Tyre pressures!

[PhilB]

Recommended tyre pressure is usually a compromise between comfort, traction, heat, cornering and wear. If you favour some aspects over others, you can change the pressure.

The safe and legal position is to set the pressures according to manufacturer's specs.
That is for a reason - safety of you and in particular other road users.

If people want to play around with pressures at the track then fine.
On public roads it's illegal and not for discussion on this forum.

[Jaguandy]

Our other car has a range of pressure options for light or fully loaded, Eco etc. So if the pressures are high for a holiday loading, you unload and drive to the beach without adjusting the pressures, it cannot be illegal, surely. I would be interested to see something official to back your illegality statement.

[Pacemaker1000]

Nearly all cars have two pressure settings, standard and laden to allow for extra passengers and luggage. I assume two seater sports cars like ours or caymans only have one setting for this reason.
However, the two litre is lighter to the tune of two twenty stone guys. Not saying it’s not right but would have expected it to have lower pressures.
Be interesting to hear from P300 owners who’ve changed tyres as to whether they wear more in the middle due to less weight.

[Alsam]

Hi Jaguandy, regrading the legality PhilB is absolutely correct. In fact I though this post was straight to the point.

The over and under inflation of tyres is covered by Section 41a of the Road Traffic Act 1988 as part of the Construction & Use Regulations 1986. I have a little experience in the forensic examination and investigation of serious and fatal collisions. Although its very unusual for police officers to issue endorsable tickets for tyre pressures it is perfectly legal for them to do so. Usually 10%+ over or under is reasonable.

Usually at court (including coroners inquests) the police vehicle examiner or forensic investigator will highlight and tyre offence including pressures and how this may have effected the handling of the vehicle pre collision. In fact one of the first checks made at serious and fatal collision scenes are that of tyre pressures. Obviously this can change if not recorded asap. The pressure given to the particular vehicle by its manufacturer are absolute! Deviate from them and your on your own both in terms of liability and insurance cover..Ive attached the CPS extract on tyres for your info.

Hope this helps.

[scm]

I'd suggest that a few psi over or under would hardly count as an offence since the pressures will change during use by about 4 psi, IME. More than about 10 would definitely constitute dangerously under- or over-inflated.

[Alsam]

I agree but it was the law clarification point I was making. Often in court you hear 50% figures. Drivers looking at the wall of the tyre and using maximum tyre pressure by its manufacturer or in many cases mixing BAR with PSI ! There is a real science behind tyre and suspension design and drivers are well advised to follow the data plate information.

[Jaguandy]

Understood. I always keep my tyres within 10% of recommended. Interestingly, I think the laden/unladen pressures for our other car vary by more than 10%, but its not here to check today.

[scm]

And, of course, over- or under-inflated tyres would be an easy get-out for your insurers, too.

[CJSJAG]

My 2021 F-Type R Dynamic P380 (20 inch) stll has OEM Pirellis.
I have kept pressures at 37psi all round - until today.
37 psi is the "recommended pressure"
However, if you scroll to tire info on the on board display, it shows "normal pressure" as 45psi front, 41psi rear.
It shows "slow speed pressure" as 37psi all round.
Today, I knew pressures were down (35psi) so Ii went to top off but forgot my psi gauge.
So, I just pumped in some air figuring I would correct later.
Turns out they were not so well balanced regarding psi after checking later.
BUT, the car felt so much better with the higher presures!!
I had inadvertently added more to the front than the back and was somewhere between "slow" and "normal" all around.
Tomorrow, I plan to check the cold pressures and I think will add about 4 psi front (41psi) and 2psi (39psi) rear and go for a ride.
I never considered myself a "slow " driver but I also never thought I was quite "normal"
Mind you, I do not think anybody who buys an F-Type is NORMAL!!

[PhilB]

My 2021 F-Type R Dynamic P380 (20 inch) stll has OEM Pirellis.
I have kept pressures at 37psi all round - until today.
37 psi is the "recommended pressure"
However, if you scroll to tire info on the on board display, it shows "normal pressure" as 45psi front, 41psi rear.
It shows "slow speed pressure" as 37psi all round.

It looks like you have stumbled upon the high speed setting.
From the manual for a 2021 it says you will get warnings if you exceed a certain limit (155 mph !) and the car's system and pressures have not been set for "high speed".