Just like using a donut tire, the difference in circumference can still be safely used at lower speed for shorter distances but may damage the car if used for long distance at high speed.

 

I sifted through several posts earlier , and couldn't find a clear answer and am hoping this group can weigh in. I recently purchased a 2023 OBW and swapped the stock wheels and tires out for Methods wheels and slightly larger tires - 245/65/17 Falken Wildpeak AT3W. I retained the spare that came with it, that is a 225/65/17.

I really hadn't thought through the unique nature of an AWD vehicle with regards to running a smaller spare tire, so I pulled out the user manual to see what it had to say. What I learned is:

1. Some outbacks come with a full size spare and some come with a donut that is smaller than full size tires
2. When using the donut it needs to be used on the front axle
3. Using different size tires can cause damage to the transmission
4. The spare should be used for a short period of time, and at slow speed

What I am trying to understand is whether or not there is a significant risk of damage if I use my full size spare (stock size 225/65/17) with my slightly oversized tires ( 245/65/17). My thinking is that its comparable to using a donut, and would need to be used for emergencies only .

I plan to buy another 245/65/17 wheel/tire combo prior to my first rotation, but haven't settled on a way I want to carry it. I am just trying to understand if I have a flat between now and then if I am going to damage my car by driving it with the smaller spare tire. Thanks in advance!
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Your #2 is backwards: a spare that doesn’t match the running tires needs to be used on the back axle, not the front. You can use your undersized spare on the back axle and follow the same guidance as for the donut in terms of speed and mileage restrictions. You might find your oversize tire fits in the spare well, but you’ll have to try to know for sure. There are a few that have posted on here what oversize they have found that fits in the spare wheel, but I don’t recall off the top of my head what that is. I know that my full-size spare does have extra room, so a slightly larger tire will fit. I would say about an inch greater diameter than the stock 225-65/17 will fit. Run a tire size calculator to see how much bigger yours is.

Update: according to the tire size calculator I just ran from discount tire, your oversize is almost exactly an inch taller than standard, so yes, I think it will fit in the spare wheel. However, do check for yourself just to be sure, i.e., make sure you can get your money back if you buy the oversize and it doesn’t fit.

Also, keep in mind that your speedometer is now 3.6% off, meaning your observed fuel economy is that much lower than actual. You’re also taking a performance hit, but since you have the Wilderness you’ll probably be fine. I don’t know of any way to adjust the speedometer and odometer on a Subaru for oversize tires. On my truck I have a Hypertech that corrects for that.

 

Added tip for getting that bigger tire in the spare well if you haven’t already figured it out: put the tire in with the top up and away from you, bottom down and towards you. Now slide the bottom further down and rearward into the well, then lower the top that is away from you into the well. If you try to put the top in towards the front first, the bottom edge nearest to you will not drop in because it gets stuck on the back lip of the wheel well trim.

With your oversize you’ll have to trim a little off the bottom of your foam insert that holds the tools. As an alternative, many of us have just ditched the foam insert and wrapped the jack and other tools in a rag. The advantage of this technique is now there’s a whole bunch more room to store stuff inside the spare under that back floor. Just be sure you don’t store your lithium jump starter under the back floor because if the car battery is totally dead, you can’t open the tailgate.

 

That's my plan when I go with 502s and a larger size tire like a Wildpeak A/T3W, to buy a 5th wheel and tire and stow it in there even if I have to deflate it completely and ditch the foam insert. I already have too much stuff in there but it's addictive to see what fits

 

Be sure you have a compressor strong enough to seat the bead if you deflate the tire. Also, I'm sure you know there is no way I am aware of to correct the errors induced in the speedometer, odometer, shifting, etc.

 

Here's another thread on the same topic with some pics of a wildpeak 245 fitting:

Gen 6 - Wilderness oversize spare tire in spare well

Okay community - looking to either find the holes in my plan and fix them or be talked out of this altogether! I’m pretty well set on swapping out the stock wheels on my OBW with a set of LP Aventure LP1s. I’d like to go with 245/65R17 tires and a matching spare in the spare well. I’ve gleaned...

www.subaruoutback.org

Generally, a 245/65/17 will fit in the spare tire well, but you'll need to deflate it a bit. Also every tire model - even in the same size - is a slightly different size circumference/diameter-wise. I have a 245/65/17 hankook dynapro at2 xtreme in my spare well, but I had to deflate it to about 14psi. I have a compressor that I can keep in the back, so its no big deal for me.

 

By deflating and squeezing a spare tire in the compartment I'm not sure it will be good for the condition of the tire.