Tires: Safety and Different Widths, runflat tires, bmw z4

Question
Thanks Barry,

One other question:  My orignal tires that came with my BMW read on the tire max pressure is 51 psi.  However, my door placard recommends 30 psi in the front and 33 psi in the back.

How can the recommendation between tire manufacturer and car mfg be SO different?  I understand car manufacturers tend to recommed less pressure as this provides a softer ride while the tire mfg wants more pressure for longer lasting and less heat build up.

What risks/trade-offs do I incur by "splitting the difference" between recommendations and putting my pressure at say 40 psi?

Thanks again Barry!

Ken
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The text above is a follow-up to ...

-----Question-----
Dear Barry,

Background:
I have a 2003 BMW Z4 with sports suspension that has the original factory Bridgestone runflat tires 225/45Z R17 W94.  They have between 5-8,000 miles left on them.

I got a nail in the sidewall which makes one of my tires unrepairable.

The tire store was nice enough to give me an old tubeless Contintental tire which actually appears to have more tread than my remaining three tires.  The ONLY difference between this tire and the original three tires, is the width is 215cm instead of 225, and the load bearing is 87W instead of my original 94W.  They put the tire on the right rear side of my rear powered car.

I will be switching all 4 tires to regular tires as I think run flats are a waste of money.

Question:

Is there a major or minor safety issue in having ONE tire with the narrower 215cm width?  The tire mechanic said I don't need to alter my driving behavoir with this odd tire.

The easy thing would  be immediately replace all 4 tires but in the interest of saving some money I feel I might as well use up the remaining 5k miles in treadlife.

I appreciate your advice!

Ken
-----Answer-----
Ken,

Assuming you have given me correct information (and there are parts that do not make sense), by going to the 215 you have lost over 200 pounds of load carrying capacity.  This is a major safety issue.

Besides, having one odd tire will cause your vehicle to skid sideways if you slam on the brakes.  This is also a major safety issue.

So I suggest you stop asking mechanics for advice, except that he was right - you don't have to alter your driving style.  These kinds of safety problems can't be fixed by changing your driving style.  

Answer
Ken,

First you need to read what is on the sidewall of the tire.  It says something like:  "Max Load XXXX pounds, Max Inflation YY psi."   That's not a recommendation, that's a warning not to exceed those values.

Personally I like 3 to 5 psi above the placard because I get better fuel economy, better tire wear, better tire durability, better steering response, better wet traction and only give up a bit of ride harshness.

However, German vehicles are a little different with regard to how they handle the placard.  I have never owned one, but the impression I have is that there may be more than one pressure listed and the pressures are characterized by different situations.  If this is true I would tend to follow those as the Germans are pretty technical folks, but may be 1 or 2 psi more wouldn't hurt.