
    Guy.Seminerio is offline Cadillac Owners Connoisseur
    Automobile(s): 2009 STS-V; 1979 Coupe Deville
    Join Date
    Feb 2013
    Location
    Whitestone, NY
    Age
    25
    Posts
    2,628

    Re: need tire size help

    Lower profile tires means stiffer sidewalls which does a better job at keeping the tread on the road and preventing the car from rolling onto its sidewall as much. This will help make a noticeable difference with turn in. If anyone has ever driven the same car on a/s versus summer tires, you know what I'm talking about, summer having the stiffer sidewall (among other differences.)

    As for suspension stiffness, a stiffer suspension prevents excess body movement which causes a shift in weight causing less control.

    If anyone has ever owned one of the older Cadillac boats, including fwd northstars, you may have been on the highway going around a curve and hit some kind of imperfection in the pavement like a dip, causing the car to wallow (secondary and tertiary bouncing movements after the initial dip.) this causes control issues in handling that curve. Also, when you take say a right turn hard on a softly sprung car, the wheels have turned right but because of inertia, your car wants to continue moving forward so your car leans left, and it even slightly rocks left and right until the body catches up with the unsprung portion of the car and your car regains it's composure. This happens in stiffly spring cars but to a much lesser and more controllable extent.

    This all is detrimental to handling and your ability to maintain control of your car when say you have to swerve to avoid something in an emergency.

    I don't believe a stiff suspension prevents the wheels from tipping versus a soft suspension. I would say that depends on other things in the suspension. Honestly I would say that if your wheels tip (a change in camber occurs) at all when you turn, something is broken in there. Wheel bearings?

    If something looks wrong in there feel free to say something. I'm by no means an expert but I think this is basically the idea.

  14. #13
    MoistCabbage's Avatar
    MoistCabbage is offline Cadillac Owners 10000+ Posts
    Automobile(s): Crimson 2001 Seville STS, 1990 Seville (RIP)
    Join Date
    Sep 2010
    Location
    East Boston MA.
    Age
    26
    Posts
    20,927
    Quote Originally Posted by daddy_o
    I say putting low profile tires on a soft suspension takes away from good road contact.
    You are mistaken. And I thought your original argument was that it would be dangerous because it would cause too much weight to be put on one sidewall??

    A wider tire increases the size of the contact patch - more grip, on any car. A lower profile tire results in less sidewall flex - more control, on any car (I'm not talking about oversized rims with 1" sidewalls).

    Tires are not wooden, they flex. A car with a softer suspension setup is not going to have one side of the rims inches higher than the other during hard cornering. The amount the wheels tilt isn't that severe. Also, the geometry of independent suspension systems is such that as the suspension compresses, camber is increased, keeping the wheels perpendicular to the ground (Ever see a car with air ride being dropped? The wheels tilt in as it goes down). The body being tilted severe during cornering (body roll) does not translate into the tires being tilted in relation to the road surface.

    Notice how the body tilts drastically, but the wheels tilt very little in comparison:


  15. #14
    daddy_o's Avatar
    daddy_o is offline Cadillac Owners Fanatic
    Automobile(s): 2010 STS, AWD,3.6 auto, Black cherry.
    Join Date
    Oct 2011
    Location
    P. R. of MA.
    Age
    53
    Posts
    229

    Re: need tire size help

    I'll maintain my position that low profile/stiff wall tires will lose a bit of traction on a turn if the suspension is not tuned for that tire.
    I will use the example offered by Guy .S.
    In a right turn, (lets go with a smooth, even road for this example), the left front tire takes a lot of weight.
    In a FE5, the suspension is set up to keep the car level, and a low profile will keep even pressure on all the sidewalls.
    On a FE1, the suspension lets the car lean a bit, giving off weight to the outside wheel (left).
    With a softer wall tire, full contact is maintained by allowing the outside wall to fully compress and the inside wall to fully extend.
    Tread contact is even.
    With a low-profile, stiff wall, the outside wall does not compress, hence, the inside wall will lift.
    (I am not suggesting that any part of the tire is coming off the ground).
    The amount would be tiny, but the contact surface becomes uneven, and contact is much lessened.
    This is why the tires are matched to the car.

    There is much more to this, the OP never stated the OEM label sizes. When buying tires, axle ratings also matter greatly.
    Putting on a tire with a 94 rating on a car with a 98 axle rating is going to cause a whole bunch more issues.

    I hope you consider this a friendly disagreement Moist, I do read your post, and closely.

    ----------

    I will add to my previous post.
    My previous drive was a 07 CTS 1SG with 18" wheels.
    The first thing I did was study and learn about tires and set-ups.
    I bought new rims and tires, Hancook V-4 235-50-18, and drove the heck out of that car.
    The cornering control and smoothness at plus 120mph was fantastic.
    Anything slower than 60mph was just a jaw-busting rattle.

    But that was then.

    I like my new 1SB, FE1, S-rated, STS, so does my back-side.

  16. #15
    MoistCabbage's Avatar
    MoistCabbage is offline Cadillac Owners 10000+ Posts
    Automobile(s): Crimson 2001 Seville STS, 1990 Seville (RIP)
    Join Date
    Sep 2010
    Location
    East Boston MA.
    Age
    26
    Posts
    20,927
    I'm just trying to understand this theory. You're saying that if we took this Toyota Echo, with skinny tires, (and suspension "tuned for" skinny tires??):



    And swapped on a set of wider, lower profile tires:



    That the handling would somehow be the same, or possibly worse? And that it might be unsafe?

    It goes against all tuning/modding practices I've ever heard of, read of, or seen. What about the hundreds of vehicle models that get larger/wider rims and lower/wider tires as part of a trim level upgrade or accessory add on, with absolutely no change in suspension components? Would this not throw off the suspensions "tune" to taller/narrower tires? Does it make the car unsafe? Some accessory rims are 2, 3, even 4 inches larger than the standard rim. I can't imagine major automobile manufacturers supplying unsafe upgrades that provide LESS traction.

    I guess we'll have to agree to disagree.

Quick Reply Quick Reply

Register Now

Please enter the name by which you would like to log-in and be known on this site.
If you do not want to register, fill this field only and the name will be used as user name for your post.

Please enter a password for your user account. Note that passwords are case-sensitive.

Please enter a valid email address for yourself.

Log-in

Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.



Bookmarks

Cadillac Posting Rules

  • You may post new threads
  • You may post replies
  • You may not post attachments
  • You may not edit your posts
Read about Lincoln | Buick | Kia Forte Forum
Need products for your Cadillac? Check out your options at the links below:

custom floor mats | Cadillac Chrome and Black Chrome Wheels | window tinting