By on August 18, 2017


TTAC Reader DeSoto writes:

Greetings, Sajeev — longtime reader, first time writer, and I enjoy the content your column adds to TTAC! (Woot! — SM)

I recently acquired an all-wheel drive 2014 Cadillac CTS with about 15,000 miles on its factory 255/35 R19 Pirelli P7 run-flat tires. Looking ahead to an upcoming 3,500 mile driving trip, I have concern for the durability/drivability of the run-flats. The CTS is not equipped with any wheel-changing tools (jack, wrench, spare tire/wheel). A spare tire assembly (for the CT6) is available online, but I believe the standard compact spare is too small in diameter to be compatible with the 19-inch wheels on the AWD drivetrain.  I prefer the freedom of having the option of changing a tire myself and continuing on my way, rather than waiting hours for assistance. I am thus considering, in order of my current preference:

1) Purchasing (along with four replacement non-run-flats) an additional wheel and tire to carry as a spare (using valuable trunk space, I know, but it’s okay). With the AWD concerns regarding uneven tire diameters causing drivetrain damage, will a five tire rotation scheme every 5,000 miles be appropriate to prevent drivetrain damage caused by unequal tire diameters? Is there a “best” jack specification available for purchase? I have found diagrams indicating the lifting points for use with a lifting rack, and these appear to be accessible for use with a single point jack.

2) Replacing the four run-flats with non-run-flats, and carrying a can of fix-a-flat. Does the use of fix-a-flat require plugging a puncture at the time of use? Will fix-a-flat harm the TPMS sensor?

3) Quit worrying so much, getting with the 21st century already and purchasing replacement run-flats. Even if the Pirellis are lousy tires, are there high performing run-flats (vs. non-run-flats) out there, in terms of handling, comfort, and durability (pothole induced blowouts, rapid treadwear)?

Sajeev answers:

Would you believe there’s a website for buying spare tires, including the jack? That’s a cottage industry I encourage in this age of rubber-band sidewalls, crumbling concrete infrastructure and run flat tires that still leave you stranded if your TPMS is on the fritz. (Which happens!)

That website lists two tires, either way too short on diameter (T125/60R-18, which is 2+inches shorter) or still short enough to say “no”” on an AWD vehicle (T155/60R-18, 7/10ths too short). Ouch. Using more Google-fu, an official GM spare exists (with jack!), but it’s still kinda short (T135/70R-18, 6/10ths too short) for your application.

Read your owner’s manual, but I reckon GM will insist on far less than a 1/2” diameter discrepancy between tires. And use this fun tool if the discrepancies are measured in circumference. So much for that, but at least we tried. 

Regarding your three options:

  1. You covered all the bases of owning 5 tires for a single car, but I am less than thrilled with it. I live in a big city and travel interstates, those who live in more remote sections of this country shall disagree.
  2. This is a good alternative, especially the cans that come with built-in air compressors. Consumer Reports says they haven’t experienced TPMS fouling from fix-a-flat, but you can expect a ton of foul language from the guy cleaning it up to patch/plug the hole. So make sure you tell them that you used fix-a-flat first, and give them a decent tip for that extra labor.
  3. Probably my favorite option, let’s spare ourselves all the drama. If you hate run-flats, modern day wheel/tire packages that can’t handle potholes, etc., just buy a truck like everyone else does.

Off to you, Best and Brightest!

[Image: General Motors]

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39 Comments on “Piston Slap: Spare Me Your AWD Drama...”

  • avatar

    Even a can of fix-a-flat AND a tire patch kit in your trunk will never offset the capability of being able to keep driving on the tire after all the air is gone. I once clipped a curb in my BMW with run-flats and I just kept driving the 10mi (or so) until I got home.

    I still keep a tire patch kit in the car, and I even went and bought a compact spare from the X3 and keep it in the back of my AWD 328XiT in case things do get bad. (along with the factory jack and tire iron) But the run-flats are the difference between driving to a safe place to change a tire and being stuck in a really bad spot on the highway dealing with it.

    • 0 avatar

      Runflats are fine if you don’t put a bulge in the sidewall, which is common when the reinforced runflats encounter a severe pothole or similar obstruction. Conventional tires have more give, so the runflats can actually work against you here. Oftentimes you can’t drive like that, and since the runflats had you thinking you didn’t need to be prepared, the result is still a call so roadside assistance. Then a big bill from the tire store – especially if the tire was worn enough that you can’t just replace the damaged tire.

