Tyre pressure - low profiles

I'm running 205/40/17 tyres and so I decided to check my tyre pressure today.

I saw it was running at a what seems to very low 15psi!
I'm not sure what the pressure norm for low profile tyres are however I know that general tyres are at around 35 psi.

I did a test and pushed the side wall of the tyre (probably ineffective) to see how deflated it was and it seemed okay. When I pumped the tyre pressure up a bit it felt like the tyre was quite pumped so i left it at around 18 psi all round.

Can anyone confirm what the correct tyre pressure should be for low profiles all around?

I'm sure 18psi isn't the correct pressure....

i say pump it up to 38psi and increase/decrease the pressure to find the sweet spot
anything below 30psi for our cars is low i reckon

i personally run 36/33psi front/rear while cold on 195/55/15

I'm probably going to go out and pump it to 34 psi all round.

With low profiles I couldn't tell if they were flat. I'll head back out to the petrol station now :)

Cheers

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Originally Posted by DC4Integra98

I'm probably going to go out and pump it to 34 psi all round.

With low profiles I couldn't tell if they were flat. I'll head back out to the petrol station now :)

Cheers

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yep low profiles are like that, they will look hard with stuff all PSI in it. just pump it up, it wont blow up on you

depends on how you drive... if you're like me and do a lot of highway driving, 38 is best... easy on the fuel...

I'm running 215/35/18s and I have them up at 38 - 40psi depending on the weather. In warmer weather I take them down to 38 or less because the heat in the road will inevitably increase the pressure in the tyres due to thermal expansion. Yeh, the ride might be a little more bumpy than normal but it will depend on the roads you drive on and will benefit your fuel efficiency by a little. I reckon the minimum you run on would be 35, but it all depends on what you like. You don't want to hit a pothole having under inflated tyres and cracking one of your rims, being the worst case scenario and costing you more in the long run.

Buy a good tyre pressure gauge, don't trust the guages on garage pressure hoses, they are often widly inaccurate. Check tyre pressures at home in the morning when the tyres are cool (not heated up by driving) and before any of the tyres have been warmed by the sun (a tyre can gain a couple of psi just from being warmed by sunshine). Buy a tyre pump so you can adjust pressures at home.

35psi ish (cold) would be a good starting point. It won't be 'too much' and it won't be 'not enough', but what is 'best' varies hugely with the exact tyre, the width of the wheel to which that tyre is fitted, the car to which that tyre / wheel combination is fitted, and on driver preference for softer ride over steering / handling or steering / handling over softer ride...

My car has a mix of tyres (on 6.5" wide wheels) with 195/60/15 on the front and 205/55/15 on the rear. I don't mind a harsh ride quality (I actually dislike a plush ride feel), and I have the fronts at 42psi, and the rears at 47psi. These pressures are found by experimentation and are different because each different tyre has different characteristics, and these characteristics are also affected by the rim width.

The wider rear tyres 'like' the higher pressure because the rim is a bit narrow for the tyre width and the sidewall is to some degree laterally unstable unless the tyre is pumped up pretty hard. The narrower front tyres 'like' the somewhat lower psi because they are a more suitable width for the wheel width, and thus the sidewalls behave in a more stable / responsive manner so don't need as much psi to behave well.

And, in case anyone asks, I have the wider tyres on the rear (rather than on the front which might at first glance seem more correct) because they don't steer as crisply as the narrower tyres do. The car handles better with the narrower 195 tyres on the front because the sidewalls are effectively stiffer on the 6.5" rim than is the case with the wider 205 tyres (even with the higher psi in the 205 tyres). When it comes time to replace the 205s I'll be getting 195s. Ideally I'd keep the 205 tyres but fit them to 7" or 7.5" rims, but $s being $s...

I love JohnL's explanations for everything. +1!

15PSI, dude thats like a flat tyre....

i would say 42psi all round..

low profiles usually require a few psi more than "normal" tyres but beware if you have crap tyres and you over inflate them.... can end up with eggs, sorta like when you kick a basketball really really hard

For 205/55 R16, I use 36 psi all round. So for your 205/40 R17, I'd go 38 psi or thereabouts. I agree with JohnL, get your own tyre pressure gauge and use that instead. The gauge at the pump might be stuffed.

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Originally Posted by aaronng

For 205/55 R16, I use 36 psi all round. So for your 205/40 R17, I'd go 38 psi or thereabouts. I agree with JohnL, get your own tyre pressure gauge and use that instead. The gauge at the pump might be stuffed.

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^^ this. I have 205/40r17's and run 38 psi for daily driving. If you're still unsure what pressure to use ask V__ what he recommends, as he works for Bob Jane.

i run 205/45/17 and have 38psi on all 4

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Originally Posted by RaZZa137

low profiles usually require a few psi more than "normal" tyres

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It seems to be common practice for tyre and car manufacturers and retailers to recommend higher psi for tyres with lower profiles than for tyres with higher profiles (i.e. "normal" tyres). However, I tend to think this is more to do with protecting the wheel rim and (shorter) sidewall from impact damage than it is with the notion that higher profile tyres work better at lower psi than tyres with a lower profile. If anything it could be argued that tyres with softer sidewalls require higher pressures to prevent excessive sidewall deflection under load (and taller sidewalls behave as if they were softer even if the construction is the same other than the sidewall height).

