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Cadillac SRX First Generation Forum - 2004 - 2009 Discussion, Tire size difference? in Cadillac SRX Forums; I'm still driving mine and almost getting used to it. The noise is different and I don't think the cupping ...
  1. #16
    Tequila Mockingbird is offline Cadillac Owners Member
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    Re: Tire size difference?

    I'm still driving mine and almost getting used to it. The noise is different and I don't think the cupping is the whole problem. I tried temporarily over-inflating to get off the cupped outside edges and the noise didn't change. It has settled into a distinctive thump thru the drivetrain like a slipped belt in addition to the cupping related growl. Better at higher speeds though. When I can't stand it anymore, its going to get a new set of 4x same size while still under warranty. Nobody else seems to notice it like I do. I mistakenly posted 09 but my AWD SRX is an 2008 and the goodyear tires suck.

  2. #17
    JackB is offline Cadillac Owners Fanatic
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    Re: Tire size difference?

    Here is what I don't get. If Cadillac is suggesting tire rotation with 255s on the back and 235s on the front then why did they put differn't size tires on in the first place?

  3. #18
    PJ1520 is offline Cadillac Owners Connoisseur
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    Re: Tire size difference?

    I don't think Cadillac does recommend tire rotation on all of the first generation SRXs. The set-up with the 20 inch rims (Sport Package) has P255/55/20 all around can be rotated, but the rest of the set-ups with the P235 fronts and P255 rears cannot.

  4. #19
    JackB is offline Cadillac Owners Fanatic
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    Re: Tire size difference?

    OK, so what was Cadillacs reason for throwing this bit of tire confusion at their customers? Was it necessary to reach some minimum weight carrying or towing capacity? They must have done it for some reason.

  5. #20
    carguy2006 is offline Cadillac Owners Member
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    Re: Tire size difference?

    Quote Originally Posted by JackB View Post
    OK, so what was Cadillacs reason for throwing this bit of tire confusion at their customers? Was it necessary to reach some minimum weight carrying or towing capacity? They must have done it for some reason.
    My guess is that GM had towing and the 3rd row seat in the mix for the wider rear tires...

  6. #21
    conedoctor is offline Cadillac Owners Connoisseur
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    Re: Tire size difference?

    It looks cool and I can think of no other reason.

  7. #22
    PJ1520 is offline Cadillac Owners Connoisseur
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    Re: Tire size difference?

    This offeset tire size thing isn't just a Cadillac approach. And not necessarily a weight decision either. Check out the OEM offset tire sizes out there on other brands, makes, and models. Something that can be altered (corrected?) when it is time for the first set of replacements.

  8. #23
    JackB is offline Cadillac Owners Fanatic
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    Re: Tire size difference?

    If it is not a weight decision then what is the reason? Hopefully GM had one other than causing aggravation for owners. My neighbor had to go to 2 differnt tire stores when he needed replacements since the first only stocked one size. The tire size difference surely is not for looks since the small difference between a 235 and 255 is really not noticeable. It is either weight or towing in my opinion. Makes one wonder if GM just got a great deal on the tires at a time they were experiencing severe financial difficulty???

  9. #24
    Tom Wheeler's Avatar
    Tom Wheeler is offline Cadillac Owners Fanatic
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    Re: Tire size difference?

    Quote Originally Posted by PJ1520 View Post
    I don't think Cadillac does recommend tire rotation on all of the first generation SRXs. The set-up with the 20 inch rims (Sport Package) has P255/55/20 all around can be rotated, but the rest of the set-ups with the P235 fronts and P255 rears cannot.
    Correction, I believe the tire size for the 20" sport package is 255/50/20 on all fours. This is the same size with after markert rims that I put on my 07 AWD (OEM 17"). I now have 58,000 miles on the Toyo Proxes S/T's and still have 7/32" tread left. They have been a great tire, have been rotated four times, and no problem with GM warranty at all.

    Tom

  10. #25
    PJ1520 is offline Cadillac Owners Connoisseur
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    Re: Tire size difference?

    Yes.....P255/50/20 ON ALL 4 CORNERS is the size used by Cadillac in the 20" Sport package. So I would challenge any tire dealer to explain why they won't put 4 tires of equal size on the SRX. It's good enough for the OEM, GM!!!! The rolling circumference, revolutions per mile, and speedo and odometer accuracy, can all be achieved by adjusting the aspect ratio selected to make sure all 4 tires are the same, as they are on the offset arrangement, whether one chooses P235, P245, or P255. Availability is a story for another day.

    Re the reasons for offset sizes, think of the physics.

    The narrower fronts still provide adequate turn-in when cornering while presenting a narrower, less wind-catching obstruction for better fuel economy. At an extreme, think formula 1 cars and their offset tire arrangement. Also, generally speaking, a narrower tire with a given aspect ratio will be more forgiving (softer ride for the front passengers) on the rougher roads. This IS still a Cadillac, and customers expect a Cadillac ride, not quite a European firmer one.

