All 4 tires should be within 2/32" tread depth.

Try to get the damaged tire fixed. If that can't be done, ask about tire shaving to see if they can get your new tire down to the current tread depth of the other 3.

If nothing else, it means you need to replace all of them in order to get them the same diameter.

 

All 4 tires should be within 2/32" tread depth.

Or 1/16"
there is no one correct answer, it depends on how new your tires are, if you only have 4,000 or 5,000 miles on them is a totally different answer than if they have 20,000 miles on them.

usually a good tire dealer can measure your tires and tell you. Obviously they may have a vested interest in selling you 4 tires vs repairing one, but ... that's why it's key to find a honest competent mechanic

 

How old are the tires? Often this question gets asked about tires with many miles on them, which should be replaced anyway.

Stan

 

Owning Subies for about 15 years, I've had it go both ways, one time the tire was repairable, another time the tire had multiple penetrations, was not repairable, and that required 4 new tires, which stinks.

 

My tires are new. But I am thinking hypothetically should a emergency occur. Does it make sense to buy a full spare--including rim--to keep instead of the donut that comes with the vehicle?

 

That's what I did! I think it makes a lot of sense. Even though it seems expensive up front, it allows you to do a 5-wheel rotation (if you keep up with rotations regularly), effectively extending the life of a set of tires. It also allows you to keep going after a flat without interrupting your schedule to find a tire shop ASAP. Certainly not necessary, but this is probably the safest option... So does it make sense? That's up to you, but IMO, Peace of mind = priceless.

Or 1/16"
there is no one correct answer, it depends on how new your tires are, if you only have 4,000 or 5,000 miles on them is a totally different answer than if they have 20,000 miles on them.

Actually, number of miles on the tires might not tell you much about how much they're worn. Depending on the type of tires, road/trail/track conditions, and the driver, it's possible that 20,000mi tires could be worn less than 5,000mi tires. The only sure way is to measure them, or have them measured if you can't do it by yourself.

The fact that the cost-saver spare tyre is a few inches smaller in diameter than the normal tyre gives the lie to the need to match tyres within fractions of an inch.

As has been mentioned, this is dangerous advice. The differential can handle tires of uneven diameter for TEMPORARY periods, but it adds a lot of strain. Please follow the recommendations in your owner's manual, which are as Josh stated:
put spare on rear only (swap with front if necessary)
low speed only
minimize distance driven on spare

Be safe, and be nice to your car!

 

The fact that the cost-saver spare tyre is a few inches smaller in diameter than the normal tyre gives the lie to the need to match tyres within fractions of an inch.

 

Yet it is also limited in use to the rear of the vehicle, the speeds under 55 MPH, and is explicitly stated to be replaced with a proper tire ASAP.

Older models require a FWD enabling fuse to be inserted before driving on the donut spare.

 

True, but using the saver spare on the front would unbalance the steering to a dangerous degree, due to its different characteristics. It is limited in use because it doesn't have the same grip, load, or wear capacity as the normal tyres. The fact that it is not necessary to disconnect the rear wheels with the fuse on the later cars demonstrates that the differential diameters is not a critical factor.

 

This is correct about the placement of the spare on the rear vs the front. It isn't so crazy of a difference that it will pull dramatically, just that it will have some difference under some circumstances, especially when you're pushing it harder.

Frankly, we got a flat on the front and just put the spare there. No sense messing about like a monkey all day for a simple swap. Once done, just take it easy, keep the speed down, and stop driving when you get where you need to go. You really shouldn't be going highway speed on these things anyway.

 

then i guess the whole thing is nonsense, thank heavens you were here

the spare is for limited use, duh

driving for extended periods with tires of different circumference will cause significant differential wear

i've been in OPs situation and have had tires shaved locally, it's not terribly expensive and it's actually a nice little luxury because shaved tires roll so well
if you have a local race track they probably have a contact number for a local tire shaver
40 bucks or so per tire

 

If you only get one new tyre make sure it's the same brand as the rest. Even though different brands might have the same size advertised, there might still be some difference in circumference.

One of my friends wrecked his centre diff on his Legacy with driving on tyres with different brands. The previous owner had just replaced the front tyres with a different brand because the tread on the back was still fine . A new centre diff is quite expensive.

The FWD fuse trick only works on AT's and not on MT's, not sure about CVT's.

 

The FWD fuse trick only works on AT's and not on MT's, not sure about CVT's.

The CVTs don't have the FWD fuse anymore. I'll have to check the manual again, but I don't believe it requires the doughnut to be mounted on the rear as in years past. I will also note that the doughnut spare is indeed smaller, but very nearly the same circumference, as a real tire.

My ex-2007 Impreza with 4EAT had a doughnut that was much smaller than the real tires. And it had the FWD fuse. And the warning to put the spare on the rear.

 

2005 manual makes no mention of tread depth. Instead, it says to measure the circumference and make sure the difference between the tires is less than 0.50". This results in a tread depth difference of about 1/16" in my limited experience.

I decided to switch to the aforementioned 5 tire rotation. Bought a 5th matching rim, and I found a tire on ebay that MATCHED my tread depth and thus my circumference quite well. It was cheap to boot, and I did not have to shave it. Tire appears to be in better shape than my others.....

The five tire rotation has a lot of merits. 1) you are on a trip and you ruin a tire (or just don't want to stop soon to patch a flat). You can put on the matching spare and keep driving with no limitations of speed, handling etc. 2) You ruin one tire and the other tires are in good shape with lots of treat. You then can take a few days to find a matching used tire on ebay for a FRACTION of what a new set of 4 tires would have cost.

Of course all this is easier if your subie can accept a full size spare in the rear compartment.