'06 XK8
My mechanic told me the inboard edge of the rear tires were worn and needed replacement. I had him mount

Michelin Pilot Sport A/S 3 255/45/18 99Y

After 8600 miles, the tread was entirely gone on both, with the steel belts visible across the tire.

I don't abuse the car, never 'burned rubber' or squealed on turns. I maintain the tire pressure as specified.

I'm hoping the tires were defective, old, etc.

What longevity is 'par'?

 

I think your alignment, especially rear toe in needs to be checked. Your rears shouldn't wear much faster than the fronts. If your roads are unusually abrasive and your driving style aggressive you could go through tires that fast, but that doesn't appear to be the case.

 

The fact that both wore in the inside points to car alignment issues, not tire issues. More specifically, your camber is off, not your alignment. Probably worn rear springs, makes car lower and tilts the camber towards the negative side.

Fix this before buying new tires.

 

What pressures are you running in your rear tires? I learned through trial-and-error back in 2012 that 28 to 29 psi is the sweet spot for my wife's 2006 XK8. Anything higher than that will wear the rear tires out prematurely right down their center tread. The sweet spot for her front tires is 32 to 33 psi....

It does appear that you have alignment issues. Worn upper shock mounts and lower shock bushings in these cars are notorious for affecting alignment. These components fail due to a combination of mileage and age causing the suspension to settle, throwing off the alignment geometry. Most of these cars that have not already had their various suspension components replaced by now definitely need the work....

My wife's 2006 XK8 received new rear shocks and shock mounts at about 83,000 miles in October 2014 and new front shocks, shock mounts, and upper control arm bushings at about 99,000 miles in January 2016. Now at 107,500 miles, her XK8's suspension is significantly better and the tires' treadwear is perfectly even across both axles again. Money and time well spent if you plan to keep the car for years to come....

 

Lots of things can be changed or adjusted, but it's mostly worn springs. At 80k miles, most Xk8's have sagging springs. I don't know of any other car that this happens on

 

Agree with jon89, along with sagging suspension issues tire pressure at "specified pressure" will wear out rear tires. I have the same tire and size as you and run 27 cold psi. Very even wear.

 

hm1,

The factory springs on my wife's 2006 XK8 are fine. We checked them when we replaced the shocks and shock mounts on both axles. The factory shock mounts are the components that typically need replacing the most....

 

Factory shock mounts last no more than 40k miles. The springs start sagging too, although they remain functional. -- just not up to spec

 

Like others I would suggest having the car aligned. They can make recommendations that you can act on or they might make adjustments that fixes the problem. I would suggest that you find the best pressure for your tires and check them frequently. I run 32psi and check the tread frequently.