    The wider rears are for weight bearing considerations when the vehicle is under heavier load......passengers (up to 7), inside cargo, stuff on the roof rack, and/or towing. Also, wider tires have better dry surface straight and lateral grip, though this vehicle is no 0-60 or autocross champ, nor was it intended to be.

    Some are making it sound as if the decision to go with the offset arrangement was purely a conspiracy between the tire manufacturers and the auto manufacturers to sell more tires. While the offset arrangement is cetainly a benefit to the tire manufacturers (tires cannot be rotated), I don't think it was the driving force behind the decision to go with offset sizes. Also, repeat after me: it is not just GM that takes this approach.

  11. #26
    conedoctor is offline Cadillac Owners Connoisseur
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    Re: Tire size difference?

    I think you guys are searching for an answer that is not there.

    It looks cool, the car was launched and marketed as a cool fast car.

    The load range change between the two sizes is like 100 -120lbs each so I can't see the extra 200lbs rating enough to say now it can tow or take more people.

    The wind resistance maybe but is enough of the tire is exposed to the wind to matter?

    Handling and balance I don't think either, they are not going to balance a car with tire size as it is too much of a variable, they just give you toe in so you end up with understeer at the limit.

    The drivetrain does not care if the front and back dont match so why not make it look nice with wider in the back?

    That what me thinks.

  12. #27
    jsiddall is offline Cadillac Owners Fanatic
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    Re: Tire size difference?

    The normal reason for staggered tires in rear drive platforms is to get lots of rubber on the road for the rear tires and to keep the front ones narrower so the steering doesn't pull so bad on the pavement grooves.

  13. #28
    PJ1520 is offline Cadillac Owners Connoisseur
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    Re: Tire size difference?

    Cone......I started reading up on this on the Benz and BMW sites (and a couple of others) and a couple more advantages came up. Road feel and rolling resistance.

    Re the load ranges and the use of the wider rears, if you talk to folks who tow regularly or tow/stow at or near the load limits, they will advise that you can never have too much tire and too much cooling apparatus (steering, coolant, and trans). They also advise to be wary of stated load limits as things tend to go South at the load limits. Plunk a few wide bodies in your SRX and a couple of height-challenged ones in the rear seats and you might find the "inside the vehicle" load limits are not as high as they might at first seem.

    Re wind resistance.....the auto manufacturers are under the gun to deliver higher fuel economy numbers and every mpg counts. If a reduced rolling resistance and wind resistance helps get them there with the narrower fronts on a staggered size setup, they will do it. These things start adding up.

    Re the vehicles handling and balance, some upscale auto manufacturers actually start with the tire sizes they intend to use (for multiple performance and handling reasons) and they build around it! And they contract with tire manufacturers to develop tires that fill their specs and requirements if one does not already exist. We've seen new tire introductions from the manufacturers that just happen to appear at the same time as a newly designed and released vehicle......provided the tire manufacturer sees a market beyond that initial vehicle.

    Also consider the use of minus sizing for snow (and wet weather) applications, even with AWD vehicles. One might make the argument for the narrower fronts on the SRX for that reason. The narrower tire cuts through the elements and provides more pounds per square inch on the contact patch. Steering precision in these elements is improved too....understeer (plowing) reduced.

    We may think the staggered tires are for looks or are a conspiracy to sell tires, but there are, on the whole, a number of practical considerations for the set up which, when put together as a whole, seem to make sense to me. We have seen the staggered arrangement on autos now for well over 20 years, spread out over a multitude of manufacturers and vehicles. And I don't see the trend moving anywhere but staggered. Even into the more mainstream, higher volume vehicles.

    Yup, I too am not a fan of the non-rotate wear implications, the inconvenience, or the additional expense to me should I choose to remain with the staggered setup. But I think we may have to get used to it as the oem's delivered choice. Or make the change to same sizing.

  14. #29
    conedoctor is offline Cadillac Owners Connoisseur
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    Re: Tire size difference?

    Very well written.

    I still think it just for looks.


    Well if you think they went to a wider rear tire for that little bit extra even if it is only 200lbs then fair enough and that could be, so then why would they add the extra expense on the non-thrid row and non-tow package models and not just make it an option?

    But yes I can see the whole "every few hundred pounds helps.


    As for better turn in and grip I would not want better front in the winter as 99% of Joe public can't deal with oversteer, more front grip in a RWD base car may not be ideal.

    Remember for the average person understeer is safe oversteer is not, the factory does not want the back coming around.

    Most of the staggered setups I have seen have had wheels to match, the SRX does not and only the tire changes.

    I think it was to look cool and they could get the tire sizes off the shelf and the wheels would not need to be changed, while all the other points are valid I don't think they were the first thought when staggering the tires.

    If they were going to do on purpose then they should have done it like the X5 did

    I will be going 255/285 or 265/285 on my new one just cause it looks cool

  15. #30
    jsiddall is offline Cadillac Owners Fanatic
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    Re: Tire size difference?

    Quote Originally Posted by conedoctor View Post
    I still think it just for looks.
    Different things for different people I guess. I personally don't like the look of staggered tires. If I had the money I would have put 255/50-20's all around.

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