Piston Slap: Spare Me Your AWD Drama

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:

  • 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.
  • 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.
  • 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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