Welcome to the North American Subaru Impreza Owners Club Tuesday September 2, 2014
Home Forums WikiNASIOC Products Store Modifications Upgrade Garage
Click here to visit TireRack
Tire & Wheel Forum sponsored by The Tire Rack

Losing traction? Need new tires?
Click here to visit the NASIOC Upgrade Garage...
Here you can view your subscribed threads, work with private messages and edit your profile and preferences Home Registration is free! Visit the NASIOC Store NASIOC Rules Search Find other members Frequently Asked Questions Calendar Archive NASIOC Upgrade Garage Logout
Go Back   NASIOC > NASIOC Technical > Tire & Wheel

Welcome to NASIOC - The world's largest online community for Subaru enthusiasts!
Welcome to the NASIOC.com Subaru forum.

You are currently viewing our forum as a guest, which gives you limited access to view most discussions and access our other features. By joining our community, free of charge, you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is free, fast and simple, so please join our community today!

If you have any problems with the registration process or your account login, please contact us.
* Registered users of the site do not see these ads.
Reply
 
Thread Tools Display Modes
Old 10-21-2004, 09:37 AM   #1
Rice & Gravy
Scooby Specialist
 
Member#: 39569
Join Date: Jul 2003
Chapter/Region: MAIC
Location: EP of WV & Leesburg sorta
Vehicle:
09 Acura TSX 6spd

Question Very poor MPG from low tire pressure?

Hi, I have had an ongoing problem with MPG in my wife's OBS. It has 205/55/16 Kuhmo Ecsta 712s on it and did then. When they were put on I had them at 36 front and 34 rear. Over a few months the MPG consitently dropped Back in July the dealer claimed the poor mileage was due to some bad gas at some point and did a fuel induction treatment on it and it seemed to improve. I had also changed the fuel and air filters just before that.

Shortly before I took it to them in July I found that the tire pressures were down around 27 PSI for some reason! I bumped them back up, took it in and then the induction was done and like I said the mileage got better for a while

But it has gradually gotten worse over the last few months. I am talking less than 20 MPG on occasion, down from around 26 MPG (pre Kuhmos).

So I am taking it back to the dealer on Friday, but last night I noticed a low tire. I checked them all again and they are again down to about 27-28 PSI!

Just wanted to give some background, but cliffnotes I guess would be:


Could PSI of about 8-10 less than normal all around on a soft summer tire cause a huge drop in MPG, to the tune of 5-6 MPG?
* Registered users of the site do not see these ads.
Rice & Gravy is offline   Reply With Quote
Old 10-21-2004, 09:41 AM   #2
Samirr76
Scooby Guru
 
Member#: 4835
Join Date: Mar 2001
Chapter/Region: Tri-State
Location: Fl33tw00d, PA
Vehicle:
2003 Baja 5MT
Yellow w/ 220,000 miles

Default

yes, bump them up closer to 40psi and check air pressure more often!
Samirr76 is offline   Reply With Quote
Old 10-21-2004, 09:54 AM   #3
Rice & Gravy
Scooby Specialist
 
Member#: 39569
Join Date: Jul 2003
Chapter/Region: MAIC
Location: EP of WV & Leesburg sorta
Vehicle:
09 Acura TSX 6spd

Default

Point taken. I usually try to check hers every few months. I need to have her do it herself, but I have enough trouble getting her to keep it reasonably clean and down to one door ding a week!
Rice & Gravy is offline   Reply With Quote
Old 10-21-2004, 11:59 AM   #4
ronan1975
Scooby Newbie
 
Member#: 72380
Join Date: Oct 2004
Chapter/Region: TXIC
Location: Austin TX
Vehicle:
2009 F-150
Green

Default

A drop in 8-10psi could make a big difference in mpg. I check mine every 2-3 weeks, you can lose upto 2-3 psi per month. I run my tires at 36front 34back and I'm considering going higher. The recommended tire psi is normaly way to low.
ronan1975 is offline   Reply With Quote
Old 10-21-2004, 12:10 PM   #5
Howl
Scooby Specialist
 
Member#: 57830
Join Date: Mar 2004
Location: Ontario CA
Vehicle:
1999 OBS

Default

I usually keep my Toyo's at 32 psi. But a few months ago I was carrying a heavy load (three people and camping gear) so I bumped them up to 36 psi for the trip. When I got back and emptied the car I noticed that I was getting better than average MPG (28 vs. 23 MPG), but the tires were noticably less grippy. So I've lowered the pressure again and now I suffer at the pump.
Howl is offline   Reply With Quote
Old 10-21-2004, 12:28 PM   #6
ronan1975
Scooby Newbie
 
Member#: 72380
Join Date: Oct 2004
Chapter/Region: TXIC
Location: Austin TX
Vehicle:
2009 F-150
Green

Default

Quote:
Originally Posted by Howl
I usually keep my Toyo's at 32 psi. But a few months ago I was carrying a heavy load (three people and camping gear) so I bumped them up to 36 psi for the trip. When I got back and emptied the car I noticed that I was getting better than average MPG (28 vs. 23 MPG), but the tires were noticably less grippy. So I've lowered the pressure again and now I suffer at the pump.
They should be more grippy at 36 than 32...less side wall flex. Could the extra load of three people abd camping gear have something to do with the loss of grip
ronan1975 is offline   Reply With Quote
Old 10-21-2004, 01:36 PM   #7
Howl
Scooby Specialist
 
