Can one new tire hurt the differential

... he had called a Subaru dealer who told him that tire mismatches take a long time to cause any harm and that it's not unheard of to replace just one tire out of the 4 ...

What any dealer employee says is immaterial. The only true authority is Subaru (i.e. Subaru of America) itself.

a cadillac dealer sold you a car with tires in the 4-5/32" range? Pretty much a worn out tire at that point. Did the car come as/is or with a 30 day warranty? Does your state have lemon law?

Also, no mechanic ever gives an estimate of miles left. Because they might be sitting in court writing a check or two. Subaru's inspection form only allows for mm of tread depth. A mismatch of more than 2 mm flags the inspection report.

A mismatch of more than 2 mm flags the inspection report.

I would hope! 2mm difference in tread depth calculates to ~1/2 inch difference in tire circumference ... twice Subaru's published limit.

Thanks for all your comments folks, the frustration is real and it's good to not think I'm crazy for feeling like I got a raw deal here.

I finally got my car back, and man I'm glad to have it back. That Cadillac sucked despite being supposedly more luxurious.

The dealer didn't budge on giving me any sort of compensation for the tire mismatch and their inspeciton mistake or deliberate inaccuracy. The most they would offer is to install a tire for me if I got one shaved down myself. I can't do much since the only thing they would cover under the 90-day warranty is something that affects the safety of te car.

I'm done stressing about this and am just going to buy nice new all season tires and give this sleazy dealer a scathing review on Google Maps.

Besides, there are more important things to focus on, like drilling a hole in my center console to install a custom volume knob for my beloved fancy aftermarket radio! Steering wheel buttons are fine but I really miss having a knob. Even better than one on the radio itself I figured is having one by where your arm naturally falls when driving. I am using an arduino nano board, a rotary encoder and a digital potientometer and plugging it into the 3.5mm wired remote port on the back of the radio. (This guy on reddit wrote the code for the arduino.) I'll put the knob on that blank square below the parking brake for a button not in use. Can't wait to show it off on this forum.