Tire Pressure and MPG

roundle · 01-11-2011, 02:06 PM

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?

dre_2ooo · 01-11-2011, 02:17 PM

Yeah good tire pressure makes a world of difference on MPGs.... this is one of the 2 best kept 'secrets' in the automotive field.

The other... snow tires!

roundle · 01-11-2011, 02:25 PM

More tire wear w/ this high(er) pressure?

XieJin · 01-11-2011, 02:34 PM

Quote:

Originally Posted by roundle

More tire wear w/ this high(er) pressure?

Actually it would be the opposite, less tire wear. Think of it as, lower pressure = softer tire. Softer tire = more wear.

carve · 01-11-2011, 02:37 PM

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.

nogard13 · 01-11-2011, 03:41 PM

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.

MassBimmer · 01-11-2011, 04:02 PM

How about you keep the tire pressure at the recommended levels and not worry about it?

hipnotic · 01-11-2011, 07:00 PM

read the door manual for the correct psi

xG35c · 01-11-2011, 07:33 PM

Quote:

Originally Posted by roundle

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?

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!

username604 · 01-12-2011, 02:26 AM

Quote:

Originally Posted by xG35c

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!

They don't sound that much higher than the normal? I would go check but its late right now.

roundle · 01-12-2011, 04:33 AM

Quote:

Originally Posted by MassBimmer

How about you keep the tire pressure at the recommended levels and not worry about it?

MABimmer,

If you review the manual, the suggested levels range by nearly 10psi...

HighlandPete · 01-12-2011, 04:58 AM

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.

HighlandPete

roundle · 01-15-2011, 02:55 AM

Conclusion: I'll keep the fronts at 42 and the rears at 46.

01-11-2011, 02:06 PM	#1
roundle EXPAT 67 Rep 889 Posts Drives: 18 VW GTI;18 CX9;11 E91 N52 Join Date: Apr 2010 Location: USA iTrader: (0)	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?
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01-11-2011, 02:17 PM	#2
dre_2ooo ... 33 Rep 373 Posts Drives: '14 435i Join Date: May 2010 Location: Minneapolis iTrader: (1)	Yeah good tire pressure makes a world of difference on MPGs.... this is one of the 2 best kept 'secrets' in the automotive field. The other... snow tires!
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01-11-2011, 02:25 PM	#3
roundle EXPAT 67 Rep 889 Posts Drives: 18 VW GTI;18 CX9;11 E91 N52 Join Date: Apr 2010 Location: USA iTrader: (0)	More tire wear w/ this high(er) pressure?
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01-11-2011, 02:37 PM	#5
carve Major 190 Rep 1,105 Posts Drives: 335i Join Date: Feb 2009 Location: usa iTrader: (0)	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, 03:41 PM	#6
nogard13 Second Lieutenant 35 Rep 221 Posts Drives: 2006 325i Join Date: Mar 2010 Location: Durham iTrader: (0)	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, 04:02 PM	#7
MassBimmer Major General 313 Rep 7,434 Posts Drives: 2023 i4 M50 Join Date: May 2009 Location: MA iTrader: (6) Garage List 2009 BMW [9.24]	How about you keep the tire pressure at the recommended levels and not worry about it?
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01-11-2011, 07:00 PM	#8
hipnotic Major General 151 Rep 6,603 Posts Drives: e92 335i Join Date: Oct 2007 Location: So. Cal iTrader: (7) Garage List 2008 335i coupe [7.00]	read the door manual for the correct psi
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01-12-2011, 04:58 AM	#12
HighlandPete Lieutenant General 6548 Rep 15,857 Posts Drives: BMW F11 535i Touring Join Date: Mar 2007 Location: Scotland, Highland Region iTrader: (0)	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. HighlandPete
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01-15-2011, 02:55 AM	#13
roundle EXPAT 67 Rep 889 Posts Drives: 18 VW GTI;18 CX9;11 E91 N52 Join Date: Apr 2010 Location: USA iTrader: (0)	Conclusion: I'll keep the fronts at 42 and the rears at 46.
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