Tyres with tall / soft sidewalls tend to give a soft ride but poor steering and handling characteristics. Such tyres also tend to be quite sensitive to psi, and increasing psi (to significantly over what most people tend to use with them) can substantially improve steering / handling at the expense of some deterioration in NVH.
Tyres with stiffer and/or quite low profiles tend to be rather less sensitive to psi, so much so that it's quite common for low profile tyres to be destroyed because the ('average') driver couldn't feel that they were nearly flat...

Keep in mind that the percieved market demographic for most higher profile tyres (and the cars to which they are likely to be fitted) will be for proverbial mother's picking up the kid's from school and doing the shopping, and ride comfort will be considered a high priority and reflected in the recommended psi. The percieved demographic for lower profile tyres will be at least somewhat more handling oriented, so this requirement will be at least somewhat higher on the priority list and reflected in the recommended psi...

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Originally Posted by RaZZa137

but beware if you have crap tyres and you over inflate them.... can end up with eggs, sorta like when you kick a basketball really really hard

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What?

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Originally Posted by 02gzm

...... what he recommends, as he works for Bob Jane.

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Some people who work at tyre retailers understand tyres quite well, but IMO, not many...

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Originally Posted by JohnL

It seems to be common practice for tyre and car manufacturers and retailers to recommend higher psi for tyres with lower profiles than for tyres with higher profiles (i.e. "normal" tyres). However, I tend to think this is more to do with protecting the wheel rim and (shorter) sidewall from impact damage than it is with the notion that higher profile tyres work better at lower psi than tyres with a lower profile. If anything it could be argued that tyres with softer sidewalls require higher pressures to prevent excessive sidewall deflection under load (and taller sidewalls behave as if they were softer even if the construction is the same other than the sidewall height).

Tyres with tall / soft sidewalls tend to give a soft ride but poor steering and handling characteristics. Such tyres also tend to be quite sensitive to psi, and increasing psi (to significantly over what most people tend to use with them) can substantially improve steering / handling at the expense of some deterioration in NVH.
Tyres with stiffer and/or quite low profiles tend to be rather less sensitive to psi, so much so that it's quite common for low profile tyres to be destroyed because the ('average') driver couldn't feel that they were nearly flat...

Keep in mind that the percieved market demographic for most higher profile tyres (and the cars to which they are likely to be fitted) will be for proverbial mother's picking up the kid's from school and doing the shopping, and ride comfort will be considered a high priority and reflected in the recommended psi. The percieved demographic for lower profile tyres will be at least somewhat more handling oriented, so this requirement will be at least somewhat higher on the priority list and reflected in the recommended psi...



What?

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tyre and car manufacturers give you pressures recommended to give even tyre wear and get most use of tyre but if you want to drive hard or track use for a normal tyre you require the higher pressure for the load.

low profiles on the other hand need the higher pressure due to the fact that they don't have the high profile to absorb impacts such as pot holes

when i was talking about crap tyres and overinflating, i was referring to 205/40R17 that i have with no brand name and im pretty sure the mechanic who fitted them didn't check the pressure he put in because when i went to check them it was somewhere around 50+ psi and after awhile the car started vibrating and when i took the wheels off and had a look, the tyres weren't round, like when you kick a basketball and it makes an egg on it, that was what my tyres looked like

so you're talking about a tyre wall bubble.. when it weakens in a particular section?

well yes except mine wasn't on the sidewall it was on the tread the whole way round the tyre

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Originally Posted by RaZZa137

tyre and car manufacturers give you pressures recommended to give even tyre wear and get most use of tyre but if you want to drive hard or track use for a normal tyre you require the higher pressure for the load.

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The idea that high tyre pressures typically cause excessive / rapid wear in the middle of the tread is a hangover from the days of cross-ply tyres (which have rather flexible treads due to the lack of reinforcing steel belts). With modern steel radial tyres it typically takes very excessive pressures to cause centre tread wear (because of the inherant tread stiffness of a radial ply tyre).

However, even with radials, the wider the tread happens to be the more prone it may be to becoming 'crowned' due to higher psi (i.e. it's easier to flex / bend a long stick than a short one), and this leading to somewhat increased wear in the middle of the tread. Fitting a wider tyre to a narrower rim also tends to 'crown' the tread to some degree, possibly contributing to increased central tread wear.

So, a radial tyre is more likely to wear in the middle if:
A) It's quite wide
and
B) The wheel rim is relatively narrow (for the tyre)
and
C) The pressure is quite high

But, if the tread is relatively narrow and / or the rim is not too narrow for the tyre, then 'C' will have relatively little effect (with most radial tyres, there are bound to be some exceptions, though I haven't yet come across one...).

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Originally Posted by RaZZa137

low profiles on the other hand need the higher pressure due to the fact that they don't have the high profile to absorb impacts such as pot holes

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This has nothing to do with how ell the tyre will work, and what pressure will result in optimal handling / grip. It is however one of a number of very good reasons to avoid excessively low profile tyres...