      Runflats are heavy, expensive, fast-wearing and provide a false sense of security. And because the sidewalls are so stiff, more impact forces are directed toward the wheel, so bending a pricey alloy is also common. The technology is getting better, but the best runflats still can’t compare to conventional rubber.

  • avatar

    we have gone the fix a flat route with my wife’s vehicle (came with runflats originally). she has used it a couple of times; she also had to call a tow service once when the hole was too large.

    the tire shop has never bothered to fix the tire with fix-a-flat in it. they just give her a new one under our road-hazard warranty we bought. i suspect if you didn’t have that they would begrudgingly try to fix it. they might even clean the original at some point and sell it as used/repaired…

    no matter what, if you don’t have a spare, good chance that at some point you will be calling a tow service or friend/relative (and be working on the side of the road a lot longer than you care to).

  • avatar

    Fix a flat is OK, but seriously, I don’t understand why people don’t just buy plugs!

    From my experience, fix-a-flat can fix about 40% of flats, a plug can fix another 50%, and then 10% or so are unfixable.

    If you care going to carry fix-a-flat, also go buy a plug kit. They cost $5-10, and are really not that hard to use. Takes less than 30 seconds to a minute to plug a tire from the outside. the best part about plug kits is they don’t damage your wheel and there’s less urgency to get to a repair shop ASAP.

    The other thing no one mentioned is the folding/expandable tires. If space is at a limit, those things are actually REALLY COOL. We have one for my wife’s car and its super compact and then blows up to a giant tire. They are available for a whole bunch of sizes.

    Lastly, I know you were concerned about trunk space, but I had your same car and I put a spare tire in the spare tire holder under the trunk… There’s enough room in the spare tire well to hold a spare tire, leaving the huge trunk empty. I was really cheap so used the camaro wheel with the spacer because that’s only $115 compared to the like $450 for the official one for your car…

    the author said he couldn’t find a compact spare the right size, but what you really just need is the compact spare WHEEL and then go buy the right size folding tire!

  • avatar

    As others have indicated, I think it depends on what type of driving you do. If most of your driving is in urban/suburban areas, run-flats are fine by themselves. If you have a long commute or regularly take long trips, a can of fix a flat, or a spare with the proper tools is the best insurance against being stranded 25 miles from a garage.

  • avatar

    Friend had a BMW 335 with factory run flat tires. He swapped them out for normal tires and the difference it made in the ride was huge. Road much nicer and absorbed bumps better without the super stiff sidewalls of run flat tires. The tires were also much cheaper and he had a wide variety to choose from.

    Funny that that BMW can no longer figure out a way to fit full size spares, or any spare into their cars anymore. The E46s had them and they were smaller. They probably do it to save weight and gain that .001 mpg.

    The downside to my friend switching to run flats is that upon trade in time the dealer he was buying new car from docked him $1k for needing to replace the tires on the car with run flats before they could sell it. Even though the tires on the car had plenty of tread on them.

    • 0 avatar

      Same here. 2011 335i had two sets of the factory tires. Car was horrible to ride in. Local BMW club members all suggested Michelin Sport 3+. WOW! BMW engineers should compare the two to grasp how horrible the run-flat factory tires are.

      BTW, this car has two different sizes front and rear. I don’t know if in a pinch you could mix-and-match to get to a tire service center.

      • 0 avatar
        DeSoto, Adventurer

        I went with Michelin A/S 3+, and the improvement in ride quality was immediately and dramatically noticeable.

        The CTS AWD has all wheels the same width, so one tire size. The V-Sport trim (RWD) is fitted with wider rear tires, so different tire size. Staggered tire sizes is another potential complication when the unexpected occurs.

    • 0 avatar
      30-mile fetch

      “Funny that that BMW can no longer figure out a way to fit full size spares, or any spare into their cars anymore.”

      Seriously? There’s no well in the trunk to put a spare? True to BMW form of charging extra for items that should already be on the car, I know you can buy a spare tire and jack for the F30, I just assumed that there would be an under-floor well to store it in.

      So if you buy a spare you have to leave it to bang around with your groceries, or tie it down like some weird imitation of a 1964 Ford Galaxy trunk?

      These cars are bigger than their predecessors, so what’s that real estate being used for in the F30?

