2013 Porsche 911 Cabriolet: Finding the Right Tire Pressure
January 18, 2013
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.
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.
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.
Mike Magrath, Features Editor