I had purchased the Michelin AS3+ in May. The tires seemed fine, but then in June I drove the car on a long trip (400 miles round trip). Most of the driving was straight highways When I got home I noticed that one of the rear tires looked low on air and I checked it, it was down to about 15lbs of pressure. The next day I checked it and did not see any leaks and spraying with soapy water didn't reveal any either. I bought the pressure up to 32 and checked it over a few days, no change. I took the car on a long trip this past weekend (500 miles round trip). When I arrived at me destination (250 miles) the same tire visually looked low so I checked it and was down to 13.5lbs. I refilled it to 35 and the next morning it was down to 33 but the tire was warm when I filled it so figured that might cause the variance. The next day we drove home and after 100 miles I stopped to check the pressure and was down 6 lbs. I bought it back up to about 33. after driving the rest of the way home (about 150miles) I was down over 10 lbs. Last night I removed the tires, sprayed the tread, rim and valve, no leaks. Checked it this morning and did not lose anything. So it seems the leak only happens under highway speed driving.

I second the rim issue. Take it in and find out. When tires get low like that the sidewall temperature increases leading to failure. That type of failure makes the sidewall come apart like someone used a knife all the way around in a circle. Does not take long to destroy the rims as well. Of course in your case the rims may already be shot!

Shop said the rim looks fine. They said they saw a small valve leak and replaced it. Will monitor it closely now to see if it worked.

Thanks for the input.