Tire Pressure

Hi all,

I have a 2004 Mustang with the V6 that came with the 16 inch wheels. Along
the way someone changed out the wheels for 17 inch. The tire size is now
245/45 ZR 17/99W. Does anyone have any suggestions on what would be a good
tire pressure to use with this tire / wheel combination ? Thanks for reading
my post.

Big Duke

"Big Duke" > wrote in message
...
> Hi all,
>
> I have a 2004 Mustang with the V6 that came with the 16 inch wheels.
> Along the way someone changed out the wheels for 17 inch. The tire size
> is now 245/45 ZR 17/99W. Does anyone have any suggestions on what would
> be a good tire pressure to use with this tire / wheel combination ? Thanks
> for reading my post.
>
> Big Duke

No matter who you talk to, proper tire pressure remains an elusive subject.
There should be a sticker somewhere that lists proper pressure for your car
and tire.
I don't think changing rim size has much affect on tire pressure. Tire
pressure is more
determined by what the car manufacturer recommends than the tire
manufacturer.
Tire manufacturers recommend what is best for longevity while car
manufacturer
recommends what is best for the handling characteristics of the car.
On either the doors or door pillars there should be a sticker that
recommends proper pressure.
My tires say 32 lbs. while the door sticker says 28 lbs.
Mine are set to 28 lbs. and I can still smoke the tires

mike

Big Duke wrote:
> Hi all,
>
> I have a 2004 Mustang with the V6 that came with the 16 inch wheels. Along
> the way someone changed out the wheels for 17 inch. The tire size is now
> 245/45 ZR 17/99W. Does anyone have any suggestions on what would be a good
> tire pressure to use with this tire / wheel combination ? Thanks for reading
> my post.
>
> Big Duke
>
>
You need enough tire pressure to provide a flat
contact patch under normal operating conditions.
"What does that mean", you ask? That means nobody
can give you an exact number, only suggest how you
might find that number.
You're correct in assuming that changing rim size
will change the pressure requirement. Also, changing
the brand or model of tire will also change the proper
pressure.
The best way to tell is with a tire pyrometer.
Drive it down the highway, then check tread temps.
SHould be pretty even across the tread.
What? You don't have a tire pyrometer? I thought
everybody had one? Isn't it a standard toolbox item?
What you want to do is drive for about 15-20
minutes and get the tires warmed up. Preferably in a
straight line, with very few curves. Like down the
highway. Pull off at the next roadside rest. Cover
the entire width of the tread with common blackboard
chalk. Now drive foreword about 10 feet.
Look at the chalk mark. It should be evenly worn
away across the tread. If more is gone from the
middle, you have too much pressure. If more is gone
from the edges, you don't have enough. If more is gone
from the outside edge or inside edge, you alignment is
off.



--
..boB
2006 FXDI hot rod
2001 Dodge Dakota QC 5.9/4x4/3.92
1966 Mustang Coupe - Daily Driver
1965 FFR Cobra - 427W EFI, Damn Fast.

"goodnigh" > wrote in message
...

> My tires say 32 lbs. while the door sticker says 28 lbs.

The pressure listed on the tire's sidewall is the maximum pressure, not the
recommended pressure for a particular application.

Ed

C. E. White wrote:
> "goodnigh" > wrote in message
> ...
>
>> My tires say 32 lbs. while the door sticker says 28 lbs.
>
> The pressure listed on the tire's sidewall is the maximum pressure,
> not the recommended pressure for a particular application.
>

The doorjamb sticker on my 6-cyl 2006 Mustang lists the original 16x7
wheels and 215 65 tires (max pressure 51 lbs), and specifies 35 lbs
front and rear.

I run GT 17x8 wheels and 235 55 tires (max pressure 45 lbs); the GT
sticker specifies 32 lbs front and rear.

I reckon the manufacturer is interested in economy for the 6-cyl
(narrower, harder tires) and performance for the GT (wider, softer
tires). All this determined on the basis of weight per tire, among
other factors. I understand the 6-cyl weighs about 150 lbs less on the
front wheels than the 8-cyl. They both understeer to a marked degree
in their natural state.

My suspension is neutral-steer with the (GT) fronts at 35 and the
rears at 32. I like the little bit crisper turn-in and the little
earlier rear wash-out. On the autocross track I run 44 front, 41 rear.

I have the GT 35mm front sway bar, a 26mm H&R bar in the rear, Steeda
Sport springs, Tokico D-Spec adjustable struts and shocks (five turns
out from full firm front and rear on the street, two turns out from
full firm in the rear on autocross track), a BMR strut tower brace. A
real treat to drive. I have yet to install the Steeda adjustable front
sway bar, reputed to increase front roll stiffness by ten percent on
the stiffest hole. The air is a little too thick with smoke and ash to
do any work today.

--
Frank ess
The "Witch Fire" is more than fifteen miles away, so far
Packed and ready
The Mustang will stay home, if we go

..boB wrote:
> Big Duke wrote:
>> Hi all,
>>
>> I have a 2004 Mustang with the V6 that came with the 16 inch wheels.
>> Along the way someone changed out the wheels for 17 inch. The tire
>> size is now 245/45 ZR 17/99W. Does anyone have any suggestions on
>> what would be a good tire pressure to use with this tire / wheel
>> combination ? Thanks for reading my post.
>>
>> Big Duke
>>
> You need enough tire pressure to provide a flat contact patch under
> normal operating conditions.
> "What does that mean", you ask? That means nobody can give you an
> exact number, only suggest how you might find that number.
> You're correct in assuming that changing rim size will change the
> pressure requirement. Also, changing the brand or model of tire will
> also change the proper pressure.
> The best way to tell is with a tire pyrometer. Drive it down the
> highway, then check tread temps. SHould be pretty even across the tread.
> What? You don't have a tire pyrometer? I thought everybody had
> one? Isn't it a standard toolbox item?
> What you want to do is drive for about 15-20 minutes and get the
> tires warmed up. Preferably in a straight line, with very few curves.
> Like down the highway. Pull off at the next roadside rest. Cover the
> entire width of the tread with common blackboard chalk. Now drive
> foreword about 10 feet.
> Look at the chalk mark. It should be evenly worn away across the
> tread. If more is gone from the middle, you have too much pressure. If
> more is gone from the edges, you don't have enough. If more is gone
> from the outside edge or inside edge, you alignment is off.
>
>
>
What a clever way to make a test. Reading the problem, I recalled a
similar scenario put to me in a Physics class. The weight supported by
the tires was given as was the contact area. For us the question was how
much to change the airpressure in the tire if the weight supported was
changed by some X amount. If you remove 500 Lbs from the load, how much
less air pressure is needed to maintain the same contact area. This
problem is similar only what is changing is the contact area and the car
weight remaining the same. I think that the factory stickers are the air
pressure at max curb weight. Changing the tires changed the amount of
contact surface area. It increased it. Figure out by what percentage
and reduce the air pressure by that amount. May want to weigh the car
and see how much lighter it is from its max curb weight. Then figure out
how much to reduce the pressure from the sticker. You now have a base
line, Now figure out how many Sq/in the larger tires are and reduce the
psi by that percentage. Check it with the Pyrometer or the chalk.