  • avatar

    What’s the difference in diameter of the run-flat at normal pressure and at atmospheric pressure?

  • avatar
    Stanley Steamer

    A tire patch kit and a cheap compressor is all I ever needed. I have had to use them 20-plus times with my old BF Goodrich tires, but never had to use the spare donut. I haven’t had to use either since getting rid of those tires.

    • 0 avatar

      the last two tire punctures I’ve had both were holes in the sidewall. can’t plug or patch that.

      • 0 avatar
        30-mile fetch

        The off-pavement tire repair kit I bought for the 4Runner has a needle and steel thread for suturing sidewall tears, with a tube of cement to slather over the surgical site.

        I have somewhere between 0 and 0.1% confidence in that, and they provide enough material for only 2 attempts whereas there are 20 standard tread puncture plugs. But it’s nice of them to provide a Hail Mary for when a glimmer of hope will be appreciated.

  • avatar
    Secret Hi5

    Don’t change anything. Stay with your existing setup, and in the unlikely event of a flat, drive on the run-flats. At that point, you can assess your options, including switching to non-runflats. Why spend the money now? (Unless . . . If you’re driving through remote areas, then maybe get a spare tire.)

    • 0 avatar

      Excellent point there “…in the unlikely event of a flat…”. I drove from San Diego to Providence RI and back in 1971, ’72, ’74, and ’76, the first two in a ’65 Impala, the other two in a ’68 Montego, both with standard 1970s steel-belted radials, and I never had a flat!.

      It was mostly interstate, but the trip included several hundred miles of detours on bumpy secondary routes, and I usually drove a few miles off the freeway for cheaper gas than the interstate stations (36 cents-outrageous!). They weren’t run flats, or rubber bands on wagon wheels, but even on side roads you can usually steer around nasty-looking potholes.

      I’m not surprised there are no full size spares, and few people under 50 who know how to change a tire. A flat is a rare occurrence, even if you drive a lot. Nineteen inch wheels with low profile tires might be an exception, though the 19×5 artillery type on the Model-T were pretty rugged, considering the state of roads in that era.

  • avatar
    DeSoto, Adventurer

    Hi all, Many thanks, Sajeev, for posting my questions! I’ve finally been published! Woot, indeed.

    Sajeev, I went down the exact same internet path you described, but felt that all the solutions offered were only close, but not a solution I’d be satisfied with long-term.

    Arach, I have never heard of expandable tires. I will check it out!

    MWebb, it is all about the type of driving I do. I like to meander, constrained only by a hotel reservation for the night, and sometimes less than that. I took the trip about 3 weeks ago. We traveled 3,740 miles over 8 days of driving (with rest in between) and did it with a full size spare in the trunk while enjoying some roads fairly off the beaten path (e.g., Missouri Route 90), with no worries about being delayed or stranded by a tire failure.

    So, I am now committed to five-tire rotations for the next 50,000 miles.

    I don’t regret my purchases. Not even the Cadillac. . .

    • 0 avatar

      I’m on your side, Adventurer. I take a 2000 mile round trip 2-3 times a year and drive through some of Montana’s vast empty spaces. I have a repair kit and plug kit, too, but no goop. You can install a patch or plug even at 4a.m. on the roadside. I believe Cadillac is being punished for past sins. Their cars are, to me, as good as any on the road today.

    • 0 avatar

      You sound like me. Despite (sometimes) doubling my expected travel time, I usually enjoy the trip more when I select “avoid highways”.

      The scenery is better, there aren’t 40 crossovers a second flying around me on a multilane interstate (doing either 60 mph or 85 mph, like there is no option in between), and I am more relaxed when I arrive.

      When I go out to west coast (I love in the Southeast), I love taking an alternate route. Little winding two lane through the mountains is far better than being driven crazy by other drivers on the main road.

      I like stopping in little towns, mom & pop stores, etc. Much better than hookers and tweakers at truck stops.

      Oh, and I have a larger spare in my car usually, I know its 15″ and my wheels are 16″, but I only put it on the right rear, and I haven’t noticed any issues when I’ve had to use it in the past. Its a full size steel wheel, not a mini spare.

      • 0 avatar
        DeSoto, Adventurer

        I live in the Southeast, but with maturity come the lowered expectations that permit me to love most anywhere.

