A Quick Tyre Pressure Question

[Hexa-dB]

If you change the size of your wheels, does the tyre pressure change? Assuming you go from stock wheels & tyres to 195/50/15 ?

I suspect it should be the same but I'm not totally sure

[vmail]

Recommended pressures 23.2 psi / 1.6 bar

Max Inflation Pressure: 44 psi / 3.0 bar

[Ryan1985]

I've got 195/45 R15 wheels and my tyre pressures are all around about the 30 psi mark! I'd imagine they should be between the 28-30 psi mark but when you take the wheels to get the tyres fitted to them (if that's what you're doing), they'll know the pressure!

I'm sure Davy will be along soon though! :)

[cfc1]

Tyre pressure goes mostly on the weight of the vehicle concerned, you only get changes when jumping from faitly small to super dooper HUUUGE!!, put your tyres at 28 - 30psi and see how it feels for you, you can increase/decrease the pressures by 1 or 2 psi to suit your own driving style.

[Bee]

If you change the size of your wheels, does the tyre pressure change? Assuming you go from stock wheels & tyres to 195/50/15 ?...

The principle is that as the dimensions are stepped up, the 'recommended' tyre pressure should also be increased accordingly, in steps.

"Where To Find Air Pressure Information" in "Air Pressure"

http://makeashorterlink.com/?A275245EC

1/3 down the page. The plus sizing referred to is explained here:

http://www.discounttire.com/dtcs/infoPlusSizing.dos

The intended stepping up does not quite follow the steps shown in the example given. Curious, I made a quick calculation. In fact, I found there would be a *reduction* of the volume of air inside the new tyre, to the tune of (250)Pi cubic centimetres, which is less than 3% of the doughnut-shaped air column ((9348)Pi) at present! So, there is no need to alter the current 'recommended' tyre pressure (3% of 30psi is 0.9psi) at all, unless someone comes along to prove me wrong (my maths are pretty rusty, to say the least!). :)

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Bee.

[cfc1]

If you change the size of your wheels, does the tyre pressure change? Assuming you go from stock wheels & tyres to 195/50/15 ?...

The principle is that as the dimensions are stepped up, the 'recommended' tyre pressure should also be increased accordingly, in steps.

"Where To Find Air Pressure Information" in "Air Pressure"

http://makeashorterlink.com/?A275245EC

1/3 down the page.

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Bee.

I won't say you are wrong, but that article goes on about the load index in relation to the tyre pressures......

Another valuable resource is the Tire Load/Inflation Tables. Your nearby tire dealer should have a copy. Not only will this document tell you the correct tire pressure for stock sizes, but it will provide the information on optional plus sizes as well. A good example would be the findings on a Honda Civic with the stock size 185/65R-14; recommended air pressure is 28 psi. Plus one size is 195/55R-15 with a recommended air pressure of 32 psi. Plus two size is 205/45R-16 with a recommended air pressure of 36 psi. Note how the air pressure increases with plus sizing to meet the load carrying capacity for the car.

If your new tyres have a higher load rating (thats the 88h bit after the tyre size.. 88 being the load index) then the car is able to carry a little more weight, if you are carrying a little more weight than normal, then it is quite normal to increase the tyre pressures by a few psi (without going over the maximum pressure as stated on the tyre itself) to accomodate this. This is why people adjust their pressures to suit there own requirements.

The intended stepping up does not quite follow the steps shown in the example given. Curious, I made a quick calculation. In fact, I found there would be a *reduction* of the volume of air inside the new tyre, to the tune of (250)Pi cubic centimetres, which is less than 3% of the doughnut-shaped air column ((9348)Pi) at present! So, there is no need to alter the current 'recommended' tyre pressure (3% of 30psi is 0.9psi) at all, unless someone comes along to prove me wrong (my maths are pretty rusty, to say the least!). smile.gif

I will try to simplify this.. and no, not the maths, i'll leave that to people that can actually count

The larger the tyre, say a 267/70x16 will take more air to get to 30psi than a smaller tyre like a 145/70x13, both tyres will still have the same pressure, but the larger tyre will take a bigger volume of air to get to the 30psi... so, to answer the original question "If you change the size of your wheels, does the tyre pressure change?" in A word.... no

Some good info on that site Bee just a pity they can't spell tYre properly ;)

[Bee]

The larger the tyre, say a 267/70x16 will take more air to get to 30psi than a smaller tyre like a 145/70x13, both tyres will still have the same pressure, but the larger tyre will take a bigger volume of air to get to the 30psi... so, to answer the original question "If you change the size of your wheels, does the tyre pressure change?" in A word.... no

Some good info on that site Bee just a pity they can't spell tYre properly ;)

Yes, you are right. The thrust to the ground to uphold the weight of the car is the tyre pressure. This thrust has to be the same as the old tyres, all other variables being equal. To maintain the same pressure, although the volume is changed, the tyre volume itself is irrelevant as long as the air density is maintained the same, as long as the 3% of air volume is deducted from the present tyre, i.e. the same slightly reduced volume can be swapped to the new tyre; there the same air molecular density being preserved, the pressure remains unchanged. Oh, well...it's the end of a long day. :)

The link has been very educational, agreed!

