[Closed] Car tyre pressures?

I always put more in all my tyres than it says on them, usually by about +10psi all around

You can safely up the pressures, yes. I don't like to go above about 3-4psi over recommendations for my Prius but then the recommendations are already 4-6psi above what you'd expect anyway. I think Toyota have already upped them for you.

Our Verso is the correct 35psi all round & they still look low. And thats a heavy(ish) 2.0 diesel.

While Im not an expert on tyres I'd say +10psi is a bad idea - you're reducing your contact patch and reducing grip. Over-pressure is as bad as underpressure, just in different ways. You might also reduce sidewall flex with higher pressure so it feels like you have better grip but really all it is is more rigidity in the system feeling more direct.
Its really hard to say without knowing the width of the tyres and the contact patch length etc, ~80kg per tyre is quite a load difference but it may be that the lighter car was spec'd with a HIGHer a pressure and yours is correct - theres no reason to assume the lighter car was the correct one?!

Tyres in my diesel focus also look low. May add a few psi next time

it may be that the lighter car was spec'd with a HIGHer a pressure and yours is correct - theres no reason to assume the lighter car was the correct one?!

True, apart from looking under inflated compared to the rears. Why should the front and rear be inflated to different levels of 'inflatedness' (obviously different actual pressures due to different loads)? Surely if they are significantly different either the front or back is likely to have less that 100% contact?

I'd be somewhat nervous going 10 psi over the manus reccomended pressure. The difference in fuel economy would surely be minimal? Hardly worth the myriad of potential dangers you're inviting.

160 kgs of extra weight might seem like a lot but it's only two adult passengers. It's not much when you consider the tolerances a car tyre is built to.

I always run higher pressure in car tyres, I tend to take the 'motorway' tyre pressures + 5 psi.

Why should the front and rear be inflated to different levels of 'inflatedness'

They do different things. If your car is prone to understeer, you could raise the tyre pressure in the rear, get less grip and even it out - maybe (note I am not an expert in car handling and this is in no way a well thought out suggestion, rather an example hypothesis)

Just about every car I've ever looked at while thinking about it has tyres that look less inflated at the front, regardless of tyre pressure or its ratio front to back, other than my dads BMW which seems to look fairly even (and has differing pressures front to rear too). Bearing in mind the rears dont need to deal with forces quite as high in a FWD car - no steering needed and relatively low braking forces. Also, having the rear tyres a bit more rigid reduces their slip angle which in turn reduces the amount of steering generated by the rear wheels mid corner. But I dont know to what extent manufacturers consider slip angles on "normal" cars rear ends.

The best way to figure out if its under or over-inflated is to monitor tread wear - wear on the shoulders says its underinflated (or you take every corner like Mansell), even wear = right pressure, middle wear - overinflated. Bit of a long term strategy though, best sticking with manufacturers pressures.

You'll see that "loaded" pressures are only slightly higher - mine specs 34 psi normal and 40psi loaded, with loaded being with 4 more adults and a their baggage (+what, about 400kg?). Any change would be +- a psi or so for 160kg. Also bear in mind that dependin on your tyre size and aspect ratio, load increases may not have a linear effect on contact patch size etc.

I wouldn't vary tyre pressure by more than a few psi.

You might get better fuel economy if you go further, but you will probably end up with the centre of the tyre wearing a lot quicker than the rest.
The car might not be as stable either as the suspension will have been tuned for the pressure values stated and grip will almost certainly be reduced as a result of less contact patch.

I'm with coffeeking regarding better grip - it probably just feels like that due to the extra pressure resulting in a 'stiffer' set-up.

Tyres are part of the suspension system and isolate some of the shock from the car - if you run your tyres too hard you will probably increase wear on suspension bushes too.

it probably just feels like that due to the extra pressure resulting in a 'stiffer' set-up.

I don't think this is the case - I've tested it beyond breaking traction.

