OLD,
I read your original post late yesterday afternoon. I’ve been trying to find the right way to phrase my hypothesis, so here goes.
A “harsh and choppy” ride and/or wandering is sometimes the result of over-inflated tires. Accordingly, the first thing to do is to check the cold pressure on your tires to verify that they are “properly” inflated per the vehicle placard on the bottom of the driver’s “B” pillar and/or the owner manual. (Page 12-5; 32 psi front and 32 psi rear, I believe).
I believe the OEM Bridgestone Potenza (RE92A’s?) tires on your 2008 Impreza were size 205/55R16 89V. I believe the new Michelin Primacy (MXV4?) tires you recently installed are 205/55R16 91H or 91V.
Although the new tires are the same size as the old ones, they are a different "service description" (load index and speed rating), and therein may be the issue. I suspect a different speed rating (V = up to 149 mph, H = up to 130 mph) alone would not be a major issue.
The old Bridgestone’s 89 load index (probably) means that its load rating was 1,279 lbs at 51 psi max pressure. Your car’s owner manual says to set these tires at 32 psi front and rear.
The new Michelin’s 91 load index means that its load rating is 1,356 lbs at 44 psi max pressure. All of these numbers can be verified in the documentation that came with your tires, on the Michelin website, and/or on the tires’ sidewalls.
Since the new tires have a higher load capacity at a lower maximum pressure, I would expect for the new tires to subjectively feel “harsh and choppy” (compared to the old tires) if set to the Subaru recommended pressure.
Accordingly, you may wish to consider trying slightly lower tire pressures. Specifically, you may want to try 30 psi all ‘round, and then maybe even 28 psi. If this experiment cures the “harsh and choppy” problem it may also cure the wandering problem.
The load index hypothesis above is just my opinion, but, as a fairly technical guy, it makes sense to me. Nonetheless, you may wish to seek out a second- or third opinion, as my hypothesis may be complete nonsense. Also, the new tires' higher load index may directly correlate to a stiffer sidewall, which in turn may deliver a subjectively harsher ride.
It’s not unusually for worn tires to handle a bit better than new tires under favorable conditions. It’s also possible that the worn tires’ slightly smaller rolling diameters were delivering false high odometer readings, and therefore delivering erroneous fuel economy results. Winter gas and/or driving conditions may have also taken their toll on fuel economy (three or four tanks on the new tires?).
We had a tire load index issue with tires on our 97 Plymouth minivan. The original pressure spec was based on tires with a max pressure spec of 35 psi. The last tires we bought had a similar load rating, but a max pressure spec of 44 psi. Driving on the the new tires at the old pressure spec was a bit too “soft and floaty” for us, so we added ~4 psi to each tire, and this seemed to cure the “problem”.
I just purchased a set of Michelin Primacy MXV4 tires (205/60R16 92H) for my wife’s Honda Accord. The OEM tires were 91V. Fortunately for our application, there's very little difference between the two indexes: max pressure for both is 44 psi, and there's only a small difference in the max load capacity (33 lbs.).
Accordingly, I have high hopes for these new tires!
I hope this helps, and I apologize for the rather long-winded response.
Jim / crewzer
