devenh

I have a 991.2 TTS. After reading the manual and searching the forums it is pretty clear what the difference is between normal and comfort tire pressures are, but the difference between partial load and full load still escapes me (for a Macan or Cayenne the distinction is more obvious).

Since the car's load capacity is 660 lbs, so my best guess is that partial load is two adults plus luggage (330 lbs) and full load is four adults plus luggage (660 lbs)? But four adults is pretty unrealistic in a 911. So under what circumstances would full load be used?

js9527

Personally, i feel the car handles better with 31/36 than normal setting. During summer, if i use normal setting, rear tires go up to 48 some times.

worf928

Short story? In the USA? Never. On the Autobahn? Probably never.

Did you find this: https://rennlist.com/forums/991/1057...-purposes.html

We beat this horse to death in that thread.

That's my guess too. Although specifying the number of adults is not useful. It could be one really, really small driver and gold bricks. It's the total weight that matters. As far as the tire's sidewall's are concerned, a screaming wild-eyed 123 pound passenger is the same as 4.5 silent gold bricks.

And, the 'number of adults' spec is not on the manual's tire pressure table. That part of your guess is just a distraction.

Think like a lawyer.

But, it is also not that simple. If you read through the above thread (the posts that make sense) you'll see that the pressure specification is about load at sustained speed. If you are not planning on sustaining speeds in excess of 125+ mph then part-load comfort pressure is all you'll ever need no matter how many gold bricks you want to take on your weekend getaway. For two normal-ish sized people and a weekend's worth of luggage, part-load comfort pressure is still good (see manual) for 165 mph sustained... which is really easy to do all over. Not.

devenh

What got me curious about the pressures is that the 991 GT3 pressures are 29/31 while the door sticker in the TTS is 38/48 -- wow, what a difference!

Digging deeper revealed all the partial load, full load, normal and comfort variations and the door sticker reflects normal full load (which in the TTS owner's manual is slightly different, 37/47).

Since I do participate in Open Road Racing events, I do have opportunities to drive at 160mph sustained. I have been increasing the GT3 tire pressures by 6 lbs from sticker per the recommendations at Tire Rack. See:

https://www.tirerack.com/tires/tiret....jsp?techid=72

So for partial load the difference front/rear between comfort and normal is 3/7 lbs. The rear difference (7 lbs) is just virtually the same as what Tire rack recommends (7.5 lbs). I can only assume that Porsche only increases the front pressures by 3 lbs because of the weight distribution of the car.

https://www.tirerack.com/tires/tiret...jsp?techid=302

worf928

Whenever confronted with a conundrum for which the model doesn't work, I always look for factors not taken into account. Here are my guesses as to why the GT3's pressure recommendations are so different:

- weight. The GT3 is roughly 400 pounds lighter. I'm going to bet that a lot of that comes out of the rear. Axle load has a non-linear relationship to required pressure at a specific speed.

- tire. What's the load rating of the GT3's tires relative to the TTS?

- top advertised speed. The GT3 is 197 and the TTS is 205. See above: non-linear relationship with required pressure and speed.

- intended use. Pressures for a road course where grip is the priority will be lower than for open road racing where safety at the tech speed is priority. This, though, is a weak argument, because Porsche's lawyers would want spec'd pressure for safe top speed.

If you follow some of the links I post in one of the above threads, there's long PDF with extensive tables that may allow you to find the required pressures given axle loads, tech speed, and tire load rating. I don't know if the tables have GT3 sizes. They might.

This is a logical assumption. This was also what was assumed in the above-linked threads.

Need4S

On a 991.1, if you check both the "comfort" box and the "full load" box, you will get an in-between setting like 33/41, which I actually like for street driving. With more air in the tires, I fear pot holes a bit less, and the car still seems to handle well.