What's your RPM?

What is the rear end gear ratio and the tire diameter.Lets assume the tach is reading correct,and that it is not still in there from having the /6.If it is,you need to open the tach and switch the switch to 8 cylinders from the six.Or if its a new tach make sure its on a 8 cylinder setting,or 4 cylinder setting.get the info and post it and I will do the math for you.Also what speedo gear are you using in the tranny,if its not the right tooth gear,its going to give the wrong speed,Mrmopartech