Tyre Pressure (M3, 19")

Hi,
I've just had 2 new rear tyres fitted to my M3 with factory fitted 19" 's on it. I had a nail in one so the pressure warning light came on.
Since having them fitted the pressure warning light keeps coming on, I keep resetting it and checking the pressures which seem to be ok.
On the inside of the door it states the 19" wheels should have between 2.4 and 3.3 BAR in them, that sounds like a lot when converted to PSI?
Google says... 2.4 bar = 34.8 pounds per square inch 3.3 bar = 47.8 pounds per square inch
I'm usually in the car alone and don't carry anything heavy, but at BMW they've put 33psi in the rear and the front have 30? Sounds like if the door sticker is right I should have them all running at about 36?
Is the warning coming on because the pressures are too low in the 1st place?
I've also notice the car seems a bit "light" on the road, as though it's not really as connected as before, is this just because I've got a lot more grip and am not driving with slicks on or is it because I've got new tyres on the back and 17,000 on the fronts?
Thanks
Daz
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new tires on back and used on front, especially if wider in rear will make the car a little less responsive. I run 33/38 on my 17" and size is not important when it comes to pressure, 17 or 19" gets the same psi. The door sticker says 38/41 I believe. the sticker is right, I have the tire shop always putting about 32psi in, because they don't know to put the proper amount, BMW does seem to take more air
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"SharkmanBMW" wrote

Well, it kind of is important, but mostly to do with load rating. If a tire has lower load rating, you might want to increase the PSI a little because at higher pressure, it can handle more load.
Also, people sometimes put in slightly higher pressure in very low profile tires to reduce the risk of pothole rim damage, but of course that doesn't have much to do with proper tire inflation for handling purposes.
Regards,
Pete
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I don't know which method that BMW is using for tire pressure warnings: 1) in-wheel sensors/transponders, or 2) comparison inside the ABS/DSC system of tire rotations.
Our Toyota Highlander uses the latter: the system works by comparing the rotational speeds of the wheels. If the steering wheel is straight ahead and one tire is slower than the others, it's low. It's *possible* that the system is detecting a rotational speed difference between front/rear tires due to the wear on the fronts.
FloydR
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wrote

Does anybody know which system BMW use? If it's the ABS/DSC system then it could explain things?
I have inflated the tyres to 40psi rear and 37psi front, I'll see how it goes over the weekend but have booked the car into BMW on Monday morning so they can check it out.
Thanks
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The light comes on because the tires are not turning at the same speeds.
The Low Air warning is an offshoot of the ABS/Traction Control (DSC?) system. When the tires are not turning at the same speeds in ABS or DSC, then it means that the tires are about to lock, or break away. But while cruising and the tires are going the same speeds, then it means that one or more is low on air, and the warning turns on.
I haven't a clue how much air there should be in a 19, but if you put 35 psi (about 2.4 BAR) in all four tires, then the Low Pressure indicator should go out. This won't be the correct pressure, but it will test the theory, then you can figure out what the correct pressure should be later.

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Jeff Strickland wrote:

I don't get it. How would that cause the light to come on if all the tires are the same pressure (and low)? What is it comparing the wheel rotation to to determine the tire pressure is low?
I see how it would work if one tire was low, but what about if all of them are?
--
-Fred W

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Well, yes, this system's design won't allow it to find low tires if all of them are low - it assumes that they start out correct. But in this guy's situation that might not be good enough, since the fronts are worn but rears new - the radii are possibly incompatible.
Floyd
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If they were the same AND low, the system (in theory) would never know. It is seldom that all four tires will be low at the same time, and if one tire was low then the system would work fine. The system assumes that somebody takes reasonable care of the car ...
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Jeff Strickland wrote:

Ah... bad assumption in most cases.
--
-Fred W

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Daz wrote:

I run 2.7bar front, 3.0 bar rear on my B10;
235/40 R 18 front, 265/35 R18 rear
33psi seems low.
A
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I no longer have my M3, but I had an identical problem when the tyre-pressure monitor kept going off after a puncture was repaired on on of my 19's. In the end I took it back to the dealer and they reset it. It can be very tempremental.
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