Incase you didn't already know... Don't ever put a space saver on the rear of a WRX.

The_Jed wrote: In reply to HiTempguy: 2/32 (1/16") of tread depth translates to a diameter difference of 4/32 (1/8"). Assuming he has the 205/55-16 tires that's about .392" more or less distance traveled per revolution. Doesn't sound like much but at 60 mph (not accounting for expansion at speed) that tire is spinning about 811 rpm. That's a 317.912" difference every minute. Lump me in with the suckers.

I would agree with you if tires didn't slip. But they do. Having a couple 32nds difference is literally no different than simply driving the car with perfectly sized tires. A drivetrain with tires on pavement is by its nature essentially in a "bind" condition to begin with. I sincerely doubt the little tread difference matters. Maybe if all 4 wheels only went straight and were mechanically forced to turn at the same speed.

Kenny_McCormic wrote: Never put a different diameter tire on any axle equipped with a limited slip or locking diff. IIRC the Subarus do come with a donut spare, and its safe for the center diff below 45mph.

Though my owner's manual says the same thing, and my 09 has open diffs front and rear.

FWIW, I replaced all four because my bad tire was at 6/32" (vs 11/32" for a new one). Not close enough to the recommended range for my liking.

I used to curse like this about owning an F-body. It was mainly concentrated at the horrible engineering practices by General Motors engineers. Only a few Honda's I've cursed at (oil filter location and ease to get to on K20s being one). I've got two Subarus now; but, both covered under warranty. I have my doubts about long term even though some people rave about their reliability.

I had a 2002 WRX Wagon for a short time in DC. It had seen it's fair share of winters and was also a beach town car in Florida for a period (according to the previous owner). I went through like 2 cans of PB blaster just to replace rear rotors and pads. Replacing one of the rear hubs? So many curse words were used. Then I had to replace the fuel lines because they leak in the cold...then I had issues with my 2012...now I have two more WTF? Still awesome cars.

Any time you have a shorter tire on one side of a regular open diff the spider gears are going to be whirring away against the diff pin. There is not a bearing there because cars are not actually designed to spend the majority of their lives turning or with asymmetric tire sizes. Sucks for a few people but think of how many, hell, BILLIONS of dollars have probably been saved not putting bearings between diff gears and diff pins over the last century?

Diff pins be gallin' and the rich keep gettin richer. THANKS OBAMA!

Why not make the donut as tall as the standard tires instead of making people play musical tires on the side of the road.

Wally wrote: Why not make the donut as tall as the standard tires instead of making people play musical tires on the side of the road.

I'm not sure about the smaller Subaru's, but the forester I used to have came with a full sized spare from the factory.

Of course that was in 03, fast forward to now and my 11 Elantra came with a can of fix-a-flat and a compressor.

As for to replace all 4 vs replace one. I understand the diameter argument and the spider gears aren't riding on bearings. But how many awd cats are running around with under inflated tires? Differences in inflation pressures can result in diameter changes that are probably bigger that the tread wear.

I recommend to have a full sized spare instead of a doughnut because they're engineered to only get you to a repair shop.

I've driven on one good tire on the front and one bad tire on the front, and the car feels awkward and uncomfortable.

The strangest thing about donuts is that the one in my (w-body) Impala was good for 2000km at 120km/h (according to manual and sticker). They exist, why can't other mfr's use them.

I agree with OP. It's inexcusable that the factory spare (assuming he used it properly) would wreck the diff. Yes he'd need 4 tires either way, but he wouldn't be dealing all the other crap.

jsquared wrote: So... you hate Subarus because of user error and because northeastern weather and salt makes things rust?

dean1484 wrote: It has 180K lived in the salt belt for 12 years what are you complaining about. Most ANY car would be the exact same.

And yet somehow Volvo figured out how not to make their cars rust profusely in the late 80's / early 90's. So did Toyota - as long as they're not US made Toyotas. What is it with Subaru and Mazda and the like that they can't seem to make a car hold up to what is a normal and known condition? And why do we forgive them for it?