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Bee.

[cfc1]

Well.. its a long day fitting tyres all day too

Education for others. ;)

[Bee]

Tyre pressure goes mostly on the weight of the vehicle concerned, you only get changes when jumping from faitly small to super dooper HUUUGE...

How do you explain that?

The article on plus sizing is misleading, causing a lot of confusion. This system only specifies wheel and tyre sizes. *Why* the recommended tyre pressures are increased in incremental steps is not stated, but is accepted as a natural consequence. The writer goes even further to state that the air pressure recommendation increases with plus sizing "to meet the load carrying capacity for the car"! Load rating is nowhere in its specification.

This caused my initial error. You drew my attention to it, giving a factual circular statement. Thank you for it, but you did not provide a reasoned physical explanation. The OP asked whether a size change would demand a pressure change. Load is not mentioned, and can be assumed constant. Please do not introduce the loading variable, which is confusing the issue as did the writer of the plus size article.

The OP's question is still not satisfactorily answered.

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Bee.

[Hexa-dB]

I've gone for the standard pressures in the Yaris handbook and it seems to be ok so far!

:)

[cfc1]

Tyre pressure goes mostly on the weight of the vehicle concerned, you only get changes when jumping from faitly small to super dooper HUUUGE...

How do you explain that?

How do i explain that going from a small tyre to a far larger tyre will increase tyre pressure?

Well, if you have a narrow tyre running at 30psi, and change to a large tyre... say 4 inches wider with a much lower profile, it (in a lot of cases) takes a few extra psi to get the wider tyres full footprint on the road.. a few psi too hard and it will wear out the centre of the tyre, a few psi too soft, and the tyre will wear out on both inner and outer edges.

The article on plus sizing is misleading, causing a lot of confusion. This system only specifies wheel and tyre sizes. *Why* the recommended tyre pressures are increased in incremental steps is not stated, but is accepted as a natural consequence. The writer goes even further to state that the air pressure recommendation increases with plus sizing "to meet the load carrying capacity for the car"! Load rating is nowhere in its specification.

This caused my initial error. You drew my attention to it, giving a factual circular statement. Thank you for it, but you did not provide a reasoned physical explanation. The OP asked whether a size change would demand a pressure change. Load is not mentioned, and can be assumed constant. Please do not introduce the loading variable, which is confusing the issue as did the writer of the plus size article.

Load was mentioned in the article.....

Another valuable resource is the Tire Load/Inflation Tables. Your nearby tire dealer should have a copy................. The writer goes even further to state that the air pressure recommendation increases with plus sizing "to meet the load carrying capacity for the car"

Load has everything to do with tyre pressures.. if you are carrying a heavy load, you tyre pressure should increase accordingly. Look at any handbook, it will state the tyre pressure for both laden and unladen. A tyres load rating also has a say in the weight that a vehicle can carry, eg. with a tyre with a load rating of 615kg the full laden weight of the vehicle cannot exceed 2460kg, as that would be more than the total weight that the tyres were designed to take.(you multiply the 615kg by the number of tyres on a vehicle to get the total load carrying capacity... although the calculations become slightly different with a double axled vehicle)

The OP's question is still not satisfactorily answered.

so, to answer the original question "If you change the size of your wheels, does the tyre pressure change?" in A word.... no

IMO the answer to the original question WAS answered satisfactoraly

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Bee.

You also said to me...

Please do not introduce the loading variable, which is confusing the issue as did the writer of the plus size article.

It wasn't me who brought this article into this thread.. it wasn't me who confused the issue in the first place.

[dimitrios]

30 psi front

27 psi back

+1psi when more than 1 passenger in the car

have been testing the yaris all summer long,in various driving conditions,from fast driving to rediculous fast driving,this is the best combination,

especially if you have no ESP,dont put more than 28 at the back tyres....

[Bee]

cfc1, cool down.

You are very confused.

Tyre pressure goes mostly on the weight of the vehicle concerned, you only get changes when jumping from faitly small to super dooper HUUUGE...

And, muddled.

Load was mentioned in the article.....

http://encyclopedia.thefreedictionary.com/circular+argument

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Bee.