        Recommendations from this trip:
        1) Roy’s BBQ, Hutchinson KS – open 11 AM until the food runs out (and it does, most every day I imagine).
        2) Norske Nook, Rice Lake WI (with locations in Hayward and Osseo!). Hot turkey sandwich with gravy, don’t forget to take home some homemade strawberry pie!
        3) Sanders Café, Corbin KY – Yeah, it’s a KFC. The original one, featuring museum displays chronicling the accomplishments of Colonel Harland Sanders, a fascinating embodiment of the American spirit (sincerely).

  • avatar
    Kyree S. Williams

    I worried about that with the X5, for the short time I had it. It had 19″ run-flats. Apparently, most of the E70s came with run-flats. They could be ordered with standard tires and a flat-tire kit to go in the rear compartment area, but most did not come that way. And the models with the folding third row and rear load-leveling air suspension didn’t even have the option for a spare tire kit (since there was no room for it), and you were stuck with the run-flats. I toyed with the idea of replacing them with conventional tires and a spare tire kit.

    Even more interesting is Volkswagen Group’s competing-sized vehicles, which are the Touareg, Cayenne, Q7 and presumably the Bentayga. Each has a giant space-saving spare tire—rather than a donut, since these are heavy vehicles—that has to be inflated using the onboard equipment. And the spare tire has a bright orange rim, too.

  • avatar

    What I would do:
    Building a zombie donut spare tire.

    Original tire:
    255/35 R19, circumference: 81.7 inch

    Fitting compact spare:
    T125/90 R17, circumference: 81.2 inch. Should fit quite good. Plus: a comfy ride (sort of).

    Get a donut spare tire from a BMW X3 (E838) (5×120), dump the 135/70 R17 tire and put on the new one.

  • avatar

    “Probably my favorite option, let’s spare ourselves all the drama. If you hate run-flats, modern day wheel/tire packages that can’t handle potholes, etc., just buy a truck like everyone else does.”

    One of the reasons I chose my Highlander was that not only does the 4wd version come with a full size spare that spare was mounted on a 5th factory aluminum wheel.

    I’m starting to find the pickup option more attractive as well. Get something in mid level trim with 18 in rims… mmmmmm nice and cushy with a full size spare.

    • 0 avatar
      DeSoto, Adventurer

      The only “truck” I’ve ever even considered purchasing was a 1971 El Camino – and I was considering it in 1976. But it did have a full size spare.

    • 0 avatar

      I wonder how extensive across Toyota’s vehicle lineup is the policy of including a full-size spare with a fifth wheel. My current LS460 (with 19-inch forged wheels) has the same setup, as did my previous car, a Lexus IS350. I didn’t realize the Highlander came with the setup. Good to know.

      I may be a bit old school in this regard, but I like the peace of mind of having a full-size spare that I could install myself in the event of a flat, though I also keep a can of fix-a-flat for redundancy.

      All this even though I have not experienced a flat tire in 20-plus years of driving.

      A cousin has had all sorts of driveability problems with run-flats on his Odyssey. They ride like crap and are super expensive to replace.

      • 0 avatar

        @Junes it was a little shocking to me given that my Highlander was basically no options except the ones that were bundled with the V6 & 4wd combo.

        On an interesting note I’ve noticed that the Grand Cherokee doesn’t come with a full size spare unless you select the towing package.

        The only vehicle I play fast and loose with on the spare is my 1967 Mustang which doesn’t have one at all. But I rarely roam more than 30 miles form home in it.

        • 0 avatar
          30-mile fetch

          “On an interesting note I’ve noticed that the Grand Cherokee doesn’t come with a full size spare unless you select the towing package.”

          That surprised me too. A Grand Cherokee on a donut would be…unfortunate looking. They include the full size spare when you opt for the package with the low-range transfer case as well, but that should really be standard on a rig that already weighs nearly 5000 pounds.

    • 0 avatar

      How many trucks come with rims that match the vehicle? I friend had a Denali with a full size spare. Four aluminum wheels and the spare is steel (same dimensions as the aluminum wheels). On my cheap Ranger, four steel wheels pained silver metal flake and a steel spare (same wheel and tire as the other four) painted black.

  • avatar

    Personally, I’m not a fan of run-flats because you lose a lot of the road cushioning that more flexible sidewalls used to give you for absorbing potholes and concrete seams. This forces the suspension hardware to work harder. This also means that the suspension itself has to be both firmer AND softer to absorb the small stuff without effort and still protect the car from more major stuff like 3″ deep or deeper potholes and ruts or equally high rocks or tree branches across the road. Even with 55-profile tires under my ’96 Camaro I hit a pothole hidden in a water puddle deep enough to dent the rims, costing me two wheels and replacement tires when I could ill afford them.