Member#: 57830
Join Date: Mar 2004
Location: Ontario CA
Vehicle:
1999 OBS

Default

No. I noticed the loss of grip when I had returned. Higher psi means a smaller foot print and thus less grip. Sidewall flex really only comes into play when the tires are severly under-inflated, but it depends on the design of the tire. In general, touring tires are designed to work best with a high psi and perfromance tires are designed to work best with a lower psi.
Howl is offline   Reply With Quote
Old 10-21-2004, 04:52 PM   #8
pilotsport
*** Banned ***
 
Member#: 71917
Join Date: Oct 2004
Default

Quote:
Originally Posted by ronan1975
They should be more grippy at 36 than 32...less side wall flex. Could the extra load of three people abd camping gear have something to do with the loss of grip
I believe that it is the opposite, you have MORE grip at lower pressure. Any pressure above 35 (max load rating, 36 for some tires) and you are actually giving up grip.
pilotsport is offline   Reply With Quote
Old 10-21-2004, 05:12 PM   #9
Rice & Gravy
Scooby Specialist
 
Member#: 39569
Join Date: Jul 2003
Chapter/Region: MAIC
Location: EP of WV & Leesburg sorta
Vehicle:
09 Acura TSX 6spd

Default

Thanks for the responses. I knew that PSI would impact MPG but just found it hard to believe that it would cut as much as 5-6 MPG. If they were down around 27-28 all that time, combined with being a soft, sticky summer tire I guess it would make sense. btw, they are being swapped out tomorrow for the Nokian WRs I have. On a side note, has anyone ever heard of Kuhmo Ecstas not holding air very well. Seems odd that they would consistently get that low that quick.
Rice & Gravy is offline   Reply With Quote
Old 10-21-2004, 09:16 PM   #10
ImprezaVA
Scooby Specialist
 
Member#: 32846
Join Date: Feb 2003
Chapter/Region: MAIC
Location: Elkton, VA
Vehicle:
1998 Impreza L-SOLD
Arcadia Green

Default

Keep a closer eye on them as it gets colder. Correct me if I'm wrong, but I think you lose 1 psi for every 10 degrees colder it gets.

-Charles-
ImprezaVA is offline   Reply With Quote
Old 10-22-2004, 09:41 AM   #11
bmorton
Scooby Newbie
 
Member#: 43528
Join Date: Sep 2003
Location: London, Ontario
Vehicle:
2004 Outback Sport

Default

I'm running 35/32 in my tires and the fuel economy is excellent (31-33 on the highway). I definitely attribute some of this to the higher pressures I'm using over the stock recommendation. I've heard that dropping the pressures down again for winter, especially in the rear, might help reduce the tendency of the car (5MT) to wag its tail when it's slippery, so I'll probably try it.
bmorton is offline   Reply With Quote
Old 10-22-2004, 11:47 AM   #12
plasticguy
Scooby Newbie
 
Member#: 70345
Join Date: Sep 2004
Default

The tire compound itself can vary the MPG by 2-3. The lower tire pressure will definitely cost you another 2-3 mpg so there is your total. I switched from all-seasons to ZR rated high performance tires and immediately lost 2 or 3 MPG. Oh well the price of performence.

OEM tires are often selected for fuel efficiency over all else, to get the manuf. CAFE averages up, at the lowest possibly cost. When you replace them almost any other tire will give lower MPG.
plasticguy is offline   Reply With Quote
Old 10-22-2004, 01:20 PM   #13
ronan1975
Scooby Newbie
 
Member#: 72380
Join Date: Oct 2004
Chapter/Region: TXIC
Location: Austin TX
Vehicle:
2009 F-150
Green

Default

Quote:
Originally Posted by pilotsport
I believe that it is the opposite, you have MORE grip at lower pressure. Any pressure above 35 (max load rating, 36 for some tires) and you are actually giving up grip.
With a lower tire presure you have better straight line grip, thats why when drag racing you lower the psi, but when you are auto crossing or spirted driving on turns people use a higher psi so the tire retains its shape on turns, serach for the coutless threads on this.
ronan1975 is offline   Reply With Quote
Old 10-22-2004, 02:02 PM   #14
Howl
Scooby Specialist
 
Member#: 57830
Join Date: Mar 2004
Location: Ontario CA
Vehicle:
1999 OBS

Default

Straight line or curve you still have more tread on the road with a lower psi, which means more grip. Every tire, in combination with the weight of the car, can handle a range of pressures. If it is too low you risk sidewall flex, which means loss of control (not loss of grip) and possibly peeling your tire off the rim, and if it is too high there is not enough tread touching the road. Within the tire's range you can play with the pressure to suit your driving style. I suspect with AutoX keeping the tire at the higher end of it's pressure range gives more accurate control on tight corners, keeps the tires on the rim under large horizontal G's and allows some drifting or oversteer (i.e. less grip) which may be the fastest way around tight corners.

