Tyre pressure

depends on the car and its weight…

what does the door card say? (usually lists recommended tyre pressures)

3.4 bar is very high, that's around 50psi! Are you sure it's not 2.4?

what car is it?

Why did you change the tyre size? Tyres also have a load rating and changing the size would possibly lower what that load rating is pkus the changes to how your speedometer reads. Plus upping the tyre pressure will not fix wear if it is one side of the tyre

What tyre sizes originally came on the car when new? Usually inside the fuel door, glovebox or drivers door pillar there will be a sticker indicating the sizes of tyre that the car came with and recommended pressures.

Sounds Like the tyre man wanted to sell what he had to hand.

They are only one step smaller but it will effect your speedo but not by alot.

2.4 bar is somewhere between 34 to 35 psi.

Listen to what the manufacturer says goes on the car. They have better idea than you tyre man.

Your speedometer will now read just 2% over your real speed, whereas on the original size it would be a higher margin.

Just be aware that you can't drive at an indicated 130 and assume you're only really doing 120 anymore!

Now, an indicated 123-124 wil mean you are doing 120 actual.

See

They have fitted a smaller rolling radius tyre so for any indicated dash speed, they will be moving slower than they originally were.

You are not interpreting it correctly.

It assumes original reading is exactly right so on the original tyre, they assume 100km per hour dash display is 100 km per hour actual speed. The revised smaller radius tyre results in 100 km per hour dash display being a slower actual speed.

If.the dash over read to start with, you.would have the situation for example of 110kmh display being a true 100 originally. Now being on smaller tyre 110kmh display becomes a true 98.

Just use Google Maps/Waze and compare it to your speedometer

Use the “4 psi rule-of-thumb”. Tyres should rise by 4psi from cold to hot. Stick 33psi in them cold, go for a spirted drive to get tyres hot, then measure. If the rise is less than 4 add air - if greater than 4 then deflate.

2 left side tyres wearing on 1 side is down to more than pressures.

The rear axles give bother on those resulting in uneven rear tyre wear

Do you rotate the tyres at set intervals. I rotate mine every 10000 km & end up with very even wear.

oops - I got that mixed up:

Use the “4 psi rule-of-thumb”. Tyres should rise by 4psi from cold to hot. Stick 33psi in them cold, go for a spirted drive to get tyres hot, then measure. If the rise is less than 4 then deflate - if greater than 4 then inflate.

I’m basing my edit on the thinking that a seriously under inflated tyre is more likely to blow-out due to the massive increase in pressure when it heats up?

Not a scientist but I don't think that is a definitive equation. All tyres increase in pressure when driven. As mentioned above there I'd no massive increase in pressure, irrespective of the cold tyre pressure. On the model 3 the tyres increase from 42psi to 44psi quite quickly - there is a constant readout on the screen. Similar increase on the Leaf from 40psi. Don't really know with the smax but they are inflated to 38psi cold. Never had an under/over inflation issue with tyres.

About half an hour at 120 this time of year or say 20 mins in summer

If you keep your vehicles as long as I do (26 yrs and 21 yrs with 2 of them here) it’s a good investment long term. (Do it in conjunction with a trip).

The point is you will know the optimum cold pressure using the 4 psi rule - stickers are unreliable/ a good starting point - car makers buy them by the millions.

Hyperbolic perhaps, but ludicrous? - hardly.

I'm with mickdw on this one. Especially the back axe issues, I'd be starting there.

Too many roundabouts on your commute to and from work.

You have Milton Keynes tyres.

There is a previous post from me somewhere about this. I had a puncture on New Year's Eve, on the same tyre as it happens, a tyre now gone. I got home with a few pumps and changed the wheel for the space saver the next day in daylight, as it wasn't a great day for getting work done. Going home was still worth it and even if I had been far from home a pump would have brought me to a more convenient place to change the wheel.