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

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.

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

quoted text -

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

VF

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.

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)

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

Carl

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

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.

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.

Why ?

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.

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.

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

-- Todd H.

2001 Legacy Outback Wagon, 2.5L H-4 Chicago, Illinois USA

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

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.

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.

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

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

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.