Check to see if the torque converter lockup clutch is still functioning.

Run a stopwatch timed test past 10 mile posts are exactly 60 mph on the speedo to see if the calibration of the speedometer and odometer is correct.
Each ten seconds fast or slow of exactly 10 minutes is 1 mph off.

 

easier way to check the spedo is to use a gps if you or a buddy of yours has one