I got a flat on my 2014 XC70 T6 and had to drive about 40 miles on the doughnut.

1) Should I replace the doughnut in case I need to use it at some point in the future?
2) I have 28,500 mile on the car/tires, should I replace both rear tires?

Any advice would be appreciated.

 

1) Does the spare still have normal tread on it? If so, its still good. Its a real tire, just skinny to lighten the wheel, so it will last. 2) Since your car is AWD, you will want to replace tires in sets of 4. (that's why regular rotations and alignments are a good idea). Tires should be replaced based on the tread depth and if they have any uneven wear or flat spots/cupping (which will make the tires noisy). Tire mileage varies widely based on brand, driving style etc - I've had sets wear out at 30K and others at 50K on the same car. Best to have the tread measured. Also if you live in a snowy climate, you may want to use snows in the winter even with an AWD car for superior braking and cornering.