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Tire Pressure and MPG
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01-11-2011, 01:06 PM | #1 |
EXPAT
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Tire Pressure and MPG
Hi All,
My 2011 328i Touring 6MT (ZSP w/ factory [staggered] RFTs) has about 10,500 miles. After about 3K miles, about when I finished the break-in, my MPG dropped from 26+ combined to around 23.5 combined. I assumed that thsi was because I was using my foot a bit more. Faster forward to 10K miles.... Last week the car felt like it was tram-lining a bit (following the grooves in the pavement, etc). I decided to check the tire pressure (after more than 6 months--oops!). Sure enough, it was below 30 PSI on all corners. As I drive in Germany once in a while (100+ MPH), as per the manual, I bumped the pressure up to 42 in the front and 46 in the rear. Since adding the air, my tram-lining has stopped and my avg MPG has jumped back up to over 26MPG. I am using the same gas (Shell 95), taking the same route, and driving the same way. Also, while the car is a bit more "crisp" and feels better, there is slightly more tire noise. Given the improved MPG and improved ride (I'm in an area with flawless roads, so I'm happy with the firm ride on RFT), think that I should stay at 42/46? |
01-11-2011, 01:34 PM | #4 |
Private First Class
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01-11-2011, 01:37 PM | #5 |
Major
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Higher pressure should mean slightly more wear in the middle, less on the edges. Since, if you corner hard, your edges wear out first anyway, I recommend staying with the higher pressure. I do, and the mpg bump is noticable.
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01-11-2011, 02:41 PM | #6 |
Second Lieutenant
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Higher pressure will reduce tire wear (lower pressure increases wear), but it will increase the "bouncy" feel in the ride. Since you have ZSP, though, I wouldn't worry about it. The MPG boost is worth it.
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01-11-2011, 06:00 PM | #8 |
Major General
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Drives: e92 335i
Join Date: Oct 2007
Location: So. Cal
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read the door manual for the correct psi
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01-11-2011, 06:33 PM | #9 |
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Can't believe you consider the ride improved - it must ride like a truck!. At those pressures there is a possibility the tires could heat up and exceed max pressure at high speeds, and the possibility of a impact blowout is greater also. I'd try a safer mid range pressure if I was you!
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01-12-2011, 01:26 AM | #10 | |
Colonel
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Quote:
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01-12-2011, 03:33 AM | #11 | |
EXPAT
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Quote:
If you review the manual, the suggested levels range by nearly 10psi... Last edited by roundle; 01-12-2011 at 05:37 AM.. |
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01-12-2011, 03:58 AM | #12 |
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I believe there is a bit of misunderstanding about the run-flat tire, compared to the experience of non run-flats. It doesn't work on low pressures as the tyre can't flex, we ride the sidewalls. It does need to be inflated enough for the crown of the tire to share the load. As the OP has found out low pressures turn to tram-lining and a heavy impact on mpg. Often with 'normal' tires we reduce the pressure a little to reduce tram-lining. Get the pressures high enough (sensible pressures) typically a bit higher than the minimum rating on the door placard recommendation, and the tire works 'softer' than at a lower pressure. Plus the wear is much more even.
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