Do all 4 tires need to be replaced when just 1 goes bad?

[Vanguard]

Because of AWD, I figured when one of the tires (front left) was damaged
beyond repair that I would have to replace both. That is, due to
difference in diameter for an old and new tire, I would have to replace
both tires on each end of the axle. That was also the recommendation
from my favorite car shop and I thought that is what the Subaru dealer
said.

Just got an oil change and that car shop said that I also need to
replace the rear tires. They are older and more worn but far from
needing replacement due to tread wear (probably more than half life left
on them). Like the idea that both tires on the same axle need to be the
same diameter, the guy at this shop says that I also should have new
tires put on the back. He sells tires but not the brand on the front so
he knows he wouldn't get my business to put them on the back, and even
mentioned that. So it wasn't like he was trying to drum up a sale and
he said the fronts are brand new and there would be no reason to replace
them to get a full set of 4 and instead go back to where I got the
fronts and get the same for the rears. He claims the tranmission would
blow due to the rears spinning faster than the front because of their
reduced diameter from being much more worn, and that the tranny would
see that as slippage on the rear.

So, is it true that if you lose one tire that you end up having to buy a
whole new set of 4 tires so they all are the same starting diameter?

 

[Carl 1 Lucky Texan]

Because of AWD, I figured when one of the tires (front left) was damaged
beyond repair that I would have to replace both. That is, due to
difference in diameter for an old and new tire, I would have to replace
both tires on each end of the axle. That was also the recommendation
from my favorite car shop and I thought that is what the Subaru dealer
said.

Just got an oil change and that car shop said that I also need to
replace the rear tires. They are older and more worn but far from
needing replacement due to tread wear (probably more than half life left
on them). Like the idea that both tires on the same axle need to be the
same diameter, the guy at this shop says that I also should have new
tires put on the back. He sells tires but not the brand on the front so
he knows he wouldn't get my business to put them on the back, and even
mentioned that. So it wasn't like he was trying to drum up a sale and
he said the fronts are brand new and there would be no reason to replace
them to get a full set of 4 and instead go back to where I got the
fronts and get the same for the rears. He claims the tranmission would
blow due to the rears spinning faster than the front because of their
reduced diameter from being much more worn, and that the tranny would
see that as slippage on the rear.

So, is it true that if you lose one tire that you end up having to buy a
whole new set of 4 tires so they all are the same starting diameter?

Technically that is true. There are some folks who have found shops to
shave one replacement tire down to match - shaving 2 is probably a false
economy since you're wasting exactly what you paid for! new tread. Some
folks say they just run newer tires a little lower in pressure. Possibly
a dangerous idea.
According to Subaru - the tires need to be within 1/4" in circumference.
Many folks say they have gotten away with varying from that number. How
much is OK and under what conditions? who knows? The risk is damage to
the center differential which detects any difference in front/rear axle
rotation as slippage and tries to engage on dry ,straight pavement
leading to 'torque bind' and possibly subsequent damage. If you have an
automatic transmission, placing the spare fuse in the FWD postion in the
fuse box under the hood SHOULD prevent damage to the center diff.

Carl

 

[Vanguard]

Technically that is true. There are some folks who have found shops to
shave one replacement tire down to match - shaving 2 is probably a false
economy since you're wasting exactly what you paid for! new tread. Some
folks say they just run newer tires a little lower in pressure. Possibly
a dangerous idea.
According to Subaru - the tires need to be within 1/4" in circumference.
Many folks say they have gotten away with varying from that number. How
much is OK and under what conditions? who knows? The risk is damage to
the center differential which detects any difference in front/rear axle
rotation as slippage and tries to engage on dry ,straight pavement
leading to 'torque bind' and possibly subsequent damage. If you have an
automatic transmission, placing the spare fuse in the FWD postion in the
fuse box under the hood SHOULD prevent damage to the center diff.

Carl

Thanks for the reminder about the FWD fuse. I think that converts to
just FWD drive, right? I'll call the shop where we got the new front
tires to ask if they would measure the diameter of the fronts versus
rears to see if the rears are within 1/4" of the fronts. Thanks again.

