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Q: Tyre Pressure again ( Answered 5 out of 5 stars,   1 Comment )
Subject: Tyre Pressure again
Category: Science > Math
Asked by: russell2002-ga
List Price: $12.00
Posted: 20 Jul 2003 10:14 PDT
Expires: 19 Aug 2003 10:14 PDT
Question ID: 233041
I previously asked -

My next related question - 

If my rear tyres are 275/40/18 and 245/45/18 for the front on my car,
What is the total volume inside each tyre, and as the tyre pressure is
35 PSI what is the total volume of pressure inside each tyre,

I apreciate you can not answer this 100% as the rim design, sidewall
size ect may distort the answer a little.

Now, what would the answer be if the tyres were 275/35/18 and
245/40/18, and with that, by how much would the tyre pressure need to
be adjusted to maintain the same volume of air in each tyre.

Request for Question Clarification by smudgy-ga on 20 Jul 2003 20:41 PDT

When you ask, "What is the total volume of pressure inside each tyre,"
what exactly are you looking for? It would be relatively
straightforward to calculate, for instance, what the volume of the air
in the tyre would be if the air were at normal air pressure. Is this
what you are looking for? Or are you looking for something else


Request for Question Clarification by smudgy-ga on 20 Jul 2003 21:55 PDT
Hi Russell2002,

Given that the 275/40/18's are at 35psi, I have calculated the
pressure at which the 275/35/18's must be to hold the same volume of
air, and likewise for the 245/45/18's and 245/40/18s. In the process I
calculated the volumes of all four types of tyre. If this is the
information you are seeking, please let me know and I will post it.
(If you are seeking other information as in my previous request for
clarification, please let me know and I will include it as well.)


Clarification of Question by russell2002-ga on 20 Jul 2003 22:45 PDT
You are correct,

I basically want to know by how much to adjust the PSI in each tyre to
maintain the same volume of air.
Subject: Re: Tyre Pressure again
Answered By: smudgy-ga on 20 Jul 2003 22:58 PDT
Rated:5 out of 5 stars
Hi Russell2002,

I hope you find the following answer meets your needs. If not, please
request a clarification and I will address any problems or concerns
you have.

There is extensive math that I have included for your interest, but if
you don't care to see it, the results are summarized as follows. Note
that these results involve some simplifications and approximations and
are not meant to be exact (details about the nature of the
simplifications is at the beginning of the justification section

275/40/18 tyres at a pressure of 35psi hold as much air
     as 275/35/18's at 43.5psi.

245/45/18 tyres at a pressure of 35psi hold as much air
     as 245/40/18's at 42.2psi.

Justification below:

We will idealize the situation slightly to make calculations easier.

Let's assume that the tyres are perfectly cylindrical and that the
sidewalls are perpendicular to the tread, etc. The resulting volumes
and pressures will not be off from the actual answers by more than a
couple of percent. Also note that the measurements neglect the
thickness of the rubber; this causes us to overestimate the volume of
the tyre, but our ignoring the bulging at the sidewalls and the volume
of the rim causes us to underestimate the volume of the tyre, so the
combination of the two factors will not cause our estimate to be -too-
far astray.

Decoding the tyre sizes, we seek information on the following four

Tread width 275mm, sidewall height 110mm, inside diameter 18

Tread width 245mm, sidewall height 110.25mm, inside diameter 457.2mm

Tread width 275mm, sidewall height 96.25mm, inside diameter 457.2mm

Tread width 245mm, sidewall height 98mm, inside diameter 457.2mm

So imagining the tyres as cylinders with holes in the center, we can
get their volume thusly:

Volume of whole cylinder - volume of hole = volume of tyre.

We will work in meters since this will make the numbers easier to work

The volume formula for a cylinder is pi*radius squared*height, where
height in this case is the tread width, radius is the radius of the
tyre, and pi = approximately 3.1415926...

Volume of whole cylinders:

275/40/18: pi * .3386^2 * .275  = .0991 cubic meters
245/45/18: pi * .33885^2 * .245 = .0884 cubic meters

275/35/18: pi * .32485^2 * .275 = .0912 cubic meters
245/40/18: pi * .3266^2 * .245    = .0922 cubic meters

Now from each of these volumes we must take away the volume of the
hole in the middle, which for the tyres of 275mm tread width is .0451
cubic meters and for the tyres of 245mm width is .0402 cubic meters.

This gives the volume of the four tyres as

275/40/18: .0539 cubic meters
245/45/18: .0480 cubic meters

275/35/18: .0460 cubic meters
245/40/18: .0419 cubic meters

Now, using the Ideal Gas Law, we will be able to determine how much
air is in 275/40's and the 245/45's, and then work backwards and find
what the resulting pressure will be in the other tyres given the same
volume of air.

The ideal gas law tells us that if temperature and total amount of air
stay the same, there is an inverse relationship between pressure and
volume-- that is, the smaller the volume, the higher the pressure.

pressure * volume = a constant, for a given temperature and amount of

(I am glossing over some details here.)

Using the convenient ideal gas law calculator at

we see that if the pressure reads 35psi at the gauge on the 275/40's,
the same volume of air in the 275/35's will produce a pressure reading
of 43.5psi on the gauge (assuming an ambient temperature of a balmy 25
degrees Celsius).

Likewise, the 245/45's at a pressure reading of 35psi hold the same
amount of air as the 245/40's at a pressure reading of 42.2psi.

(It is interesting to note here that as far as the gas law is
concerned, we need to take into account the nominal 14 psi that the
tire and gauge are both under due to the pressure of the atmosphere.
The gas law calculator takes this into account if you enter the psi in
the entry marked "gauge".)

To summarize: 

275/40/18 tyres at a pressure of 35psi
hold as much air as 275/35/18's at 43.5psi.

245/45/18 tyres at a pressure of 35psi
hold as much air as 245/40/18's at 42.2psi.

I hope this answers your question satisfactorily. If not, please
request a clarification and I will do my best to answer more

Good luck,

Google search terms:
<tire dimensions>
Relevant results used for above:

<ideal gas law>
Relevant results:
russell2002-ga rated this answer:5 out of 5 stars
Excellent answer,

However, an answer which only opens more questions, its now obvious
you cant simply scale up the PSI, the highest I have tried is 41PSI
which causes the tyre to start to bulge and cause some handeling

O well.....

Subject: Re: Tyre Pressure again
From: bluebike-ga on 17 Sep 2003 21:38 PDT
Very impressed by the detailed explanation of the calculations! My
point/question is what would one hope to gain by equalizing volume?
Keep in mind that your tyres basically have two functions; the first
is to serve as the friction interface area (contact patch) while also
acting as a (pneumatic spring) component of the suspension. Increasing
the pressure in a smaller tyre to equalize volumes will effectively
reduce the friction/traction of that axle, while increasing the spring
rate, resulting in handling changes as you noted - which I would
imagine was more exciting than you'd prefer. Rather than worry about
volumes, it would be more valuable to consider tyre temperatures; if
the tires on both axles are approximately the same temperature after a
vigorous drive, you can be confident that neither axle is being
'overworked'. Just keep in mind the relationship between T, P, and V,
and use your air pressure as the variable to achieve an equalized
temperature, and ignore the volume issue altogether. Hope that is of
some interest in regard to the basis of your original question.

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