I have one bad tire. Don't feel like buying four to replace one damaged one. Can I put two new tires of the same size/tread on the rear, run the used tires up front, and not have any AWD issues?

Subaru makes one tough car, but the weakest part of that car is the AWD unit, be it a automatic, or manual. Running mis-matched tires is just asking for trouble, trouble easily avoided by either running 4 identical tires, or 4 tires that you have physically measured as to being identical in diameter.

 

Your car, your call.

Yeah, I've wondered how sensitive the sytem was. I'd never run two different styles of tires, but I'm sure there will be some slight differences in the two. I can't afford to buy four tires just to replace one bad one, though.

 

A question, though: can your have bad effects w/ the AWD system if you put four identical tires on a Sube, but one of them is at a significantly lower psi than the others? (Say 10 or 15 lbs less pressure) Or if you had a bad alignment and one of the four tires had premature wear? Just how sensitive in reality is the AWD system to differences at the wheels when it comes to tires?

BAD BAD BAD

 

Its the quickest way to guarentee that you need to spend 800 or 1200 dollars later.

 

By two tires, put one on each axle, thats the best solution. physically measure the old tires against the new and maybe you can luck out, or find a used tire shop and have them measure and match.

 

 

If you have an automatic put the fwd fuse in.

 

nipper

Just how sensitive in reality is the AWD system to differences at the wheels when it comes to tires?

 

I can only speak to automatics...........

 

The driving force split, front/rear, is controlled by your transmission's computer, this computer takes into consideration a variety of things, engine load, gear the transmission is in, etc, but it does not take into consideration tire size. In the back of your transmission there is a small clutch pack controlled by the transmission computer that does the torque splitting between front and rear, by running different size tires you are forcing this clutch pack to slip. It was not designed to slip, at best you will burn up your automatic transmission oil (by overheating it), at worst you will tear our the clutch pack totally contaminating your transmission with bits, requiring a complete overhaul and torque converter replacement. There is a spot under hood to force your car into front wheel drive only, in this spot you plug in a standard fuse (passengers side firewall), this is for just such a situation. I can't speak to prolonged use of this (as it was never intended for prolonged use), but it will get you out of immediate trouble.

As nipper said, and as I have said and been attacked for in the past, if you can only afford 2 new tires, put them on the same side of the car. This way the front and rear differentials take up the difference and the center diff (which is the one that burns up) sees nothing.

 

The only exception to this with subaru AWD is if you have a rear LSD(limited slip differential). In that case, you have to buy tires in sets of 4, no iffs, ands or buts. Makes a good case for rotating your tires every 6k miles.

All tires MUST be within 1/4 inch circumference as measured with a narrow blade tape measure or you are asking for problems. If you have an auto and must run tires outside this spec then install the FWD fuse.

Search torque bind if you dont want to listen to us.

 

 

nipper

I have one bad tire. Don't feel like buying four to replace one damaged one. Can I put two new tires of the same size/tread on the rear, run the used tires up front, and not have any AWD issues?

 

Some tire suppliers which cater to racers offer "tire shaving" services. (I.e. they grind down tires to reduce the tread depth for better handling. You might be able to find a tire shaving service which can shave down your "new" tire to match the circumference of your old tires. I think "The Tire Rack" online might offer this service- dunno.

 

I really think Subaru should come up with torque bind warning/prevention software for the automatics. It would be a relatively simple matter to have the TCU watch for a constant speed differential between the front and rear speed sensors. If it sees this for an extended period of time (like an hour's drive) it should respond by either locking the car in FWD and illuminating a trans warning light to warn of this condition.

 

Nathan

Some tire suppliers which cater to racers offer "tire shaving" services. (I.e. they grind down tires to reduce the tread depth for better handling. You might be able to find a tire shaving service which can shave down your "new" tire to match the circumference of your old tires. I think "The Tire Rack" online might offer this service- dunno.

 

I really think Subaru should come up with torque bind warning/prevention software for the automatics. It would be a relatively simple matter to have the TCU watch for a constant speed differential between the front and rear speed sensors. If it sees this for an extended period of time (like an hour's drive) it should respond by either locking the car in FWD and illuminating a trans warning light to warn of this condition.

