440 Automatic. What gears are you running?

If your swapping gears in rears...here is the transmission speedo gear chart...keep that speedo in check as well.
Go to the link and use the Automatic or Manual trans link (at the top) to insert tire sizes etc for correct gear choice... cool
This will also tell you expected RPM @ speed

Thanks to Brewers web site

Mopar Speedometer Pinion Numbers