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      05-04-2011, 11:24 AM   #1
DARDO
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Tire Pressure For Run Flats

I just got my new X3 35i, with the Dunlop Sport Maxx 245-50 R18 Run Flat tires, what is the right pressure, as I got it with 30 Psi, but the recomendation on the door says 27, and the dealer, whom I just called said 35

what to do?

thank you
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      05-04-2011, 11:36 AM   #2
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Originally Posted by DARDO View Post
I just got my new X3 35i, with the Dunlop Sport Maxx 245-50 R18 Run Flat tires, what is the right pressure, as I got it with 30 Psi, but the recomendation on the door says 27, and the dealer, whom I just called said 35

what to do?

thank you
Your Owner's manual should specify pressures of 32 psi Front and 35 psi Rear for those tires at sustained speeds of under 100 mph with an average load.. the F/B differential is important. My 35i handles significantly better at 34 F and 37 R.

For higher loads (more than 3 pass.) or for higher speeds, the pressures should be raised, but you should still maintain the F/B difference.
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      05-04-2011, 11:44 AM   #3
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Tyre pressures for all combinations of factory fit tyre sizes are also printed on the car's B-Pillar. If you open the front door on the driver's side, you'll see them
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      05-04-2011, 01:11 PM   #4
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I agree, dealers are notorious at delivering cars with improperly inflated tires.
Go by the figures cited door and manual.

I should pick mine up tomorrow or Friday. I will bring my tire gage and verify the correct pressure before I drive it off the lot.
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      05-18-2011, 03:55 PM   #5
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Two questions about tire pressure in the X3 (35i).

Door says 32 front 35 rear. Cold. Basically, don't drive the car for a few hours and measure THAT tire pressure correct?

How bad for the tires is it to run the pressure at something like 30 up front and 33 in the rears.
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      05-18-2011, 04:04 PM   #6
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Quote:
Originally Posted by bluesmachine View Post
Two questions about tire pressure in the X3 (35i).

Door says 32 front 35 rear. Cold. Basically, don't drive the car for a few hours and measure THAT tire pressure correct?
Pretty much. Couple of hours should give them plenty of time to cool off to "cold pressure" temp.

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Originally Posted by bluesmachine View Post
How bad for the tires is it to run the pressure at something like 30 up front and 33 in the rears.
That's a miniscule amount, but lowering the tire pressure will give the following:
Softer ride
Tires will warm up a bit faster
Run a bit hotter (heat cycles wear out the tire, too)
Be a tad more sticky with aggressive driving.
They'll also wear out sooner.
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      05-18-2011, 04:45 PM   #7
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As the manual says, the 32F/35R recommended pressures are for a light to medium load (up to 3 passengers).

If you prefer slightly "crisper" handling, less understeer at light throttle, and a better safety margin for heaver loads you might try 35F and 37R. Higher pressure will give you slightly faster "turn-in" response, and lower pressures will soften the vehicle's transient response because of sidewall deflection, and of course, will give a slightly softer ride. However, with run flats, the sidewall vertical compliance is so bad already, running the tires very low doesn’t improve the ride significantly.

When you measure pressures, the car should be at outdoor temperature (in the shade) and the tires, wheels and brakes should be cool to your touch. If they feel even slightly warm, you should wait before measuring the pressure. Every 10 deg. F temp. increase will cause the pressure to read approximately 1 psi higher). Use a good (never dropped) dial gauge or more likely a digital gauge. Most of the digital gauges can resolve a difference of 0.5 psi and even the cheap ones ($10) are usually accurate to about +/- 1 psi.

A good race-quality gauge with 0.1 psi resolution and 0.2 psi accuracy will cost at least $60, but is totally overkill for regular street use.

Always remember to reset your TPM (procedure is simple and is in the manual) whenever you adjust any tire pressures.

Last edited by Lotus7; 05-18-2011 at 11:20 PM.
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