987 Boxster 18" Wheel Questions
 

Recently, I found a set of new OEM 18" Boxster S wheels. I currently have new 19" wheels on my car and instead of replacing the tires, I thought I would replace them with 18" wheels and store the 19" wheels. My questions are:
1. What is the recommended tire pressure (front and rear) for the 18" wheels/tires?
2. Are the tire pressure sensors interchangeable, so that I can remove them from my existing 19" wheels and install them in the 18" wheels.
Thanks,
Vince

Hi Vince,

Let's start with Q#2 first. The sensors are probably interchangable but your better option is to buy a set from Tire Rack or your favorite discount Porsche dealer (Suncoast or Sunset). You can get a set for just over $400. Then you don't have to worry about a tire tech messing one up trying to pry it out of one of your 19s.

Now for Q#1. I have my 18" snows mounted on a set of Cayman wheels. I put sensors in them. When you put them on, the TPMS asks you if you've got 18s or 19s. When you enter 18, it tells you what the pressures should be for 18s (26F / 33R).

I believe you will find a sticker on the car when you open your door that tells you what the pressures should be for the entire range of offerings from Porsche.

By the way, I still have a set of 18" 987 wheels if anyone is interested. :)

Will those fit 99 boxster? Thanks!

[QUOTE=eslai]I believe you will find a sticker on the car when you open your door that tells you what the pressures should be for the entire range of offerings from Porsche.

On my 2008 987S, the sticker only refers to 19" wheels, as that was the original equipment on my car. I contacted the dealership to purchase a replacement door sticker, reflecting 18" wheels, and was quoted $235.00 for the sticker! Apparently, the sticker needs to be very specific as to the serial number of the car, etc. I think I shall pass on that one, and I appreciate the information from Husker Boxster, providing me with the correct tire pressure for 18" 987S wheels/tires, at 26 lbs front, 33 lbs rear.

Vince

I run my 18's at about 29/36 lbs. They seem to wear more evenly. At the recommended (lower) pressure, they were getting the classic under-inflation wear pattern (it was subtle, but enough that it shortened the useful life of the tires by about 10%)

On long trips (I've done a couple 1,000 mile round trips to Oregon) I've run them as high as 32/38. I got 29.8 mpg on the way up, and 28.8 mpg on the way back with the AC on :-)

Your car looks great! (I'm pretty sure I've seen it around the Richmond district)

WOW seriously!? I can't believe they would have a different sticker! Mine shows the pressures for 18" and 19", although mine came with 18's. That's just ridiculous! And the price for a replacement is damned nutty!

Savanh: They will fit, and here's my for-sale thread if you want more info.

The sticker in my door also only refers to 19" wheels.

I wondered if the TPMS would send a warning if it thought you'd overinflated the tires but a quick check of the owners manual says it wouldn't. It only sends a signal if your tire is low (3-6 psi below recommended) or if it detects a tire is flat. So adding extra air wouldn't bother it.

BTW...
Why don't you want to keep the 19s on? They look great on your car.

BTW...
Why don't you want to keep the 19s on? They look great on your car.[/QUOTE]

No real reason, other than the 18" wheels look good as well and ride a little smoother on San Francisco pot-holed streets. I will probably rotate them back onto the car after the tires wear out on the 18" wheels.

Vince

Well, the conversion of wheels was very easy, and the Tire Pressure Monitoring System was very easy to reprogram to 18" wheels, as outlined in the manual. Again, I appreciate the recommendations for tire pressure. On my car, a cold temperature of 26 psi front and 33 psi rear seems to inflate to 29 front and 36 rear when warmed up. Keeping in mind, my car is equipped with PASM and factory 5mm spacers, my impression is that the ride with 18" wheels/tires is virtually the same as with 19" wheels/tires, on smooth stretches of road, and as expected the 18" wheels/tires are smoother when the road conditions get rougher. All in all, if you must have the look of the 19" wheels and your road conditions are good, the penalty in ride compliance is not all that significant. As for handling, the 18" wheels seem to make the car lighter on its feet, and a bit more tossable on the backroads, which I like. As for looks, I like the 19" wheels/tires better, and will keep them in storage for future use.
Vince

Did you swap the monitors or purchase another set? If you swapped, was it easy?

my 06 Boxster S says 30 psi front/37 psi rear for the 18's.
I also run them at 28/36 for better wear pattern. They were wearing in the middle from over inflation.

I swapped the monitors from the 19" wheels to the 18" wheels, and yes it was easy, but the installer seemingly knew what they were doing. Ask them if they have a Hunter Balancer, while it has nothing to do with the Tire Pressure Monitors, it is a good indication that the shop is up to date with more recent tire/wheel technology.
vincesf

I called my Porsche Dealer and they also stated that proper inflation on a 2008 Boxster S with 18" wheels is 30 psi front / 37 psi rear. I am going to look in my 2008 manual again. In regards to your '06, I also have a Boxster, Boxster S manual for 2006 and the recommended tire pressure for cold tires is 29 psi front / 36 psi rear. Looks like I should put more air in my tires as 26 psi front and 33 psi rear may be a bit too low, and am a bit puzzled as to why someone stated that 26/33 was the correct number as per the TPMS.
The differences between the sticker and the manual are rather trivial, but why can't Porsche at least be a bit more consistent? Different tire brands specify different psi's?
vincesf

I went back to my 2008 owner's manual and found that the recommended cold tire pressure for 18" tires is 29 psi front and 36 psi rear. Those owners that have a sticker that references 18" tires and loading information on the inside of the driver's door jam tell me that the recommended cold tire pressure is 30 psi front and 37 psi rear, no problem as 1 psi is not a big deal, and there is also a reference to: "See Owner's Manual For Additional Information". What is confusing to me is that when my car was cold (not driven overnight), I manually checked the tire pressure with a tire pressure gauge, the tire pressure is 29 front 36 rear, and the Tire Pressure Monitor System read 26 front 33 rear or an alternative reading of -0- on each wheel, indicating perfect pressure. Driving the car about 5 minutes, the TPMS read 29 front 36 rear and again indicating perfect pressure -0- to be added at each wheel. Makes me believe that the TPMS is not completely accurate when the car has been sitting and cold, that the truer reading can be obtained after driving the car for 5 minutes to provide a "cold-operating" psi of 29 and 36.
vincesf

Be careful about TPMS
 

Remember TPMS did not come with early 987s. So depending where you got your 18" wheels they may not have the indents designed to hold the TPMS sensors. In that case fitting the 19" wheel sensors will be a pain.

I imagine that could be an issue. Fortunately, these 18" wheels/tires came off a new '08 987S, and the TPM's are the same for 18" and 19" wheels/tires. The TPMS allows for a set up programming for both 18" and 19" wheels/tires. It's actually quite a nice system. Someday I need change my avatar, as my car is pictured with the 19" wheels/tires.
vincesf