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Discussion Starter · #1 ·
I have a BMW 335i coupe with the M Sport package. The tires and air pressure requirements are as follows:

Front - 225/35 R19 88Y - 38 pounds

Rear - 255/30 R19 91Y - 45 pounds

I purchased a new Base Cayman last year. The tires and air pressure requirements are as follows:

Front 235/40 ZR19 92Y - 29 pounds

Rear 265/40 ZR19 98Y - 30 pounds

Both vehicles have Pirelli P-Zero UHP summer tires installed. The BMW has run-flat tires.

Why is there such a difference in the air pressure requirements for these two vehicles? Do run-flat tires require more air pressure? If so, what is the technical reason for this higher requirement? Thanks.
 

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re: Recommended Air Pressure for 19"? Confusing information.

I've never worked out why, but some tires like higher pressures than others. It also depends on the tire size.
 

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re: Recommended Air Pressure for 19"? Confusing information.

The last few BMWs I've owned (both with run-flat & go-flat tires) all seem to call for higher pressures than most other cars. My '07 328i wagon specs 32/39 pounds for maximum load (1100 lbs.) and speeds up to 100 mph. But for a better ride and less understeer, I usually run 34/37 lbs. And since I have 16" wheels, it's not surprising that your 19" rubber requires more psi.

And there is no difference in recommended pressures for RFTs & non-RFTs, although BMW likes to insist that RFTs be fitted on virtually all of their current models.
 

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re: Recommended Air Pressure for 19"? Confusing information.

Why is there such a difference in the air pressure requirements for these two vehicles? Do run-flat tires require more air pressure? If so, what is the technical reason for this higher requirement? Thanks.
1. One tire is n-spec, the other not. While the brand and model might be the same, the internal construction is not. This might have something to do with it. read http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=26&

2. The label on the BMW door is the maximum pressure. See What Air Pressure to Use When Changing from Run-Flat Tires to Non-Run-Flat Tires - Make Driving Fun with Performance Tires & Wheels | Tire Rack. The tire pressure on the Cayman is the "comfort setting", the softest setting.
 

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re: Recommended Air Pressure for 19"? Confusing information.

Think the p zeros can safely range 30 cold to up to 40 hot. Usually going to be 35ish. Each 10 degree F change in weather will mean +1 or -1 in cold. 70 F at 35 cold, means 30 cold at 20F winter. If you went with comfort pressure (must have 20 inch wheels), you can do 30 at 70F and they'll drop to 25 at 20F, which is kinda low and you'd have to add air during the winter. It all depends on where you live and the season.

I run ~40 hot in the summer as it find they handle better. They are 37-38 'cold' during the summer near 100F and as low as 30 at 20F (about the coldest it gets here in NC), but realistically ~33 driving during in the winter.
 

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Discussion Starter · #6 ·
re: Recommended Air Pressure for 19"? Confusing information.

Think the p zeros can safely range 30 cold to up to 40 hot. Usually going to be 35ish. Each 10 degree F change in weather will mean +1 or -1 in cold. 70 F at 35 cold, means 30 cold at 20F winter. If you went with comfort pressure (must have 20 inch wheels), you can do 30 at 70F and they'll drop to 25 at 20F, which is kinda low and you'd have to add air during the winter. It all depends on where you live and the season.

I run ~40 hot in the summer as it find they handle better. They are 37-38 'cold' during the summer near 100F and as low as 30 at 20F (about the coldest it gets here in NC), but realistically ~33 driving during in the winter.
My Cayman has the 19-inch Boxster S wheels. On page 257 of the Owner's Manual as well as the door pillar, the standard tire pressures are shown for front as 29 and for rear as 30 --- so those amounts are the ones that I am using. Confort level psi must be even lower and I think only apply to 20-inch wheels.
 

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Discussion Starter · #7 ·
Need Help --- Confused

With all of my prior vehicles, I have tried to maintain tire air pressure according to the psi amounts shown inside the driver's side door pillar. My new Cayman has the 19-inch Boxster wheels with UHP summer tires and the psi amounts on the door pillar are 29 for front and 30 for rear. Therefore, I have been trying to maintain those air pressures.

On page 93 of the Owner's Manual, it states that "(t)ires are inflated to comfort pressure at the factory, which is not suitable for driving at very high speeds. Always adjust the pressure in the tires to the standard pressure setting when driving at higher speeds. Remember to adjust the settings in the Tire Pressure Monitoring System accordingly."

On page 257 of the Owner's Manual, the table shows "(s)tandard tire pressure for summer tires" and 29 psi for front and 30 psi rear. These amounts match those shown on the door pillar.

I am not sure what "high speeds" means --- on page 93 of the Owner's Manual it mentions 165mph while on page 95 of the Owner's Manual it mentions 100 mph.

I would think that the psi amounts shown on the door pillar would be those for "normal driving speeds" yet they appear to be the amounts for standard pressure for "very high speeds."

I am not driving my Cayman at "very high speeds" so am I keeping the air pressure too low by using the 29 psi for front and 30 psi for rear?
Something that should be so simple to understand seems to have been made more complicated and confusing.
 

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Re: Need Help --- Confused

In Germany you will find different values on the sticker - autobahn speeds, etc. In the US the info sticker has been dumbed down.
 

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Re: Need Help --- Confused

Those super precise Germans! 29/30 is perfect for everyday driving - not too loo, and not too high.

On the track, the tire pressure builds easily to over 40lbs when starting at 30lbs due to the heat.

More important than perfect pressure is be aware that the compounds used by the tire manufacturers on the factory OEM tires do not grip well in the cold. So, if you are planning on spirited driving, and the outside air temp is below 50, be sure get some miles in to heat up your tires first. Huge difference in grip between hot and cold.
 

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Re: Need Help --- Confused

Tires can range 30-40 and its preference really in normal driving. Higher = tighter (better) handing, but less traction. Lower = more comfortable and sloppier handling (though better traction). I messed around with both and find more air makes the car more agile/ feel faster. Low air seems to confuse traction control at the rear and you actually have worse launch... seems typical for rears to be recommended to have more air.

When going higher speeds, you need firmer tires to reduce the rubber from stretching or it might stretch too much and fail. Depending on the weather and type of driving you do, they will go from 30 cold to 35 or maybe near 40 on the track. You shouldn't start going 120 MPH with cold 30 tires though. Its not that they will fail, but the chance of failure increases when they are cold and not 35+. If you aren't tracking, you don't have to worry about this at all...please don't go 120 MPH on roads. ><
 

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Re: Need Help --- Confused

I thought comfort pressures were only for 20" wheels which you don't have. My manual gives different pressures for 19" and 20", the 20" have to the two separate pressures. The regular 19" pressures are fine for driving right up to max speed. For the tracK i reduced the pressures a bit so they heated up to about the same pressure.
 
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