Incidentally, why do manufacturers recommend increased pressure for prolonged high speed use? Presumably to reduce over heating through sidewall flex?

I essence I'd say its probably safe to say most people (ourselves included) dont know enough about the handling changes to make a qualified judgement on whether to stray from the manufacturers spec. Personally I'd rather sacrifice 1mpg on motorway than grip when someone makes a mistake.

Incidentally, why do manufacturers recommend increased pressure for prolonged high speed use? Presumably to reduce over heating through sidewall flex?

According the what I've read from tyre manufacturers, its because you'll reduce the heat put into the air in the tyre. If you didnt the heat would increase the tyre pressure to beyond ideal. My head says they'd stabilise somewhere but I cant decide if it would stabilise above or below the slightly higher "highway" pressure they spec.

I don't think this is the case - I've tested it beyond breaking traction.

You can reduce slip angles (feels stiffer and more responsive) at the same time as not changing the friction-slip point - so in effect you can have stiffened up the ride (making it feel more grippy and less "loose") while not having changed the ultimate grip point of the tyre. "slip angle" isnt (necessarily) anything to do with the tyre actually slipping.

I always thought you could get done by the Police for having the incorrect TP?

Having ready up on slip angles a bit I suspect the reduction in slip angle at the front due to higher pressure has resulted in less understeer and thus improved feel (which I think is what you are talking about when you mean grippier feel).

However, I don't think I've goo to far and reduced the contact patch with over pressure because the point where traction is finally broken does not seem to be at a lower cornering (or braking) force.

I wonder if the typical lower 'inflation' on the front is to give a 'safer' understeer charateristc?

[i]I wouldn't vary tyre pressure by more than a few psi.

You might get better fuel economy if you go further, but you will probably end up with the centre of the tyre wearing a lot quicker than the rest.[/i]

I've been doing this for years and never had uneven tyre wear in various cars and a van.

I tend to run mine at the motorway pressures as the car goes on the motorways a fair bit. Our car has a difference front to rear of 4 psi 36F and 32R and there are diffent setting depending on what engine you have and body type. As ours has pressure monitors i can see the pressures go up an ddown with both use and the weather as in th erecent cold snap they went down by 3-4psi where as in france last summer one of the fronts hit 41psi.

I once upped the pressures from 35/33 front and rear to 44/42, which is apparently what some American geeks run in their Priuses. Not only was the ride harsh as hell but on a lane near my house with a very rough road surface (think bad pothole repair over many years) on a corner, the car skittered sideways noticeably. Let them down again pretty quickly.

The rear of my estate skitters sideways of its own accord even at the correct pressures 😀

Sounds exciting 😯

Pug 306 handling for you 🙂 Thats when it's not indulging its love of the outside edge of a bend - my god can it understeer! You know that feeling when the steering goes light and you pucker? I seem to get that daily, regardless of speed or conditions lol.

The manufacturers recommended pressures are designed to make the tyre contact patch as large and evenly weighted as possible. Over inflating the tyres a lot will feel like the handling and grip is sharper because at low cornering forces there is less steering weight. However, ultimate grip, which depends on maximising the contact patch in strong braking or lateral G situations will be reduced. i.e. ultimate braking and cornering grip will suffer, even if it doesn't feel that way. As an extreme example look at how sensitive F1 cars are to tyre pressure to understand the effect on grip.

Not likely to be checked, but seriously over-inflated tyres are a prosecutable offence.

glenh - I would guess the reason for the heavier model weighing 160kg more but having the same pressures is wider tyres which contain more air and therefore can carry a larger load for the same pressure

Having said all that, sometimes, 2 or 3 PSI extra (but no more) does help, as long as you don't see excess centre tread wear. Also remember that garage and track pump guages can be very innacurate, so get an accurate guage to check them properly

CM has a good point, the tyres on a heavier car may indeed be wider, but I had assumed the OP would have spotted the different specs for different tyre sizes.

haha! but do you seriously think that forecourt air pumps give accurate readings?