different air pressure for different sized tires?

I just bought a new set of tires for my '70 bug. Since size 165/80R15
tires are getting harder to find, I switched over to size 185/65R15
tires. My question is:

Do I need to use a different air pressure in the tires than the ones
recommended by VW?

Looking at the label on the inside of the glove compartment lid, it says
the tires should be inflated to 18psi in front and 27psi in the rear.
I just checked the pressure on the new tires and found that they've been
inflated to ~35psi. Should I deflate them to VW specs, or should I
leave them inflated to 35psi?

--
Chip
San Francisco, CA

(Please post replies to the newsgroup: My spam filter
often rejects mail from unfamiliar addresses ... )

SF Briarpatch wrote:
> I just bought a new set of tires for my '70 bug. Since size 165/80R15
> tires are getting harder to find, I switched over to size 185/65R15
> tires. My question is:
>
> Do I need to use a different air pressure in the tires than the ones
> recommended by VW?
>
> Looking at the label on the inside of the glove compartment lid, it says
> the tires should be inflated to 18psi in front and 27psi in the rear.
> I just checked the pressure on the new tires and found that they've been
> inflated to ~35psi. Should I deflate them to VW specs, or should I
> leave them inflated to 35psi?
>


You picked a nice tire size, which would work great on a 5.5" wide rim.
You didn't mention your rim size. If you used the most common stock VW
rim, it's only 4.4" wide - your new tires are too big for them. Illegal
even, in some places.

Even so, 35PSI is too much. Set the rears to around 30-32, and the
fronts lower, maybe 22-25. You can go higher in the front if you don't
like it. Observe hos it handles and brakes, especially in corners. The
front end is so lightweight, that you may need more contact patch
against the pavement to maintain decent grip, hence the lower pressure.

Modern tires don't like running underinflated.

I normally use 185/65 15 in all 4 corners, inflated to around 30psi, and
on 5.5" or 6" rims. I like it, it's a nice all purpose tire/rim combo
for a daily driver.


As a general rule, if you have no other info to go by, passenger cars
run 32PSI and SUV's and minivans usually 35PSI. Some passenger cars run
as low as 27PSI.

Higher pressure makes the steering lighter and saves you some fuel. Too
high pressure wears out your tires, and so does too low pressure. And as
I mentioned earlier, the tire contact patch size grows when tire
pressures are lowered. That gives you more grip, and is needed in a
beetle for example to maintain the proper brake balance etc.

Find a place where you can test drive your car, push it a little at
controllable speeds, to see what it does in different braking
situations. Simulate emergency/panic braking or practice cornering.

Jan

SF Briarpatch wrote:
> I just bought a new set of tires for my '70 bug. Since size 165/80R15
> tires are getting harder to find, I switched over to size 185/65R15
> tires. My question is:
>
> Do I need to use a different air pressure in the tires than the ones
> recommended by VW?
>
> Looking at the label on the inside of the glove compartment lid, it says
> the tires should be inflated to 18psi in front and 27psi in the rear.
> I just checked the pressure on the new tires and found that they've been
> inflated to ~35psi. Should I deflate them to VW specs, or should I
> leave them inflated to 35psi?
>


Personally, I like them down. With 195/65R15 on 5.5" bus rims I like
19psi front, 23psi rear. Once I read somewhere if you take the tire
load rating and max psi and ratio that down to actual load, you can get
a recommended psi. I don't know how valid that is, but thought it was
interesting.

max psi / max load = actual psi / actual load

or

max psi / max load * actual load = actual psi


If I plug in my numbers for my Dunlop G/T qualifiers:
35psi/1190lbs * (1850lbs/4 wheels) = 14psi

Now I don't really believe it, and that assume equal weight
distribution, which a bug is far from.

in a bug with radials and 165s, I like 28-30psi rear, 20-22 front.

So with 185s, we convert using Eric Allred's formula....

Rear - 28-30 * (165/185) ~ 25 - 27psi

Front - 20-22 * (165/185) ~ 17.5 - 19psi.

this works very well for all vehicles when changing tire sizes to
account for the changed contact patch.

John
Aircooled.Net Inc.

On Wed, 03 Sep 2008 18:55:41 -0700 SF Briarpatch
> wrote:

>I just bought a new set of tires for my '70 bug. Since size 165/80R15
>tires are getting harder to find, I switched over to size 185/65R15
>tires. My question is:
>
>Do I need to use a different air pressure in the tires than the ones
>recommended by VW?
>
>Looking at the label on the inside of the glove compartment lid, it says
>the tires should be inflated to 18psi in front and 27psi in the rear.
>I just checked the pressure on the new tires and found that they've been
>inflated to ~35psi. Should I deflate them to VW specs, or should I
>leave them inflated to 35psi?

You'll actually want to deflate them to LESS than the OE VW specs,
because these tires are wider. Wider tires have a larger contact
patch, so you need less pressure to support the car over the increased
contact patch.

35 psi probably made this car handle pretty squirrelly.

As Jan mentioned, these tires may be too wide for stock 4.5" rims.

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Jim Adney
Madison, WI 53711 USA
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