I recently purchased a 04 400 with 1800 miles. I've noticed constant improvement in both acceleration and smoothness. How long before break-in is accomplished? I also checked my mileage. 57mpg. This is probably being a little generous since my speedometer runs exactly 5 mph fast (according to gps) and my odometer is a tenth or so fast over a measured mile. The exact figures were 72.6 miles and 1.26 gallons 91 octane.