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Old 11-15-2007, 04:16 PM   #1
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Tire Pressure and Sag

Lately I have been running my tires at 44/42 PSI (F/R), and although I have not done any actual coast-down testing, my impression is that rolling resistance has been reduced by a decent amount.

That's obviously a good thing. I'm a bit worried about how this will affect my drivetrain, though. My car is all wheel drive with a viscous coupled center differential. This means that unlike Subarus with automatic transmissions, my car can't be driven in 2wd by pulling a fuse. As I understand it, with AWD cars it is important to keep tire diameters equal. Get too much difference in rolling diameter between the front and rear wheels, and the center differential will constantly be trying to lock up, damaging tires, fuel economy and drivetrain. I observed this first hand once when I drove a short distance on a donut spare with a whining sound starting around 25 MPH and getting louder as speed increased.

The suggested tire pressure for my car is 32/29 PSI (F/R). I have always assumed that this difference in pressure was to equalize rolling diameters between the heavier front end and the lighter rear end by making them sag an equal amount. I may be wrong about that, though.

Can anyone tell me to what extent pressure actually affects a tire's rolling diameter? Should I really be concerned about this? My thought is that as I increase pressure, the amount of tire sag will decrease and so will the amount of required difference in pressure. Any thoughts? Thanks in advance.
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Old 11-15-2007, 04:39 PM   #2
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cafn8 -

I don't know what year Subaru Impreza this guy has, but maybe this applies to you. Read his website and maybe you can figure out which year/model he has :

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Mr Incredible -

... I like this website for learning about tires :

The wheel and tyre Bible
http://www.carbibles.com/tyre_bible.html
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Tyre pressure and gas-mileage.

For the first two years of our new life in America, I'd take our Subaru [Impreza] for its service, and it would come back with the tyres pumped up to 40psi. Each time, I'd check the door pillar sticker which informed me that they should be 32psi front and 28psi rear, and let the air out to get to those values. Eventually, seeing odd tyre wear and getting fed up of doing this, I asked one of the mechanics "why do you always over-inflate the tyres?" I got a very long and technical response which basically indicated that Subaru are one of the manufacturers who've never really adjusted their recommended tyre pressures in line with new technology. It seems that the numbers they put in their manuals and door stickers are a little out of date. I'm a bit of a skeptic so I researched this on the Internet in some of the Impreza forums and chat rooms and it turns out to be true. So I pumped up the tyres to 40psi front and rear, as the garage had been doing, and as my research indicated. The result, of course, is a much stiffer ride. But the odd tyre wear has gone, and my gas-mileage has changed from a meagre 15.7mpg (U.S) to a slightly more respectable 20.32 mpg (U.S). That's with mostly stop-start in-town driving. Compare that to the official quoted Subaru figures of 21mpg (city) and 27mpg (freeway) and you'll see that by changing the tyre pressures to not match the manual and door sticker, I've basically achieved their quoted figures.

...
...
Also, I would go here for more info on your Scooby-roo :

http://bbs.scoobynet.com/driving-dynamics-354/

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Old 11-15-2007, 04:44 PM   #3
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I would assume the PSI recommendation difference is such so that the car will ride the best. I have seen the same type of recommendation on RWD and FWD cars also, and they don't have to worry about the front or rear tire diameter matching.
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Old 11-15-2007, 04:47 PM   #4
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I would assume the PSI recommendation difference is such so that the car will ride the best. I have seen the same type of recommendation on RWD and FWD cars also, and they don't have to worry about the front or rear tire diameter matching.
some could if they have ABS (senses different wheelspeeds and thinks ones slipping)
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Old 11-15-2007, 05:51 PM   #5
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I would have to think that there isn't much danger in having mismatched tire pressures. The average person rarely if ever checks their pressures. The engineers at Subaru know and understand this. Tires also go bad and get leaks. This is an unnavoidable fact of owning a car. Having serious damage occur because of this just seems like something that someone would have caught and put measures in place for.
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Old 11-15-2007, 06:12 PM   #6
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I would have to think that there isn't much danger in having mismatched tire pressures. The average person rarely if ever checks their pressures. The engineers at Subaru know and understand this. Tires also go bad and get leaks. This is an unnavoidable fact of owning a car. Having serious damage occur because of this just seems like something that someone would have caught and put measures in place for.
yea especailly if they supply a doughnut spare tire, its obviously gonna be alot smaller...
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Old 11-15-2007, 06:30 PM   #7
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You would need to have a much bigger difference to hurt anything...your tire whether at 30 psi or 40 psi still has the same circumference...just a different shape.
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Old 11-15-2007, 10:06 PM   #8
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You would need to have a much bigger difference to hurt anything...your tire whether at 30 psi or 40 psi still has the same circumference...just a different shape.
I've always wondered about that. I've measured rolling distance of a bicycle wheel while unloaded and loaded with my bodyweight, which results in a difference of approx 1". Of course bicycle tires are very flexible, while the steel belted tread of a car tire is not. Has anyone measured how far a car tire rolls with one revolution while inflated to different pressures?

