My car currently has two different tyre sizes, 275/40/18 for the rear
tyres and 245/45/18 on the front of the car. BMW recommends I use 35
PSI of air in each tyre.
 
If I replace the tyres with 275/35/18 on the rear, and 245/40/18 on
the front keeping the same alloy wheel, which would effectivly reduce
the volume inside each tyre, would the pressure needed remain the
same.

Hello there

"Pressure is related to the volume of air necessary to carry load.
More
pressure is required to allow a smaller volume to carry adequate air."
From "Re: (rx7-fb) [1] 14 and 15 inch tires"
http://www.wankel.net/rx7-fb-1999/msg03313.html

"Understand that the pressure does not hold yor vehicle up. Nope. It's
the volume of air. A larger tire will require less pressure to contain
the same volume and support the same weight compared to a smaller
tire."
From "Finding the Best Street Air Pressure"
http://tinyurl.com/hgys - PLEASE NOTE - this is a cached page.  the
original is no longer up.  You may want to make a copy of this page if
you need it as soon as possible.

"Remember that it's the volume of air in the tire that supports the
weight of the rig, not the pressure. A large tire has more volume than
a small one, which is why more air volume needs to be crammed into a
little donut to support the same weight as a big tire and hence
different weight ratings. Generally the lighter the vehicle and the
bigger the tires, the less pressure you can run."
From Oasis Off Road Mfg.
http://www.oasisoffroad.com/airDown.html

Once again, that was another reference to smaller volume needing
higher pressure.

So, by changing your tires to ones with lower internal volume, you
will also increasing the amount of pressure needed for best operation.

Now the above information is based on normal regulation tires.  That
is not to say there might not be specialty tires which operate with
different pressures.

One company even has a line of zero pressure tires.
http://www.michelinman.com/difference/innovation/zeropressure.html

Search - Google

Terms - tire pressure tire volume

If I may clarify anything before you rate the answer, please ask.

Cheers
digsalot

Excellent, answered the part I was stuck on very well.

Auto makers recommend tire sizes and pressure based on many factors
like vehicle weight, turning radius, springs and shock absorbers, and
things I have no idea about, and you should continue to use those
recommendations for safety reasons.

Hi,

With modern cars, the aspect ratio (ratio of sidewall heigh to tyre
width) plays a big part in determining the correct tyre pressures. In
your case, you are moving to an even lower profile (35/40% from
40/45%) and gaining width to maintain the diameter.

Generally, the lower aspect ratio, the higher the recommended tyre
pressure. As such, I would suggest adding 1 or 2 PSI to maintain the
status quo.

However, the whole issue is more complicated that this. The
"recommended" pressure is generally a compromise between comfort, tyre
wear, and performance. The recommended tyre pressures tend to be
favour comfort over performance. The lower the pressure, the greater
the flex in the sidewall. This flex acts as additional suspension,
providing greater comformt and producing less road noise transmission
from the tyres. However, the flexing also causes the tyre to move
around more on the road under heaving g-forces - thus making the car
less accurate to drive, and reducing overall grip.

On my BMW M3 with 245/40/17 rear and 235/45/17 front, I always run
40PSI rear and 36PSI front. This provides me with exellent
performance, but the ride is a little harder and tyres don't quite
last as long. The rears tend to wear out in the middle quicker due to
the slight convexing caused by the tyre pressure.

At the end of the day, it depends on what you want to do with the car.
If you prefer cruising and easy driving, then stick with 35 PSI all
round. If like me, on most days you drive hard enough to slide your
car around, or enjoy very sharp turn in, then run them higher.

Hope this helps,
Tim

eppy,

I thought that was the case, too -- higher is better right?

Nope.

For maximum traction, you want the lowest pressure possible such that
the tire doesn't roll onto the sidewall under the hardest cornering
that you do. (To determine this point, you can draw a line on your
sidewalls with a piece of chalk, go out and do some cornering, then
see how much of the chalk's still there.)

You're correct that a higher pressure will create a more "direct"
steering feel through less sidewall flexure, but you're losing contact
area, and hence traction. Of course, if you enjoy the quicker turn-in
and don't mind giving up a little traction and tire life, that's fine
too. For street driving, it is largely a matter of preference. On the
track, though, the difference is substantial.