Deciphering the tyre pressure label

[Sybaris]

Just a quick one here.

Inside the door is the tyre pressure guide.  However its not clear.

 

What is the difference between ~160km/h and 160km/h ~  I know one is over and one is under but which is which?

[NemesisUK]

First line 160kph~ is for speeds above 160kph the second ~160kph for speeds up to 160kph

[Sybaris]

Ah ok thanks.

I have 20 inch wheels on at 2.5/2.6 bar but that is the rating for 18".  However it rides well.  If I change my tyres over, should I be ok sticking to the same pressure. Iim guessing that if 3 0 is safe at high speeds it should be safe at lower speeds and having a bit less pressure in the tyres would be ok too.

[NemesisUK]

These are the manufacturer's recommended pressures and one must experiment if one finds the ride unacceptable or the tyre wears unevenly.

Personally I have always found the recommended pressures have worked fine for me and my driving style. I also have the 20" wheels and have left the pressures at the recommended 2.3 bar, I don't see myself travelling at 100mph for any extended periods, if at all (officer!).

Tread wear appears even on what I presume are the original tyres, so time will tell. Hopefully an all season tyre will become available before they need replacing.

[Sybaris]

No, I can't see myself ever doing 100mph either.  Oh well, its good at 2.6 then I will keep it at that.

Raises the question though if Lexus say you can put 18s on the car, then does that open up a wider range of tyres?

[ColinBarber]

5 minutes ago, Sybaris said:

Raises the question though if Lexus say you can put 18s on the car, then does that open up a wider range of tyres?

Base model RXs had 18" wheels as standard fitment which is why the size is on the label. 

[Spock66]

29 minutes ago, Sybaris said:

Raises the question though if Lexus say you can put 18s on the car, then does that open up a wider range of tyres?

Probably, but I think they would look crap 🙂

[Zotto]

Besides, it's rather difficult to change pressure while at 160 km/h.

[Sybaris]

2 minutes ago, Zotto said:

Besides, it's rather difficult to change pressure while at 160 km/h.

True

[NemesisUK]

1 hour ago, Sybaris said:

No, I can't see myself ever doing 100mph either.  Oh well, its good at 2.6 then I will keep it at that.

Raises the question though if Lexus say you can put 18s on the car, then does that open up a wider range of tyres?

I thought the same and started looking for 18" wheels on the Lexus website but no RX seems to come with that size anymore.

[ColinBarber]

3 hours ago, NemesisUK said:

I thought the same and started looking for 18" wheels on the Lexus website but no RX seems to come with that size anymore.

That's correct, since the facelift they all come with 20" wheels in this country. Before then 18" were fitted to the lower spec models:

 

[Sybaris]

I like the 202s, size matters 😁

[Lydiamight]

Wouldn't a higher pressure result in less rolling resistance and therefore a lower fuel consumption?

[NemesisUK]

32 minutes ago, Lydiamight said:

Wouldn't a higher pressure result in less rolling resistance and therefore a lower fuel consumption?

Quite possibly but also quite possibly at the expense of increased tyre wear in the centre section, harsher ride and increased braking distance especially in the wet

[Sybaris]

I would have thought less pressure would increase resistance and more surface area of the tyre would be in contact with the ground causing more wear.

[Linas.P]

20 minutes ago, Sybaris said:

I would have thought less pressure would increase resistance and more surface area of the tyre would be in contact with the ground causing more wear.

Both are true, hence the previous suggestion of experimenting with pressures and monitoring tyre wear. I don't know about RX, but previously on IS I was running +3-4 PSI because that resulted in most even wear on the tyres. But not only that - each tyre will have slightly different wear characteristics, so sadly there is no universal pressure which would work for all, meaning wear needs to be monitored ad pressures adjusted to maximise grip, tread-life and economy. 

Generally speaking +/-2 PSI not going to have massive difference either way i.e. with -2 PSI tyre not suddenly going to be "under inflated" and cause significant wear on the outside and double the fuel consumption, nor tyre with +2 PSI going to suddenly wear the centre and aquaplane. But those are tendencies when over/under inflation is extreme or tyre are used for extended periods of time in that way.

[RXtoNX]

@Sybaris, more wear on the outer edges yes, higher pressure = more wear in the centre, but 2psi will be negligible in either case.
For a road car I wouldn’t change the recommended settings by more than 3-4psi, it is more important to consider the effect of ambient temperature changes on tyre pressures, particularly between Winter and Summer

If you want the full analysis you need to consider the effect on tyre temperature and, from my track days, how reducing the front only will help under steer etc. and then we can bring spring rates and geo changes in to it 🙄

Its an RX, a road car, stick to the basics then you’re not giving your insurance company an excuse to refuse e a payout if the worst happens.

[Rabbers]

As a general rule I exactly follow the manufacturer’s recommendations for my summer tyres but add 0.2 BAR on changing to winters in anticipation of the colder ambient temperatures and consequent decreases in pressure to be expected, for example, after overnight parking.  I used to do the same during the colder months even when living in places where seasonal tyre changes were unnecessary. 

[Sybaris]

In the winter they do lose more pressure than in the summer but I agree 0.2 bar is not going to impact it hugely.

Maybe I should fit monster truck tyres to it 😀.

 

[Rabbers]

1 hour ago, Sybaris said:

… 0.2 bar is not going to impact it hugely.

Perhaps not, but the figure is not entirely without logic.  The recommended pressures for my RC are 2.5 BAR on all four corners.  My TPMS readings after overnight parking can vary but, except for occasional bigger drops because of serious frost, they rarely fall below 2.3-2.4 after being set at 2.7 in winter, thus matching what I see with the recommended setting of 2.5 in the warmer months.  So the extra 0.2 seems about right.

While on the subject I am reminded of a story about Alain Prost, the F1 driver. Legend has it that he was so sensitive to his cars’ setup that he could immediately tell when he got in whether one of the tyres was under- or over-inflated by as little as 0.1 BAR. This was less impressive when you consider that the pressure on F1 tyres is usually no more than 1.0-1.1 BAR, but it was impressive nevertheless.