It's like blaming alcohol in gas for trouble with a car - they've been putting alcohol in gas for decades. If it's causing trouble, the car is to blame. Road salt is a little more hit and miss, but it's telling that some automakers have figured it out and others just don't seem to care.

dculberson wrote:

jsquared wrote: So... you hate Subarus because of user error and because northeastern weather and salt makes things rust?

dean1484 wrote: It has 180K lived in the salt belt for 12 years what are you complaining about. Most ANY car would be the exact same.

And yet somehow Volvo figured out how not to make their cars rust profusely in the late 80's / early 90's. So did Toyota - as long as they're not US made Toyotas. What is it with Subaru and Mazda and the like that they can't seem to make a car hold up to what is a normal and known condition? And why do we forgive them for it? It's like blaming alcohol in gas for trouble with a car - they've been putting alcohol in gas for decades. If it's causing trouble, the car is to blame. Road salt is a little more hit and miss, but it's telling that some automakers have figured it out and others just don't seem to care.

x2.

I've lived in this part of MI my whole life and this is definitely not my first time playing with high mileage salty winter cars. I've never had this kind of issue on rusty high mileage Prizrolla's, Probe's, DSM's and other crap. You'd think for the premium you pay and the fact that they are popular in these climates, Subaru's would handle it a bit better.

In reply to dculberson:

Planned obsolescence and cost control, Ford Motor Company is literally built on top of a salt mine, and look at how their stuff looks after 10 years.

So, can someone tell me why a car with a front and rear diff even has a space-saver spare?

I know better than to run one of those on any axle with a differential, but on a AWD car, its not like you have a choice. Flat tire=new diff if you use the space-saver.

They have an open front diff fyi. That being said, we don't have a clear indication of the original posters actually driving situation. How many miles driven the spare, and did they go above the 50 posted limit on the space saver spare. That being said I've driven with the space saver on the back for longer than I should have in my 02 wrx wagon, with no ill effect. That rear diff is currently in my race car, albeit with new fluid. It did 172k miles in the street car before it went to my race car shell. It's been abused with all kinds of race tires since, and mix of tires, though in uniform size. I don't doubt the original problem, but can't help but think some other factor may have contributed to the death of that diff.

Wally wrote: Why not make the donut as tall as the standard tires instead of making people play musical tires on the side of the road.

the donut on my 09 wrx is exactly the same diameter as my stock tires. YMMV.

On a related tangent, if I were to have 215/50/16 tires on 8" wide front wheels, and then the same tires on the 9" wide rear wheels, would that be bad on the diffs on a dsm?

The_Jed wrote: Assuming he has the 205/55-16 tires that's about .392" more or less distance traveled per revolution. Doesn't sound like much but at 60 mph (not accounting for expansion at speed) that tire is spinning about 811 rpm. That's a 317.912" difference every minute. Lump me in with the suckers.

This. I've seen Jeep Grand Cherokee transfer cases get borked from running two new and two worn tires.

You might be able to get around it by putting the new tires on the front, but you're supposed to put new tires on the back for safety reasons.

IMO, tires should always be replaced in pairs or sets of four, but sometimes you gotta do what you gotta do to get back home.

Opti wrote: I know of a certain local shop that paid for a brand new transmission in a 380K mile camry because they didnt check the "diff fluid". Hint: Camrys diff fluid is seperate from trans fluid.

How old? A lot of Toyota transaxles do have separare differential fills. Right pain in the ass to get to the check/fill port, too, so a lot of people don't know it's there. Have seen a few cars junked because the diff bearings were shot and the car wasn't worth a transmission.

dculberson wrote: And yet somehow Volvo figured out how not to make their cars rust profusely in the late 80's / early 90's. So did Toyota - as long as they're not US made Toyotas.

