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      10-31-2010, 08:12 PM   #1
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Drives: F80 M3
Join Date: Dec 2008
Location: Denver

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Determining the correct tire pressure

If this has already been covered I apologize but I didn't find anything when I searched.

Some basic things that everyone knows is that OEM tire sizes and recommended tire pressures for that size are listed on the card on the inside of the drivers door and in the manual. But when upgrading wheels and/or changing tire sizes there isn't much info, at least I haven't been able to find much.

When upgrading wheels and changing your tire sizes things to take into account are keeping the correct rolling diameter and and matching the load rating. There's plenty of info on this and is pretty straight forward. For example take the front tire of an e93 335:

225/45 R17 91H

225 is the tire width
45 is the aspect ratio (sidewall height in percentage of the tire width)
17 is the wheel diameter
91 is the load rating (91 is a load carrying capacity of 1356lbs)
H is the speed rating (H is up to 130mph)

There are tables listing all the load indexes and speed ratings on tire rack.
The recommended tire pressure from BMW is 32psi.

You can use this info to get the rolling diameter to match with a new tire and also match the load rating. However nothing on determining the correct tire pressure. I'm sure a lot of you are saying its just pressure what the big deal, but tire pressure is one of the important things thats commonly overlooked. Correct tire pressure is important for safety, correct tire wear, ride comfort, performance and grip.

This is where I'm hoping all of you can chime in so this info is easy accessible to everyone. How can we determine the correct tire pressure when changing tire sizes?
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      11-01-2010, 04:54 AM   #2

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Join Date: Sep 2009
Location: Holland Europe

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I kept myself busy with that since 2007.
I got hold of the equation and system then, the European tyre- companies use themselfes to make those advice stickers.
Learned myself Excell to make spreadsheets for it and translated a few to English, from my native speek Dutch.
in this map spreadsheets and examples for re-calculating when other then original tires.
If you cant work it out, ask me here.

I asumed a long time that this system was the only one worldwide, but by reacting on American fora, I discovered that it was diferent in America before 2000 . After 2005 ( coincidently after the Ford Firestone affaire ) America graded up to the European power in the Equation.
Came by an American article about a new way of calculating, with the meaning that the tire-sidewall keeps the same indentation for lower loads then maximum , and comparison with the in 1928 introduced power-equation and its diferent powers used in Europe and America.
Saved it also on my skydrive of hotmail, in case it suddenly dissapears.
From these links you can navigate my complete public map on this skydrive , for more spreadsheets and articles, some in English.

But I did not have to register on this forum, so you did not search with the right words , becaus I apearently reacted earliėr on this forum with this information 13 times.
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