 

Nathan

 

Actually the new VDC systems dont have this problem. And they would need to use a steering angle sensor to let the car know its going straight as well.

 

nipper

 

I really think Subaru should come up with torque bind warning/prevention software for the automatics. It would be a relatively simple matter to have the TCU watch for a constant speed differential between the front and rear speed sensors. If it sees this for an extended period of time (like an hour's drive) it should respond by either locking the car in FWD and illuminating a trans warning light to warn of this condition.

 

 

GM's Versatrak system, at least as used in the U-body vans, can tell when you're running the donut spare and turns the AWD off. So it's been done before. Of course, the difference between regular tires and a donut is pretty extreme, so it's easier for the software to notice the difference.

For the record, its not just subaru. Jag and volvo have the same warining, just to name a few.

 

nipper

Not to disagree with everyone, but in the last 3-4 years, I have run all different sizes of tires on my Outback. Many times I've had to purchase a used tire or two to get by, and I have mixed different sizes and wear levels. I currently have over 273,000 miles on my awd the auto transmission in my 97 Outback. When I could afford it of course, I have tried to keep them all them same, but it never seems to work out. Just wanted to throw that out there:eek:

Not to disagree with everyone, but in the last 3-4 years, I have run all different sizes of tires on my Outback. Many times I've had to purchase a used tire or two to get by, and I have mixed different sizes and wear levels. I currently have over 273,000 miles on my awd the auto transmission in my 97 Outback. When I could afford it of course, I have tried to keep them all them same, but it never seems to work out. Just wanted to throw that out there:eek:

I am very glad for your luck, but I wouldn't take this chance myself. To me, Subaru drive train parts cost a fortune, moreover, if one relies on a mechanic the costs sky rocket! Tires are really cheap insurance to me:burnout:

All tires MUST be within 1/4 inch circumference as measured with a narrow blade tape measure or you are asking for problems. If you have an auto and must run tires outside this spec then install the FWD fuse.

 

ditto---

 

can you find a used tire(s)? measure with a thin metal tape, or cloth one from you wife's sewing kit, (you'll need at least 6.5 feet) ensuring that there is less than 1/4" difference in circumference. You'll be fine, otherwise go with the fuse, but only for a short time.

 

Lewis

BAD BAD BAD

 

 

By two tires, put one on each axle, thats the best solution.

 

nipper

 

is that the best (least worst) solution out of all of your bad possibilities, or is this an acceptable solution long term.

 

i assume this solution requires 2 matcing tires on one sdie and 2 matching tires on the other. so over the long haul the rotations will equal out.

is that the best (least worst) solution out of all of your bad possibilities, or is this an acceptable solution long term.

 

i assume this solution requires 2 matcing tires on one sdie and 2 matching tires on the other. so over the long haul the rotations will equal out.

 

More like the only solution (aside from all 4 new). Professionally I really dont like it, but if your in a crunch for money and have no choice, you have no choice.

 

nipper

Thanks for all the feedback.

 

I was forced to replace one tire on my 95, and knew I couldn't do that, so that's why I opted for two instead. I just got two put on this morning. Even if I'd wanted all 4, they only had 3 in stock. I'll have to run only two until the next payday, and then I'll go back and get two more put on. So, it looks like I'll be driving around town for two weeks like this.

 

I'd prefer to wait and get all 4, but as is I had a damaged tire that required pumping up with air twice a day to keep moving. I figured two new tires of the same size might be better for the AWD system than having one tire that is either running way too low on psi or way too high (from the cycle of pumping twice a day).

 

I am putting the FWD fuse in as soon as I log off the 'net. Thanks for the advice on that.

Tires don't always wear evenly. That's why we're supposed to rotate them. But in the real world, I wonder how many average folks really do rotations. Is the Subaru tranny/differential truly unable to cope with differences in wear with the same size and make of tire?

This isn't just a rhetorical question. I found myself in a similar situation a few years ago and went the 2-tire replacement route, using the exact same tires as what was on there. I'm still running them...the fronts have roughly 80% tread left to the rear 40% I don't know if that falls within the 1/4" tolerance but I'll measure. It's a standard transmission BTW.

I rotate mine twice a year. Tire rotation has been around as long as there have been tires.

 

nipper