2013 Porsche 911 Carrera Cabriolet Long-Term Road Test


2013 Porsche 911 Cabriolet: Finding the Right Tire Pressure

January 18, 2013

2013 Porsche 911 Cabriolet

The long-term road test on our 2013 Porsche 911 Cabriolet started off on a sour note. I was the one to break the news that our car had terrible ride quality, but it was the buzz of the office.

We placed the blame entirely on the optional 20-inch wheels. Sidewall, after all, is your friend. But after my long-term update and (our Editor in Chief complained about the issue on Twitter) we found out that there may have been another culprit: Our tire pressure.

On the door jamb of every vehicle is a sticker indicating the manufacturer recommended tire pressure. This is a pressure picked by the manufacturer for optimum ride quality and fuel economy. At least, that's the way it normally works.

2013 Porsche 911 Cabriolet

Next to our mandated-by-the-government sticker which reads 36 psi front and 44 psi rear, is a small auxiliary sticker that reads, "Use the inflation pressure on the label to the right. For better driving comfort, the tire pressures in the owner's manual may be used." Next stop, the owner's manual.

The first thing we noticed when checking the multiple charts that make up the suggested tire pressure section is that the numbers that match our door sticker are recommended by Porsche for "Full Load" and 20-inch wheels. There was also a setting for standard tire pressure at part load, and a "Comfort Pressure" for, again, both part and full load.

2013 Porsche 911 Cabriolet

A quick two-hour call to Porsche cleared up all of these different options.

First things first: The additional window sticker is a perfectly legal move and their compliance department has confirmed that multiple factory tire pressure settings will not set them up for any liability. (See: Ford / Firestone.)

Next up, full load. Unlike most brands, Porsche's recommended tire pressure setting isn't there as some sort of compromise. Instead, the 36/44 setting is for running with full fuel, the full 660 pounds of cargo/passengers AT TOP SPEED. This is a max-duty setting that Porsche's putting out there just in case.

Further, the car wasn't engineered with that setting in mind. According to Porsche, the chassis engineers develop the cars according to the comfort, part-load tire pressure (on 19-inch wheels, but we'll get to that later) which for this car is 31 front, 34 rear. Oh, and Porsche says this tire pressure will make the car handle better, to boot.

So we let the car cool down (Porsche recommends the tires cool to 68 degrees, our garage is 66. Close enough) and set the pressures to the 31/34 "comfort, part load" specs. After doing this, we had to reset the tire pressure monitoring system (TPMS) in the IP. It couldn't have been easier. We selected 20-inch wheels with summer tires and then the comfort setting and BAM, it knew we needed 31/34. Perfect.

Selecting this option also reveals the only downside to the comfort part-load setting. Top speed is now reduced to 165 mph instead of 176 mph. Porsche says this isn't a hard limit, at 165 a warning will illuminate that we could, if feeling brave, power through. We don't have an Autobahn, we don't care.

The other potential downside to this tire-pressure reduction. FuelEconomy.Gov says that MPG can drop 0.3 percent for every psi drop. We went down an average of about 7.5 psi. This translates to about a two-percent drop in overall fuel economy. We'll test this moving forward.

So we took a top speed hit and we may take a hit in fuel economy. Did the drop to the comfort setting fix our ride quality issues?

In a word: Absolutely.

The Porsche is still firm, no doubt, but the impact harshness and overall unpleasantness is gone completely. It's a different animal. Previously, this wasn't a car you'd want to drive to the local florist, now you'd beg to drive it to Florida and it still kills it on mountain roads.

We didn't get away from our conversation with Porsche without some bad news, though. It turns out that Porsche doesn't necessarily like the idea of non-PASM-equipped cars (which have adjustable suspension and ride slightly lower) on 20s and that the 20s we have are actually the heaviest wheels available for the 911.

A set of 19s is likely in our future, but for now we're thrilled with the new character of our 911 and we're looking forward to track-testing the car on multiple suggested tire pressures to see what the performance difference actually is.

