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In Reply to: Help on tire pressures please posted by Professor on June 03, 2001 at 15:40:51:
As for varying your air pressure, remember unless you have a "certified-accurate" pressure gauge like what I use at work, the difference between 32 and 34 psig may be insignificant. I would say all tire gauges whether it be inline or gauge style will read +/- 2 psig (the "g" stands for gauge). How can you tell if your gauge is 100% accurate? Cost should be the factor. How much? At least $1,000 for a good Heiss electronic pressure gauge that uses differential pressure sensing.
How much is your gauge worth? I test and calibrate pressure gauges all the time and have found even $300 glycerin-filled industrial gauges to be off by as much as 2 psig, usually reading under.
So what do you do? Nothing really. Just keep your tire pressure up to BMW's recommendations for the geographic region you're in. Never go by the tire manufacturer's pressure rating. Usually they only list the maximum pressure for a given load. Typically if you're in a cool to colder climate (under 60 degrees farenheit), then you'd probably want to add less air pressure than what is reflecting on your gauge. Above 60 degrees farenheit then a bit more than what you're reading.
Why? Because air expands in warmer climates and condenses in cooler climates.
But what if you change tire specs? Well then go by a standard you feel comfortable with. 34 psig all around seems good. Once you get on the road, the difference in weight on each tire (passenger loading)will affect the pressure on each tire as well as rolling resistance due to more right or left hand turns heating up the driver or passenger side tires.
At the end of your trip your four tires will reflect different tire pressures and you will be confounded.
Sounds like rocket science? No, simple physics but too cumbersome for most to determine what tire pressure to use. So you just eyeball it.
One more thing to confound you: General rule of thumb is that for every one psig of pressure you drop from your recommended tire pressure rating, that equals to one MPG loss on each tire! So supposedly if all four tires are under by 2 psig, that's an 8 MPG loss to your efficiency.
Go figure.
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