 

[houndman]

Technically that is true. There are some folks who have found shops to
shave one replacement tire down to match - shaving 2 is probably a false
economy since you're wasting exactly what you paid for! new tread. Some
folks say they just run newer tires a little lower in pressure. Possibly
a dangerous idea.
According to Subaru - the tires need to be within 1/4" in circumference.
Many folks say they have gotten away with varying from that number. How
much is OK and under what conditions? who knows? The risk is damage to
the center differential which detects any difference in front/rear axle
rotation as slippage and tries to engage on dry ,straight pavement
leading to 'torque bind' and possibly subsequent damage. If you have an
automatic transmission, placing the spare fuse in the FWD postion in the
fuse box under the hood SHOULD prevent damage to the center diff.

Carl

I heard the center diff. could be damaged if there were different
sized tires on fr and rr. Heard a guy went through 3 before they
figuired it out. Maybe it was long ago, or no one knew.

Didn't know there was a fuse to change the drive axles. Is that with
an Auto, or all of them?

VF

 

[houndman]

Thanks for the reminder about the FWD fuse. I think that converts to
just FWD drive, right? I'll call the shop where we got the new front
tires to ask if they would measure the diameter of the fronts versus
rears to see if the rears are within 1/4" of the fronts. Thanks again.- Hide quoted text -

- Show quoted text -

Carl said 1/4" Circumference, NOT Diameter!!))

VF

 

[Vanguard]

Okay, I've been through 3 different Subie owner manuals (for 1992, 2002,
and 2003) and even a Chilton repair book. I've found mention of the FWD
fuse (which was not by direct mention in the owner manuals but instead
regarding a dash light). I have yet to find out what SIZE fuse goes
into the FWD slot.

 

[Carl 1 Lucky Texan]

I heard the center diff. could be damaged if there were different
sized tires on fr and rr. Heard a guy went through 3 before they
figuired it out. Maybe it was long ago, or no one knew.

Didn't know there was a fuse to change the drive axles. Is that with
an Auto, or all of them?

VF

Auto tranny, - it tells the ECU (or TCU?) to engage the duty solenoid C
constantly which places the center diff in FWD max bias.


if I recall correctly.

Some folks have rewired their cars to a toggle switch to 'force' FWD,
4WD or AWD at their discretion.


Carl

to reply, change ( .not) to ( .net)

 

[Carl 1 Lucky Texan]

Okay, I've been through 3 different Subie owner manuals (for 1992, 2002,
and 2003) and even a Chilton repair book. I've found mention of the FWD
fuse (which was not by direct mention in the owner manuals but instead
regarding a dash light). I have yet to find out what SIZE fuse goes
into the FWD slot.

Pretty sure its 15A - the spare (or larger of the spares.)


Carl

 

[houndman]

Auto tranny, - it tells the ECU (or TCU?) to engage the duty solenoid C
constantly which places the center diff in FWD max bias.

if I recall correctly.

Some folks have rewired their cars to a toggle switch to 'force' FWD,
4WD or AWD at their discretion.

Carl

to reply, change ( .not) to ( .net)- Hide quoted text -

- Show quoted text -

I take it that 4WD has the same power going to all the wheels all the
time.

I have thought it would be interesting to have a car that started out
in RWD, maybe switch to 4WD, and then cruise in FWD, and then switch
to AWD when needed.

VF

 

[Vanguard]

Pretty sure its 15A - the spare (or larger of the spares.)

The FWD slot is inside the fusebox inside the engine compartment.
Inside the lid are 10, 15, and 20 amp fuses together and a 30 amp fuse
farther away. You'd think they could put the amperage fuse in the same
icon as "FWD" on the inside of the lid.

 

[Body Roll]

So, is it true that if you lose one tire that you end up having to buy a
whole new set of 4 tires so they all are the same starting diameter?

No, you distrust the manual, run one new tire and ruin the center
diff. Then yet another very slightly used subie hits the used car lot.

 

[bugalugs]

I take it that 4WD has the same power going to all the wheels all the
time.

I have thought it would be interesting to have a car that started out
in RWD, maybe switch to 4WD, and then cruise in FWD, and then switch
to AWD when needed.

VF

Why ?

 

[B A R R Y]

I have yet to find out what SIZE fuse goes
into the FWD slot.

Probably almost any size.

That fuse is more of a jumper that tells the computer to do something,
as compared to carrying a decent amount of current to actually power
something.