2013 Porsche 911 Cabriolet

2013 Porsche 911 Cabriolet

<

2013 Porsche 911 Cabriolet

Mike Magrath, Features Editor

Comments

  • duck87 duck87 Posts:

    Porsche doesn't like the idea of a non-PASM car with those 20's, and yet that's exactly what you bought off the dealer. For an extra $2730. Is it just me or is there something wrong with this picture?

  • bassracerx bassracerx Posts:

    i see you fixed your "crappy tire guage" issue eh?

  • bassracerx bassracerx Posts:

    @duck there is a demand for them so they sell them. porsche is in the business of making money. its an extra 2 grand because the wheels and tires are more expensive then the 19s but honestly they could not be more then a grand more expensive but i guess t

  • jederino jederino Posts:

    Informative post, and interesting how much difference was made. A 10 PSI range for the rears - holy cow! Man, those Porsche engineers are a fussy bunch. Too bad the marketing bosses aren't with the program - 20-inch wheels are just nonsense. I'm reminded that the Chevy Corvair had differential PSI recommendations, which owners frenquently ignored (to their peril).

  • sharpend sharpend Posts:

    Sigh. It's been said before. RTFM. It's amazing that Edmunds continues to let editors evaluate and actually write publicly about vehicles when they do NOT even know how to properly operate them! A RTFM procedure should be MANDATORY before Edmunds lets their editors get the keys to a vehicle.

  • stovt001_ stovt001_ Posts:

    Sad how wheel size is the full determinant of self worth that so many car owners feel. I one time had a person tell me straight out that no one in his neighborhood would respect him if his car had less than 20 inch wheels.

  • vetting1 vetting1 Posts:

    Is this a joke?...You guys actually drove around with 39/45 AP...

  • duck87 duck87 Posts:

    @stovt001: Does he live in Donkville? @ vetting1: You'd think they were towing a large trailer with a cow on the roof or something

  • quadricycle quadricycle Posts:

    @jederino: Those engineers ARE fussy but no-one would argue that they are also some of the best in the business. It kills me that even at Porsche the bean counters have more say than it's flagship engineers.

  • jederino jederino Posts:

    @quadricylce: I know what you mean about bean counters, but I think we are not talking about accountants. In the automotive context, it's really marketing and sales people who rule the roost over the engineers.

  • davisdvm davisdvm Posts:

    I recently leased a 2012 C4S coupe (997) and had almost an identical experience (I now have 18k miles on the odo). Not only was ride harshness a problem but also road noise, esp over concrete surfaces. The tire pressure had been set to the "full service" level and after reviewing the manual I reset the tires to the lesser pressure (the car has 19s and runs Bridgestone tires) and the difference was very noticeable, at least for ride quality. The car is still noisy on concrete but better. The other thing the dealer told me was that the suspension components would "soften" a bit after break in and I have to say, albeit subjective, that the car did improve in ride quality after a couple thousand miles. I think the take home is that this car is VERY sensitive to the type and size of tire/wheel and especially to the pressures you run. I certainly cannot discern any performance loss with the lower pressures but I do not track the car or push it beyond my amateur driving limits.

  • quadricycle quadricycle Posts:

    @jederino again: I had no idea that was so, that's interesting. Care to elaborate? (Not arguing just curious). Is it so because Marketing looks at what consumers want? And by sales do you mean dealer network..... Like I said I would just like to know mor

  • autotragic autotragic Posts:

    Whoa, heavy duty tire pressure meter. Two decimal places? Nice!

Leave a Comment

Research Models

ADVERTISEMENT
ADVERTISEMENT
ADVERTISEMENT

Edmunds Insurance Estimator

The Edmunds TCO® estimated monthly insurance payment for a 2013 Porsche 911 in VA is:

$210 per month*
* Explanation
ADVERTISEMENT
Have a question? We're here to help!
Chat*
Chat online with us
Email
Email us at help@edmunds.com
*Available daily 8AM-5PM Pacific