I know alot of AWD systems like Subaru symmetrical will have issues with binding if the tires aren't the same size.
I don't believe this to be an issue with our cars because shawd has several clutch packs in between that are capable of varying the amount of transmitted power through them variably.

When I got my car it had mismatch tires but all 245/45/18, not all the same tread depth, front and back sets were different. At first I got a full set of winter Michelin tires and then in the summer I replaced 2 tires with Michelins. So I still was mismatched but at least my steering tires would have good traction. Then I tossed the 2 old tires at the end of the season.

This year I had planned on buying 2 brand new tires to match those 2 that now have 1 season of use on them. However I'm strapped for cash and now have a deal for $50 Canadian I will have a pair of 235/50/18 Firehawk gt tires with 6mm tread depth remaining. New they have 11mm but can be driven on to about 2mm and I just need something for the summer so I don't keep running my winters, I already did a couple big roadtrips (2 seperate 800km+ trips) on them plus my normal driving all in the heat which can't be good for them or my gas mileage.

They don't match perfectly but they only have a 2.2% larger circumference when new and they're significantly more worn than the 2 Michelin tires to compensate for that.
Overall I'll be giving up about 3% width for about 7% more sidewall.

I figure if I didn't have issues running brand new tires on the front and dead old tires on the back previously then I won't here either

I'm wondering what other people think? Have you done something similar?

 

I haven't done this, but I've been told not to do it because it causes excessive load/wear on the SH-AWD clutch packs. I have noticed that if I have a tire that's low on air it's far more noticeable on my SH-AWD TL than on any other car I've owned and mucks up the driving/handling/stability.

 

I'm not saying it's ideal, however the difference between the tire circumference is what matters here. My mis match before didn't cause any harm, I know because I did my front and rear differential fluid this month and both were in good condition. And that circumference mismatch was an estimated 1.5%-2%

After accounting for treadwear on these tires the circumference mismatch is an estimated 1.6% difference.
Meaning I know I won't explode the clutches in the couple months before I put winter tires back on in the fall ( I live in a very snowy area and they get put on early) I also don't plan on using them past this season. Just need something to pair the Michelins with for a couple months in the summer heat.

Thanks for the response, it made me double check my math and it actually made me more confident in my decision

I'll let everyone know if I destroy my differential

 

Well, hope that doesn't happen!