With my tires I find that I get better milage at 36 psi and better grip at 32 psi.
Howl is offline   Reply With Quote
Old 10-22-2004, 02:50 PM   #15
bull3964
Scooby Specialist
 
Member#: 48928
Join Date: Nov 2003
Location: Pittsburgh, PA
Vehicle:
'02 WRB WRX Sedan
'11 DGM WRX 5dr Premium

Default

I'm just floored that you go months between checking pressues. It should be a weekly thing really, especially this time of year with varying temps.
bull3964 is offline   Reply With Quote
Old 10-22-2004, 03:28 PM   #16
ronan1975
Scooby Newbie
 
Member#: 72380
Join Date: Oct 2004
Chapter/Region: TXIC
Location: Austin TX
Vehicle:
2009 F-150
Green

Default

Quote:
Originally Posted by Howl
Straight line or curve you still have more tread on the road with a lower psi, which means more grip. Every tire, in combination with the weight of the car, can handle a range of pressures. If it is too low you risk sidewall flex, which means loss of control (not loss of grip) and possibly peeling your tire off the rim, and if it is too high there is not enough tread touching the road. Within the tire's range you can play with the pressure to suit your driving style. I suspect with AutoX keeping the tire at the higher end of it's pressure range gives more accurate control on tight corners, keeps the tires on the rim under large horizontal G's and allows some drifting or oversteer (i.e. less grip) which may be the fastest way around tight corners.

With my tires I find that I get better milage at 36 psi and better grip at 32 psi.
At the stock psi on the stock tires, you will be running on the word dunlop[ on the side wall if you turn hard the presure is to low at 32 psi
ronan1975 is offline   Reply With Quote
Old 10-22-2004, 04:23 PM   #17
Rice & Gravy
Scooby Specialist
 
Member#: 39569
Join Date: Jul 2003
Chapter/Region: MAIC
Location: EP of WV & Leesburg sorta
Vehicle:
09 Acura TSX 6spd

Outback Sport

Quote:
Originally Posted by plasticguy
The tire compound itself can vary the MPG by 2-3. The lower tire pressure will definitely cost you another 2-3 mpg so there is your total.
I think you are right. Z rated summers that were underinflated for quite awhile is what I am attributing it to. Had some of the 30k service done today (what I had not done myself) and the Nokians WR put on. Since the wife drives this car now and does not drive nearly the miles that I did, I am gonna just keep those on it as the year round tire. They have another 40k or so treadlife left in them. Probably going to sell the Kuhmos next Spring. Thanks everyone.

Last edited by Rice & Gravy; 10-22-2004 at 04:29 PM.
Rice & Gravy is offline   Reply With Quote
Old 10-22-2004, 04:28 PM   #18
Rice & Gravy
Scooby Specialist
 
Member#: 39569
Join Date: Jul 2003
Chapter/Region: MAIC
Location: EP of WV & Leesburg sorta
Vehicle:
09 Acura TSX 6spd

Default

Quote:
Originally Posted by bull3964
I'm just floored that you go months between checking pressues. It should be a weekly thing really, especially this time of year with varying temps.

Well, I do on my car...about every 2 weeks. This is my wife's car that used to be mine, so I don't think of it often since I am not driving it, or when I do think of it has been sitting a while and there is really not much good in checking pressure in cold tires. Lesson learned the hard way.
Rice & Gravy is offline   Reply With Quote
Old 10-22-2004, 04:29 PM   #19
bull3964
Scooby Specialist
 
Member#: 48928
Join Date: Nov 2003
Location: Pittsburgh, PA
Vehicle:
'02 WRB WRX Sedan
'11 DGM WRX 5dr Premium

Default

Quote:
Originally Posted by Gray
there is really not much good in checking pressure in cold tires.

Uh, that's when you are supposed to check tire pressure.
bull3964 is offline   Reply With Quote
Old 10-22-2004, 04:41 PM   #20
Rice & Gravy
Scooby Specialist
 
Member#: 39569
Join Date: Jul 2003
Chapter/Region: MAIC
Location: EP of WV & Leesburg sorta
Vehicle:
09 Acura TSX 6spd

Default

Wow, you know what, I am a real dumbass then. I have always thought that you check pressure and inflate tires after they were driven on a little bit.
Rice & Gravy is offline   Reply With Quote
Reply

Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Very Poor MPG boost_wagon Factory 2.0L Turbo Powertrain 46 01-11-2010 11:22 AM
Getting tires up to temp - low tire pressures? Back Road Runner Autocross 9 10-04-2009 04:45 PM
Turbo Shuts off from Low Oil Pressure Meyers Factory 2.0L Turbo Powertrain 41 06-12-2009 11:57 PM
Low Tire Pressure Buckeye Kid Tire & Wheel 3 01-14-2004 10:18 AM


All times are GMT -4. The time now is 12:19 AM.


Powered by vBulletin® Version 3.7.0
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Powered by Searchlight © 2014 Axivo Inc.
Copyright ©1999 - 2014, North American Subaru Impreza Owners Club, Inc.