Good to know. Why did they hoard this technology instead of using it on production cars?

sachilles wrote: They have an open front diff fyi. That being said, we don't have a clear indication of the original posters actually driving situation. How many miles driven the spare, and did they go above the 50 posted limit on the space saver spare. That being said I've driven with the space saver on the back for longer than I should have in my 02 wrx wagon, with no ill effect. That rear diff is currently in my race car, albeit with new fluid. It did 172k miles in the street car before it went to my race car shell. It's been abused with all kinds of race tires since, and mix of tires, though in uniform size. I don't doubt the original problem, but can't help but think some other factor may have contributed to the death of that diff.

Exactly. There's usually more going on than is divulged.

Former Subaru service advisor here, but I worked for them pre electronic transfer case. I used to get asked this question all the time and the official Subaru line:

1) Use the space saver ONLY on the front.

2) The space saver tire should be run for a maximum of 50 miles at 50 mph. If longer distances are necessary, Subaru said to let the transmission cool off for about an hour then continue another 50 at 50, etc.

3) All 4 tires need to be within 3/32" of each other. That means replacing all 4 tires at the same time with the same brand etc.

4) It is possible to get away with tire height discrepancies on DIRT roads since the tires can slip but NOT on asphalt, that's because they will NOT slip enough.

FWIW, the Mitsubishi 3000 GT VR4 had the same restrictions. When I ran an AAMCO shop, we had a guy drive one from North Carolina with a space saver on the rear, the diff seized up due to heat. That happened after he came off the Interstate then stopped for a traffic light, the car refused to move when the light changed.

Jeep's QuadraTrac has the same tire diameter warnings and yes I dealt with this question constantly for 6 years. Here's the deal: all 3 diffs are limited slip. That means plates which have to slide against each other and they are NOT designed to do that constantly. The diff fluid does not last forever either, it should be replaced at 30k intervals. If not, the center and rear diffs will stick and the thing will 'crab walk' in a turn on asphalt, making the tires bark and all kinds of goofy stuff. (Typically the front won't stick, dunno why.) Changing the fluid in all 3 and driving it in figure 8's will get the diffs freed up.

Now, if the tires are different diameters front and rear, the center diff will be constantly 'working', i.e. the plates slipping against each other and oh yes this WILL cook the center diff. That's because the center diff plates will be 'slipping' at highway speeds, like 80 MPH, and no it is NOT designed to do that for, say, 300 miles.

Isuzu's TOD (Torque On Demand) wasn't as sensitive to tire discrepancies because the transfer case was designed to send power to the front wheels only as needed, it would 'decouple' at highway speed on dry pavement for instance. Isuzu still recommended having all 4 tires the same size.

Curmudgeon wrote: Former Subaru service advisor here, but I worked for them pre electronic transfer case. I used to get asked this question all the time and the official Subaru line: 1) Use the space saver ONLY on the front. 2) The space saver tire should be run for a maximum of 50 miles at 50 mph. If longer distances are necessary, Subaru said to let the transmission cool off for about an hour then continue another 50 at 50, etc. 3) All 4 tires need to be within 3/32" of each other. That means replacing all 4 tires at the same time with the same brand etc. 4) It is possible to get away with tire height discrepancies on DIRT roads since the tires can slip but NOT on asphalt, that's because they will NOT slip enough. FWIW, the Mitsubishi 3000 GT VR4 had the same restrictions. When I ran an AAMCO shop, we had a guy drive one from North Carolina with a space saver on the rear, the diff seized up due to heat. That happened after he came off the Interstate then stopped for a traffic light, the car refused to move when the light changed. Jeep's QuadraTrac has the same tire diameter warnings and yes I dealt with this question constantly for 6 years. Here's the deal: all 3 diffs are limited slip. That means plates which have to slide against each other and they are NOT designed to do that constantly. The diff fluid does not last forever either, it should be replaced at 30k intervals. If not, the center and rear diffs will stick and the thing will 'crab walk' in a turn on asphalt, making the tires bark and all kinds of goofy stuff. (Typically the front won't stick, dunno why.) Changing the fluid in all 3 and driving it in figure 8's will get the diffs freed up. Now, if the tires are different diameters front and rear, the center diff will be constantly 'working', i.e. the plates slipping against each other and oh yes this WILL cook the center diff. That's because the center diff plates will be 'slipping' at highway speeds, like 80 MPH, and no it is NOT designed to do that for, say, 300 miles. Isuzu's TOD (Torque On Demand) wasn't as sensitive to tire discrepancies because the transfer case was designed to send power to the front wheels only as needed, it would 'decouple' at highway speed on dry pavement for instance. Isuzu still recommended having all 4 tires the same size.

