The tyre pressure rating in the manual, especially for very fast cars, is the lowest pressure that the engineers deem safe for the maximum speed that the car is able to achieve. This is already a high pressure for most geographies with speed limits. Most grip is probably achieved at quite a bit lower pressure but at high speeds that tyre/pressure combination would lead to overheating and earlier failure of the tyre. Going even higher than the book rating is almost always pointless in practice.

Winter tyres and summer tyres have virtually the same tread depth (7-8 mm) but a different pattern. However, increasing pressures on a winter tyre has the same effect as increasing pressures on a summer tyre. The guidance in the above post is misleading as it suggests that winter tyres simply need more pressure. Most people will assume that the starting point is the recommended pressure in the firstpalce. The downside of overinflation is that there is less tyre on the road and grip is lost which under unfortunate conditions could prove to be a serious mistake. We should be responsible with our suggestion or at least clarify their consequences.

 

of course, but it's also for the recommended tire. that includes speed and wear ratings. a substitute winter tire cannot have the same characteristics, was my point there.

er, the market must be different where you are, and/or the winters. i know nothing about this. US winter tires start at around 12/32" *(9.5mm) in tread depth. trucks probably get even deeper ones. i assumed the UK would be like the US but actually i have no idea.

yes, i'm curious about this as well, which is why i replied. he must have some idea of what the specific need is.

 

Not everything we read on forums is correct! If you look at the Jaguar guidelines for both summer and winter tyres for the 150 mph setting, they are the same for a number of different OE tyres even though the max. speed ratings are different. The mechanical characteristics (strength) must be the same. They all have to do the same job - support the car safely at 150 mph. Randomly changing safety critical parameters seems unwise to me. However, the practical safety net is that most of the world never drives at 150 mph so the real load is so much lower that you can do pointless stuff and it doesn't really matter. We are still allowed to drive at these speeds in Germany and as the loads on tyres are phenomenally high at these speeds, hence, I tend to follow the prescribed settings. But over inflation does reduce grip which may be an issue in the wet even at sane speeds.

 

Wow had no idea that my overinflation was going to cause such debate.

Let me explain, both alloys and tyres were indoors in the warm 10 mins prior to fitting. I have just check them in the cold (minus 2 C currently) and they are now sitting at

Rear: 2.4 bar = 35psi
Front 2.5 bar = 36psi

To my knowledge (and my door sticker) this is within specs, albeit the higher end of the scale, notwithstanding the softer compound I think I've got it right.

But then have been running winter & AS tyres in the UK for over 15yrs (there is no legal requirement to do so) long before it became popular here, and they've served me well on 7 different cars

So think many are blowing things* out of proportion

* See what I did there!?

 

Bill did, and every boy in high school has been grateful ever since!

 

Thank you, that is really helpful information

 

definitely not attempting a debate just commenting on an interesting subject. still not sure i understand. you said winter tires but now have said all-seasons, by winter tires did you just mean a separate set of tires for winter? not actual snow tires?

 

this all makes perfect sense to me.

ah, okay, but that's not true of winter/snow tires when you are on clean asphalt as when the seasonal weather abates for a while. re-reading my first post i see that it's not clear that that's what i'm thinking of, so, my apologies. i also cannot see any reason to overinflate exceptin this case. like we both stated, this would firm up the center band again, and literally burn both rubber and money. however, because what it's firming up is an intentionally softer compound, it would give you some of your mechanical grip back. here is a demonstration of the grip loss:

and certainly as you've pointed out, these minute distances in stopping power multiply greatly at 150mph. incidentally i'm sure they don't ignore their speed laws in the UK but we do in the US.

 

To clarify i've used both types, on my C-HR and Peugeot I've had AS tyres, on my previous Audi A6, X100, MX-5 and Mondeo (I think you would call a Contour stateside*) and X150 I have had winter tyres.

The great thing is my Summer's have lasted a full 4-5yrs when I've used winters for 4-5 months (Nov-Feb/March)

This is probably the best explanation, simply put winter tyres don't have studs, snow tyres do, all season are just a halfway house tbh, bu fine for a low/light mid powered vehicle.

https://www.autocar.co.uk/car-news/advice/winter-tyres

*just proving I know my cars both sides of the pond

PS: And there was me thinking it didn't get cold enough in DFW for you to trouble yourself with such things?

 

thanks for the info! that's correct however i have only lived here since i got smart enough to leave the northeast. i bought a set of blizzak's every winter so that i could drift around the rutted town roads of the massachusetts countryside.

 

Worse, they can get freezing rain or ice storms which lets all the idiots on the roads slam into each other with wild abandon! The snow would be much better to drive on.

 

I don't see that in the video or logically. Take a winter tyre in warmer conditions with the correct pressures and it does not perform as well as a summer tyre. Every test says the same and hence the recommendation not to use winter tyres in summer. Now over-inflate your winter tyre and it has less tyre on the road making things worse not better. Manufacturers do millions of miles of testing before releasing a car. This includes selecting the right pressures and the calibration of the ABS and ESP systems. We then go and mess it all up by using the wrong pressures and the wrong sized tyres but we think we have done better

Marky's actual pressures and the his guidance don't match hence it is misleading. For a 2015 XKR with the standard size 20" tyres available for both summer and winter Jaguar recommend:

Summer:
Front 2.3 bar / 33 psi
Rear: 2.5 bar / 36 psi

Winter:
Front 2.3 bar / 34 psi
Rear: 2.4 bar / 35 psi

If anything the recommendation is slightly lower, not higher! (I expect that the psi conversion is rounded differently too )

The other possible source of error can be from the actual tyre used. Some cars have the 295/30ZR20 tyre at the back. This is a stronger tyre (101Y instead of the usual 99Y) and needs less pressure: 2.3 bar / 33 psi. If you switch to the 285/30ZR20 winter tyre you do see a 2-3 psi rise but this is not because the tyre is a winter tyre, it is because it is a weaker tyre.

It doesn't matter how long people have been using winter tyres, over 20 years for me, we don't know more that the people who engineered the car. There is no correlation between winter tyre and more pressure required. Another forum myth and can be safely ignored

 

We have freezing rain forecast for tomorrow

 

Ready for paint not a fan of the dickie two tone effect

 

Much better

 

Details please Steve. That looks great. Did you do it or have it done for you? What paint did you use etc?

 

Hi Sean

thanks it looks absolutely fantastic I'm totally floored .

I very very careful popped out all the buttons from the rear of the fascia (obviously this was removed from the head unit first ..which is accessible from the front ) take extra care with the chrome dial there's springs on the rear .

Once all the buttons / dials removed I had the painter at work spray it for me

 

I'm anxious to see it installed. Do you have piano black trim on the console?

 

Only have a pic of a trial fit at this stage however it looks great

 

I like the gloss black for the nav panel surround (that grey plastic just looks cheap), but I'm not sure yet if having the whole thing black works for me. When you have her finished, can you add a head-on shot including the center console?