Eric a couple of things you can add for cross country comfort: Taller profile tires besides lowering the RPM's added gas milage the softer sidewall will add to the ride. Also you can add an air dam. Also because our cars aren't long on insolation check McMaster-Carr or even Loews and add the material under the carpet at the firewall and floor. Adhesive backed closed cell rubber or foam inside the door skin. (pull of door panel roll up window and add to outer skin) On a 510 I had for a while I used 2 layers of the stock firewall insulation and 1/8 acoustic foam under the carpet..............loads better.

As for the speedo error, your car should have a 3.90 rear gear, use the online programs right down the RPM for 25, 35, 45, 55, 65, 70 &75 then before your trip you can make note of what the speedo says, then take little labels and stick em to the speedo face (On my B210 I used little red dots about 3mm in size) After a While you'll know that 27=25 or 55=62 etc.

Tom

Thanx for the advise on the insulation, I had thought about that. I am going to try under the hood, inside the doors, all of engine compartment. I am triing to figure away out to insulate under the rear wheel vinyl arches, I think that a good amount of road noise is produced there.

I will try to post some pics of my progress.