 

[Vanguard]

No, you distrust the manual, run one new tire and ruin the center
diff. Then yet another very slightly used subie hits the used car lot.

The owner manual actually says that all 4 tires must be the same
circumference (or diameter since a reduced circumferance would mean
also a diminished diameter)? I'll check again but don't remember the
book saying anything about that.

 

[Todd H.]

The owner manual actually says that all 4 tires must be the same
circumference (or diameter since a reduced circumferance would mean
also a diminished diameter)?

Yup, they're directly proportional. Circumference = pi * diameter.

 

[Al]

The owner manual actually says that all 4 tires must be the same
circumference (or diameter since a reduced circumferance would mean
also a diminished diameter)? I'll check again but don't remember the
book saying anything about that.

So then why isn't there a problem under normal use? After, all off/on
ramps to the highways are right turns. The left wheels travel further
than the right wheels. The front wheels travel further than the rear
wheels as the rear wheels don't track the front wheels. Doesnt' this
cause a problem?


Is this significant for someone who does most of his driving on highways?

I learned recently that UPS has a computer program that organizes the
routes of the delivery vans. They try to make all turns right turns as
left turns burn gas while waiting for traffic to clear. Apparently this
does save them significant amounts of fuel.

How would this impact a Subaru?

Al

 

[y_p_w]

So then why isn't there a problem under normal use? After, all off/on
ramps to the highways are right turns. The left wheels travel further
than the right wheels. The front wheels travel further than the rear
wheels as the rear wheels don't track the front wheels. Doesnt' this
cause a problem?

I guess it's a matter of magnitude. The biggest potential problem is
alleged to be the electronically controlled center transfer clutch
being overworked in the automatic transmission models, especially on
dry pavement. It could be like a clutch with a reasonable life if
only used when needed, but easily trashed if the driver insists on
shifting every 10 seconds. The manual transmission models are thought
to be more robust, although I have heard of premature failure of the
center diff with mismatched tire sizes left on way too long.

Is this significant for someone who does most of his driving on highways?

I learned recently that UPS has a computer program that organizes the
routes of the delivery vans. They try to make all turns right turns as
left turns burn gas while waiting for traffic to clear. Apparently this
does save them significant amounts of fuel.

How would this impact a Subaru?

BTW - manual transmission Subarus don't come with a FWD fuse. The WRX
automatic transmission version (at least the 2004 model year) doesn't
have a FWD fuse. I've used the compact spare a few times, and the
only factory recommendation is keeping it under 50 MPH.

 

[strchild]

Pardon my ignorance, but normal front and rear differentials, not the
locking sort, seem to have no problems running for the life of most cars,
and to tell somebody you can steer your car but, not constantly or else your
front or rear diff. would burn up, would be silly. Why is it Subaru does
not use one of these style differentials in the center, if it is serving
essentially the same purpose, and doesn't seem to wear out so quickly based
on usage? Sure, it might take a tiny bit of engineering to use that sort of
differential, but maybe someone else can enlighten as to the reason not to?

~B

 

[Blair Baucom]

That would be an open differential. The power would go to the one wheel
with the least traction. It would be one wheel drive.

With a locking or limited slip center differential, you have two wheels
driven, one on each end, with the least traction.

On my Forester XS with manual transmission, as the center slips it heats and
locks the clutch pack, and with limited slip rear differential, it drives
the three wheels with the least traction.

There are only a few vehicles that lock all four wheels, I saw a Mercedes
SUV that has three air lockers on front, center, and rear, that you can
switch when on sand or rock crawling so that all four wheels are pulling.

There are aftermarket parts to add locking or limited slip to vehicles.

There are others systems made to direct power to the wheels that have
traction.

The Subaru site shows the 5 systems that they use.

Blair

 

[Vanguard]

The FWD slot is inside the fusebox inside the engine compartment.
Inside the lid are 10, 15, and 20 amp fuses together and a 30 amp fuse
farther away. You'd think they could put the amperage fuse in the
same icon as "FWD" on the inside of the lid.

I called the Subaru shop today and asked them. They said to use a
15-amp fuse. Of course, they also said that FWD mode is only for when
doing diagnostics or when towing but I've seen many folks discuss
staying in FWD mode all the time.