On the subject of viscous couplers- my other car is AWD (92 Eagle Talon) with a viscous center diff. In the DSM community it's highly recomended to keep all 4 tires the same. Not only the same size, but the same brand too, since there will be differences in actual tire dimensions among different manufacturers. I'm suprised Scubie USA would supply a spare with a different diameter.
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Old 11-16-2007, 05:32 AM   #9
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Hi all, thanks for all the input.

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I'm suprised Scubie USA would supply a spare with a different diameter.
Actually the spare is a full-size (diameter, not width) spare. I'm not sure if the protesting drive train was caused by worn tires or an underinflated donut. I did have the spare on another time recently (roofing nail near the sidewall- 4 new tires *cha-ching* ) and made sure the spare was properly inflated- no noises were heard.

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You would need to have a much bigger difference to hurt anything...your tire whether at 30 psi or 40 psi still has the same circumference...just a different shape.
Kind of what I was thinking. Most likely the square vs. round profile makes a difference. The spare that I mentioned above has a round profile.

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I would have to think that there isn't much danger in having mismatched tire pressures. The average person rarely if ever checks their pressures. The engineers at Subaru know and understand this. Tires also go bad and get leaks. This is an unnavoidable fact of owning a car. Having serious damage occur because of this just seems like something that someone would have caught and put measures in place for.
True, there has to be some level of wiggle room. Otherwise it might as well be a locked up 4wd. I'm generally pretty anal when it comes to my tire pressure, which, I guess, is why I'm concerned about this in the first place. I've heard that 0.125" to 0.1875" difference tread wear front to rear can cause damage, but I guess that just brings me back to whether air pressure does the same thing as treadwear.

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Originally Posted by Danronian View Post
I would assume the PSI recommendation difference is such so that the car will ride the best. I have seen the same type of recommendation on RWD and FWD cars also, and they don't have to worry about the front or rear tire diameter matching.
May be true.

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Originally Posted by cfg823 View Post
I don't know what year Subaru Impreza this guy has, but maybe this applies to you. Read his website and maybe you can figure out which year/model he has
Sounds like the same specs as my car (at least in old EPA MPG) my car did have 40 PSI in the tires when I first brought it home, but after that every time I had it serviced, some monkey in the back would adjust the pressure to the numbers on the door frame (and always when the tires were hot ) I have never had MPG as low as that guy, but I have noticed rounding shoulders on my tires in the past. I always assumed that came from my tendency to enjoy a little trailing-throttle oversteer when I get a chance. I've cut back on that habit a bit, and hopefully the high pressures will help too. BTW, good links, I didn't find anything specific to my question on the tire bible page, but there's a lot of good info. I plan to read through the other car bible pages when I get a chance.
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Old 11-16-2007, 06:00 AM   #10
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I spent a fair bit of time hanging out on the Subie message board
http://www.ultimatesubaru.org/forum/

Learned that 4wd Subies really need matching diameter tires. Gotta rotate them at every oil change, about 3-5K miles. Subie has specs for how much difference is allowed in treadwear between tires. If not tended to you wreck the "differential", or whatever it's called where the front/rear drive trains meet.

As for the different pressure spec'd for front and rear, my guess is that the ~10% difference is just what's needed to end up with matching diameter front + rear. Most of the weight is in the front, thus slightly higher pressure there.

I tend to believe the post reporting success at 40 psi. That's high enough to avoid lots of tire squish and reshaping as it rolls. Having the fronts just a bit higher psi makes sense to me. I would expect things to go well at 44/42 as you have it now.

Would I go higher, like 50/48 or higher? Dunno. My rwd car is 52/50 now but that's my car, not your 4wd Subie.
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