My early Rocket did something similar when I disconnected the battery to install my Gerbing's/Battery Tender hookup. The bike had 2900 miles on it when I disconnected the battery and 900 when I hooked it back up. Then for an added twist, when the ODO should have turned over to 1,000 miles, it turned over 10,000! Went through 2 more speedo's under warranty, but their trip meters wouldn't reset. Finally just put the original back on when I found it sitting in a box in the back of the old Cycle Mart. The mileage read high, but at least it functioned.