What tire pressures are correct for 15" wheels?
I've been running 215/60/15's on my '87 Westy for a number of years, but
I've never been positive about the correct tire pressures.  I've recently
lowered the pressures to be F29 R34 based on an assumed F45/R55 weight
distribution and some trust in the archive post below. According to the
formula, I could even lower the pressure some, but I'm skeptical.

>>> Posting number 72406, dated 22 Oct 1998 12:02:31
Date:         Thu, 22 Oct 1998 12:02:31 -0700
Reply-To:     YauMan Chan <YauMan@CCHEM.BERKELEY.EDU>
Sender:       Vanagon Mailing List <vanagon@vanagon.com>
From:         YauMan Chan <YauMan@CCHEM.BERKELEY.EDU>
Subject:      Re: tire pressure - How to calculate
Comments: To: vanagon@VANAGON.COM, vanagon@VOLKSWAGEN.ORG
Mime-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: quoted-printable

The correct tire pressure has nothing to do with the diameter of the =
wheel.. It can be determined by the Width of the tire and the weight of =
the van (+load).  Here is a sample calculation:

The typical 2WD Vanagon (non-camper) is about 4400 lb wet!  The weight =
distribution of most rear engine vehicle is 45/55 (45% weight carried by =
front wheels, 55% by rear)  I guess it's probably 50/50 for synchros.

Assume you carry a load of 1000 lb (7 passengers at ~ 150 lb each - =
1050lb)  So the total weight carried by the tires is 5400 lb with 2430 lb =
in front and 2970 lb rear (45%/55%)  So EACH front wheel carries 1215 lb =
(2430/2) and  EACH rear wheel 1485 lb (2970/2) (see why reinforced side =
wall tires are recommended!!)

Now for the stock 207/xx-14 tires, the WIDTH of the tire is 205 mm which =
is about 8 inches (205/25.4)  The typical tire make a "footprint" of about
=
5 inches long This means that the area of surface contact is about 40 sq =
inch (8x5)  so the tire pressure for the front wheels is 1215/40 =3D 30 =
psi  and for the read is 1485/40 =3D 37 psi (hey pretty close to vanagon's
=
manual of 32psi front and 36 psi rear - they must be using a different =
weight distribution ratio!) =20

So for your tires which are 225mm (8.9in) wide, your contact surface is =
close to 45 sq inch.  So the front is 1215/45=3D27psi and rear is =
1485/45=3D33psi.

From this calculation you realised that the pressure formula is self =
correcting.  If you put less pressure in the tire, it will flatten out and
=
increase the contact surface area...until the area is big enough to hold =
up the van.  Thus, the more pressure the lest contact surface (more =
slipping but better gas mileage) and the less pressure the more road =
contact!)  That's why big Catapillaf earth moving truck with 24" wide tire
=
requires only 6psi and skinny bicycle tires requires 100-90psi.

Yau-Man Chan
87 GL

George Becker
e-mail: George.Becker@sce.com