I really screwed up that message -- that's what I get when I try to think at 7:00 AM, before my morning breakfast!

First, I wrote (excerpted) that your 205/55-15 tires, with a 606.5 mm diameter (compared to the OEM's 614.6 mm), had been "...very slightly overreporting your mileage and speed (i.e., when your speedo says you're going 50 mph, you're really doing 50.7 mph)..."

This is wrong. It's a smaller tire (and smaller circumference), and therefore your car doesn't go as fast or as far as the speedometer and odometer indicates -- it has been UNDERreporting your speed and distance, so that if it says you're going 50 mph (or you traveled 50 miles), you're really going only 49.3 mph.

Likewise, your planned newer tires, 205/60-15, with a 627 mm diameter, are larger, and therefore your car really goes 2% faster (and farther) than your speedometer indicates.

I'm very sorry for my mistakes.