Some higher profile tyres need higher psi in order to remain stable under load. In my experience, higher pressures are often essential for reasonable stability if the tyre is fitted to a rim on the narrower end of the tyre manufacturers recommended wheel fitment range.

An increase in rim width of only 1" can make a profound difference to the behaviour / performance of a given tyre. As an example; a 195 tyre fitted to a 5.5" wheel tends to require quite a lot of pressure to remain adequately stable (and responsive) on the rim, but on a 6.5" rim the same tyre tends to be much more stable (and responsive), even if the psi is substantially lessened.

Quote:

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Originally Posted by RaZZa137

when i was talking about crap tyres and overinflating, i was referring to 205/40R17 that i have with no brand name and im pretty sure the mechanic who fitted them didn't check the pressure he put in because when i went to check them it was somewhere around 50+ psi and after awhile the car started vibrating and when i took the wheels off and had a look, the tyres weren't round, like when you kick a basketball and it makes an egg on it, that was what my tyres looked like

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Be careful of assuming that a given experience with a specific tyre will hold true across the board...

Thanks for all the advice!

I ended up pumping all tyres to 36 psi. However.... I've seen a loss in fuel economy. Could be because it revs much better now.

if anything the fuel economy should go up
afaik theoretically there should be less rolling resistance

what i thought too. Probably me being a lead foot thinking i've gotten a power boost lol

Try 38psi, i run it on 205/40/17's. It usually goes down a psi every week.

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Originally Posted by DC4Integra98

I ended up pumping all tyres to 36 psi. However.... I've seen a loss in fuel economy. Could be because it revs much better now.

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The drop in economy won't be caused by the raise in tyre psi. By definition higher psi will reduce rolling deformation of the tyre (such deformation causing internal molecular friction / heat generation in the rubber and various plies etc) so will lessen heat generation and loss in the tyre (that heat energy originating from burning fuel...), i.e. higher psi lowers rolling resistance and somewhat improves economy because the tyres generate (and dissipate) less heat.

You're either driving the car slightly differently or some other co-incidental problem has cropped up.

Quote:

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Originally Posted by DC4Integra98

Thanks for all the advice!

I ended up pumping all tyres to 36 psi. However.... I've seen a loss in fuel economy. Could be because it revs much better now.

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Fill up to a full tank and then drive. Fill up again and then measure fuel consumption. I doubt you would be able to see the difference in fuel economy over a span of only a week unless you are a sales rep and finish your whole tank of fuel in 5 working days.

im running 205/40/17 also, i inflate 40psi no probz and regulary check once a fortnight.

Quote:

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Originally Posted by JohnL

The idea that high tyre pressures typically cause excessive / rapid wear in the middle of the tread is a hangover from the days of cross-ply tyres (which have rather flexible treads due to the lack of reinforcing steel belts). With modern steel radial tyres it typically takes very excessive pressures to cause centre tread wear (because of the inherant tread stiffness of a radial ply tyre).

However, even with radials, the wider the tread happens to be the more prone it may be to becoming 'crowned' due to higher psi (i.e. it's easier to flex / bend a long stick than a short one), and this leading to somewhat increased wear in the middle of the tread. Fitting a wider tyre to a narrower rim also tends to 'crown' the tread to some degree, possibly contributing to increased central tread wear.

So, a radial tyre is more likely to wear in the middle if:
A) It's quite wide
and
B) The wheel rim is relatively narrow (for the tyre)
and
C) The pressure is quite high

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Actually, the bit about tyres wearing in the middle when over-inflated is more to do with running tubes, as all tyres used to long ago, than anything to do with crossplies vs radials. Just for the record, radials also wear in the middle when running tubes and they don't need to be running very high pressures for it to happen. Tubeless tyres generally wear relatively evenly across the tread at most pressures. The fact that you were more likely to hear stories about over-inflated tyres wearing in the middle 50 years ago is because most tyres were tubed back then, not because they were crossply tyres. Because the tubes have a round cross-section, they are higher in the middle than at the edge, which pushes the centre of the tread up.

I only know all this because I had a wheel rim develop a slow leak some years ago, and the local service station mechanic fitted a tube, until I could find a rim for the relatively rare European car I was driving at the time. That tyre wore more in the middle:(, but the other tubeless tyres (all radials) didn't, even though they were all running at the same pressures. When I worked out what was happening:confused: I banished it to spare wheel duties, and bought a good rim off an interstate wrecker to eventually replace the leaking one.

And for the record, I would say 35 psi would be a good pressure to start at. When I did an advanced driving course some years ago, the recommended minimum pressures for safety were 32 psi in all tyres. Some years ago I owned a 1984 Holden for which, according to the tyre placard, 26 psi was the ideal tyre pressure. That wasn't my experience - I ran them at 35 psi. So following what the tyre placard says for pressure isn't always ideal, although recent model cars, from the last 15 years or so, tend to have more realistic (safe....:D) pressures on the placard.