I went to stretch the Jag’s legs a bit today. Ran up to about 140 on the first run and about 165 on the second run. Slowing down from the 165 mph run, a yellow triangle popped up with a message stating “Tire pressure too low for high speed travel.” I stopped and checked the pressure and it was 34 psi on all corners. I aired everything up to 37. The message did not appear on the earlier 140 mph run.

I have searched quite a bit and cannot find any reference to this message. Has anyone else seen this message? Is there a spec for tire pressure vs. speed?

 

It should be in your Owner Handbook, it's in mine on page 179.
Firstly on the tyre placard (on my car on the door jamb of the driver's door) it says words to the effect "36 PSI all round for speeds up to 155 mph/250 km/h, over that speed see your Owner Handbook".
The Handbook then says "41 PSI all round for speeds over 155 mph/250 km/h".
So the message you got was correct/valid for 165 mph coz the car reckons you should bump the tyre pressures up to 41 PSI for that speed.
I don't have TPMS on my car so I would never get that message, not that there is anywhere in Oz I can do over 155 mph without risk of a ginormous fine, loss of licence for at least 12 months, car seizure and possibly gaol time!

 

My recommended pressure is 37psi and on a run the pressure increases to 41psi anyway, so looks like I'd be safe to lose my license (and possibly car) without any warning messages.

 

My interpretation of that pressure for high speeds would be for something like an Autobahn where that might be legal and where the tyres are not getting very hot (i.e. no hard braking and turning).
I had the pleasure of driving around Mount Panorama at speed last year (Challenge Bathurst) and as is normal for track use, lowered the tyre pressures when cold so that they would be operating at the correct pressure once hot.
That hot pressure is below either of the handbook cold pressures for normal or high speed driving and I can confirm that the warning does come on above 250 km/h on Conrod Straight, which I could have done without just before going into the Chase.

 

According to my 2021 P380, JLR handbook, tire pressure spec. is 37psi all round (for slow driving), with 45psi (front) and 41psi (rear) for "NORMAL" driving (over 155mph).
Not sure where 155mph is "normal" driving.
I guess I am not "normal", though I did get very close to normal once on the autobahn in a Mercedes.

 

I split the difference and run mine at 39psi all the way around cold, which will get them above 41psi warm.

 

I’ve liked the feel of 38psi all around with the MPS4S, and I’ve used that for years. My XE has wacky specs, but 38 all around works there too.

This is the first report of that warning that I can recall, though others have reported those speeds. I don’t think I’d be looking at the dash!

 

Then the pressure must have been calculated back to an index temperature of 68 or 70 degr F.
What computer of car looks back in is the cold pressure. And the 34psi was the warm pressure.
Porche also uses that, and resets tmps system for comfort and sport setting, based on higher speed used, so higher cold pressure needed.

In 2007 I got hold of the official European formula and system to calculate pressure for a certain load on tire and speed used, and went running with it.

Dont use that system anymore, but my own determined tighter system.
Roughly for speed, every 10kmph/6.2mph different speed, 1 loadindex step different, higher speed /lower loadindex, lower speed/higher loadindex.
So for 250kmph I substract 250-160= 90/10= 9 loadindex steps from given loadindex on tire, and use that in the formula, to calculate the needed pressure for load on tire.

160 kmph is the referencespeed of tire if speedcode above Q. Some exeptions ( R 170kmph, W190kmph, Y and (Y) 220kmph), but I use 160kmph/99mph for every speedcode.

I made a spreadsheet to make pressure/loadcapacity lists, with build in reserve.
Custom made, so yust say for wich speed, howmuch reserve, per axle or wheel given.
Can give 2 loadcapacity's per pressure.
For instance front and back, if different sizes, or for 160kmph, and 260kmph.

Need from tire next, and if different front and back, 2 times.
1. Maximum load or loadindex
2. Kind of tire to determine the referencepressure.
3. Speedcode, less important, have my own system.

Sizes would be nice too,.
At 2. Here 2 choices
A. Standard load AT 36 psi/2.5 bar.
B. XL/ reinforced/ extraload. AT 42psi/ 2.9 bar.
Most low hight/width division tires, as used for Jaguar, I think, are XL//.

Then, maximum speed you use, and wont go over for even a minute.

Then you only have to determine the axleloads in your use 99,9% acurate, and search them back in my made list, and use the cold pressure with it.

Succes with that axleload determining, the most tricky part in it all, and your responcibility.
Carmakers use GAWR's, with no reserve, to look back the recomended pressure.

 

I’m sure you guys have heard of the Bonneville Salt Flats. There is a four lane highway running just along side the Flats. It is dead flat and perfectly straight for about 30 miles. You can see to the horizon. The state of Utah recently installed a 3’ tall fence running along each side the highway and along the median for several miles.

Occasionally you will have no traffic in front of you on your side of the highway as far as you can see. If a police officer were to be traveling in the opposite direction, the fence (and recent rains) would prevent him or her from crossing the median to come get you.

I’m certainly not suggesting anyone exceed the speed limit, but all the ingredients are there and folks have gone much, much faster on the adjacent salt flats…

 

I asked the question about the cold tire pressures recommended for very high speed driving right after I bought my F Type. It seemed counter intuitive to increase the pressures, I figured the higher speed would generate more heat and therefore increase the temperature when the tires are warm. And I also thought it was odd that the pressures would not be the same for front and rear like they are when the normal cold reading is 37 PSI all around..

But someone explained the reasons for the increased pressure requirements at high speed. And I also figure that the Jaguar engineers have determined those specifications on the test track. So if I even plan on going more than 150 MPH in my Jag I'll be sure to heed the factory recommendations.

 

Anyone who hasn't heard of Bonneville and/or hasn't seen the movie, should watch "The World's Fastest Indian." Although, I expect the New Zealand crowd knows about Burt Monro. He had his own "method" of prepping tires for a high speed run that still gives me shivers. Great flick. Anthony Hopkins at his best.

 

Pretty much my favourite film of all time, watched it for the umpteenth time again yesterday!
PS - it's Bert - his real name was Herbert - Munro.

Spoiler alert - Bert had a two step method to prep the tyres for a run:
1. Cut as much tread off as possible (but not so much as to damage/cut the cords) using a kitchen carving knife.
2. Fill in the age cracks (Bert couldn't afford new tyres so he used very old ones) with Kiwi brand black shoe polish. I used the exact same polish on my school shoes when I was a little tacker.

 

The road in question. Straight and flat as far as the eye can see. I’m traveling west, so the famous salt flats are on the right, but it’s dead flat in all directions for miles.

 

Did you try the land speed record??

 

You mean this message?
This was in my XJR and I was running 34psi cold.

 

The Jag is quick. Those guys are quicker!

 

That would be the one!

 

AS posted previously, on the P380 6 cylinder , at those high speeds it would be 45psi front and 41psi rear (37 psi all round ).
Perhaps different for 8 cylinder??

 

My MY18 handbook lists different pressures for "closed road use" between the non-SVR and the SVR. SVR shows 29 front, 32 rear, while for the non-SVR it's 32 front and 29 rear. For public roads. SVR is 37 all round up to 174 mph, 50 front, 45 rear beyond 174 mph. For non-SVR it's 37 all round up to 155 mph, and 45 front, 41 rear beyond 155 mph. All for 20" wheels.

 

All a bit confusing, particularly the reverse pressure difference for SVR vs non SVR front to back.!!