    That said, I am aware that they have improved enormously. Originally they were so hard that you realized relatively poor traction, but newer designs weld the firm sidewalls with high-traction rubber for the tread making for reasonable durability and less need for carrying a spare but I still don’t feel comfortable about not having a spare and I still don’t trust those leak-stop cannisters. I agree with Sajeev that cleaning out the tire and repairing any leak after that is a mess that deserves giving the tech some sort of tip. While tire shops don’t normally permit tipping, I tend to carry small gift cards to local coffee shops or fast-food places that seem to let them get away with it.

  • avatar
    cimarron typeR

    Our AWD Sienna came with run flats and I quickly replaced them with Crossover SUV Bridgestone Ecopias, with a considerable ride improvement/noise improvement.
    Our 2012 Sienna came with a jack which is curious.
    I was able to find a space saver kit w/ wheel/canvas bag on Ebay with the assistance of SiennaChat forum (believe it, there is a Sienna forum)
    It’s within 1/4 in same diameter but I figure I could air the contralateral tire down to make up the difference. I have a small patch kit w/ 12 volt compressor also.
    I’d buy the closest spacesaver to your diameter, and keep a portable compressor and store it deflated in the trunk to make it fit better.

  • avatar
    30-mile fetch

    Stiff walled runflats and cans of hokey aerosol glue that *freeze solid in winter*. Ridiculous. We had a solutions to flat tires before and they were called reasonable sidewall and full-size spares. Now some plebeian 4-pot Accords have 19-inch rims.

    I suppose we’re usually in cell range of someone now.

    My old 96 Camry had a full-size spare in the trunk well. That came in handy on several occasions. 2010 Jetta wagon carried a temporary that was the same diameter as the full. Our 2012 Altima only has the donut, but the tires are 60 series so they can take a pothole hit. 4Runner obviously has a full size spare.

    Runflats and glue cans are not automotive advances.

  • avatar

    Honestly, I don’t know what I would do in a similar situation. Probably just make the trip with what I have and take my chances. However, packing a spare tire, even a donut spare will get you to a place safer than by the side of the road. That would most likely be my choice, since that’s how our cars are equipped.

    I’ve never owned a modern-day performance car that requires or is equipped with run-flat tires, but I’m rather skeptical of them. For every good idea, there’s a downside just like all the modern medicines currently available. Even if they help you, you might die in the process or suffer a worse condition.

    Since I always tend to play it safe, I’ll stick with a mainstream car that covers the normal range of driving. I’m getting older, so I no longer care, I just want a nice, comfortable car.

    Now, saying that, IF I had my druthers, a sporty Caddy would be nice or even a Corvette, but me being retired, it just isn’t going to happen.

  • avatar

    For this situation, I recommend a temporary spare, as close as possible to the size of his tires. I imagine the CTS uses an open differential up front, and a LSD in the back. I would be more worried about the LSD with unequal tire sizes. If there is a flat in the back, I would put the donut on the front, and move the good front wheel onto the back. Both rear drive axles spin at the same rate now.

    Obviously, this is only a temporary solution to get you to a shop.

  • avatar

    Circumference is more important than diameter. Easy way to measure this is put a dot of paint in the center of the tire tread and roll the car on a flat surface. Then measure between the two dots.
    Plugging tires is a method to get you home or to a tire shop, not a long term fix. Yeah, I know some plugged tires will hold air and function till they wear out. However some also fail in the worst ways. Plugging a puncture from the outside means there is no inspection possible of the inside of the tire. Often when a tire loses air and is driven, even a short distance, there is damage to the tire. Cord fabric can be pushed through the inner liner.
    Long ago, when I was in the auto business, there were warnings in all the publications intended for shops. Do not plug tires, they wrote, and related several horror stories of tires that later failed causing crashes.
    Also notify any repair people if you have used a “flat fix” pressure can in a tire. The propellant is propane or some other flammmable hydrocarbon. Fire and flame are not good ideas in repair/tire working areas, but it still happens (smoking). Some people have been burned by the flamethrower when a valve core is removed from a tire with that stuff in it.

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