Piston Slap: Spare Me Your AWD Drama

Sajeev Mehta
by Sajeev Mehta

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]

Sajeev Mehta
Sajeev Mehta

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  • Igve2shtz Igve2shtz on Aug 18, 2017

    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.

  • Pwrwrench Pwrwrench on Aug 19, 2017

    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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