Ground speed doesn't really matter during breakin....don't lug the engine and don't exceed 4000 -4500rpm for the 1st 1000 miles. Also don't cruise at freeway speed holding the throttle unchanged for any long periods of time...like more than 15 minutes...vary the engine speed slightly from time to time. For example...drive 60mph for 10 min, then 65 for next 10, then 70 fornext 10 then back to 55-60 etc. I would manually shift into 4th at 60-65 to vary engine speed once in awhile and mix it up so to speak.