Date: Sun, 24 Jul 2005 17:51:22 -0700
Reply-To: Larry Chase <roadguy@ROADHAUS.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Larry Chase <roadguy@ROADHAUS.COM>
Subject: Calculating Proper Inflation PSI for Tires
Content-Type: text/plain; charset="us-ascii"
John,
You asked for recommendations on the correct air pressures to use for your
Michelin XCA 205/75/R14C tire on a 1988 GL Vanagon.
Mark and Dennis made suggestions and my calculations are in the same
ballpark.
Larry Chase
35-40 front / 45-48 rear
- - -
Mark Drillock
36-40 front 43-48 rear
- - -
Dennis Haynes
36 front / 40-45 rear
- - -
But I want to show how I did the math.
To calculate proper tire pressure for a non stock size tire we need four
things:
1) VW Silver Sticker (door jam) tire info
2) Tire Load Inflation Tables (Weight a specific load range tire carries at
various psi values)
3) Max Load Capacity & Max Inflation Pressure for the OE Tires
4) Max Load Capacity & Max Inflation Pressure for the current tire.
Your starting point is the VW Silver Sticker (on door jam) from which you
determine the manufacturer's target tire load capacity with safety reserve.
By looking up the specified load capacity of the OE tire combinations in the
load inflation tables, you then match the new tire inflation to that value.
Doing this identifies the safety reserve VW used.
Fortunately I have been able to acquire the "Load Inflation Tables" and
because of the various tire info suveys folks responded to (Silver Sticker,
GVWR & OE Tire)I now have the data needed.
Your Vehicle: 1988 GL Vanagon
Original Equipment Tire Option 1 (if we used the other OE Tire Option for
that year we would get similar results)
Size: 185R14 C 6PR
Brand: Continental
Model: Transport RS 771
Load Index: 99
Load Capacity: 1710 lbs
Max Inflation Pressure: 55 psi
The VW Silver Sticker for a 1988 Vanagon = front tire inflation of 39 psi /
rear inflation of 48 psi for this tire.
Using the "Load Inflation Tables" this tire has the capacity to carry ~1400
lbs at 39 psi & ~1600 lbs at 48 psi.
BTW ... the vehicle GAWR Capacities 1323 lbs front and 1433 lbs rear per
tire.
The difference is the safety margin VW determined was needed (1323 vs 1400
frt & 1433 vs 1600 rear)
So for any tire size other than OE sizes .. we calculate what PSI is needed
to carry ~1400 lbs & ~1600 lbs.
Example .. your current tire
Michelin XCA
205/75/R14C
Max Load Capacity: 2270 Lbs
Max Inflation: 65 PSI
Consulting the "Load Inflation Tables" we see something that looks like
this,
Inflation PSI vs Load Capacity in lbs
35 / 40 / 45 / 50 / 55 / 60 / 65 psi
1415 / 1555 / 1695 / 1835 / 2110 / 2115 / 2270 lbs
Since we need to be able to carry 1400 lbs frt & 1600 lbs rear the correct
PSI values for this tire are;
Front = 35 psi
Rear = 45 psi
From that point you would make minor adjustments for actual load, handling
and ride.
I'd suggest a range of
Front = 35-40 psi
Rear = 45-48 psi
Hope this made sense and wasn't too terribly boring.
Larry Chase
www.roadhaus.com
www.roadhaus.com/shops.html
www.roadhaus.com/tires.html