Thank you so much. It's always nice to get the straight (correct) poop.

I understand running extreme differences will be outside normal operating parameters, but but I would not be surprised to see a bad alignment or lack or tire rotations resulting in tread depth differences that could exceed 3/32" (.093") over the life of a set of tires.

And what happens to the diffs when when turning corners?

The only time all four wheels turn the same exact speed is in a straight line. Any turn results in a difference between the inside and outside wheels, as well as different speeds for front and rear.

So during an autocross, track day, or a drive on a nice curvy road the diffs are constantly working.

There's a difference between an autocross and driving on the highway.

Two interesting points before I go:

It's general consensus among the oldtimers on speedtalk that going from a limited slip diff to a spool will cost a tenth. Spool is a tenth slower. Extra tire friction since no two tires have the same rolling radius, the chassis is going to be loading the tires differently anyway, and limited slips still slip.

I realized that I had a low REAR tire on my limited slip RX-7 when it started pulling heavily to the right. After fixing it, I could still feel it when one tire would get 10psi lower than the other. Just to reiterate, this was a rear tire going low, which ordinarily you can't feel.

Three, three things: On my 4wd Subaru, I experimented with tire pressures by seeing what pressures would allow me to shift in and out of 4wd at highway speeds with no effort on the shift lever. I ended up with 44psi front and 35 rear. So the front tires definitely do roll out less than the rears, pressure being equal. Now, the difference in rollout would be on the order of tread depth differences between "looks okay" and "new".

Four, four things! : That Subaru had a full size diameter compact spare. My Mom's Subaru, same year/model but front wheel drive, had a smaller diameter spare. I thought anything AWD/4WD would have had a full diameter spare.

Five is right out.

I understand there are differences between autocross and highway driving. But unless your going in a straight line and everything else is perfect then tires are rotating at different speeds.

The question becomes how much variation in wheel speeds can the diffs tolerate, and for how long?

The mfg specs are based on reliability, risk, and warranty.

The issue with tire shops is that I've had them insist on 4 new tires without even take a measurement or looking at the specs on a new tire.

Correct, they do work constantly in turns, when changing lanes etc. There is also a small amount of slippage even in a straight line due to torque input. The engineers have already allowed for this. Just as an FYI, the 'normal' amount of slip for a clutch pack is somewhere in the 2-3% range. That means for each 100 RPM in, the clutch pack will have 97 RPM out as a matter of course. That's where most of the heat is generated from an automatic transmission, for instance and the same is true of a limited slip clutch pack. The difference is the LSD plates are metal on metal rather than paper on metal and are thus more susceptible to galling due to spot overheating. The automatic transmission also has a constant flow of cooled fluid, again something a differential does not have. Some differentials have cooling fins on them to try to keep temps down.

But when more 'slip' is added through tire diameter discrepancies etc the slip can get out of normal ranges. Again as an FYI, Chrysler's transmission controllers are designed to allow up to about 8% slip (IIRC, don't have quick access to the manuals at the moment) before taking corrective action by putting the trans in limp mode etc., sorry I don't know what Subaru says on that subject.

Quick dumb math on my part: If a tire is 24" OD, the RPM per mile is 840.336. Once worn 3/32, the RPM is 847.229. So if the rear tires are worn 3/32" more than the fronts then the rear tires are turning 7 RPM more than the fronts per mile. That doesn't sound like a lot but add that to the 'built in' and expected 3% slip mentioned above and the lane change etc slippage and you can see where things can start to get hot.

I'm sure that Subaru has built a safety margin into that recommended 3/32" difference. It's even possible to get lucky a few times. To protect my wallet (I'm a cheapskate ) I'd rather not take that chance too many times.