u7135 speedometer help

[malcolm47]

so i got my predator yesterday got it all hooked up. I notice as i was driving down the interstate and i got my foot off the gas just to see will it slow down well it did but it speedometer was barely moving while decreasing almost like it was idle while around 40 mph. is this normal?? but when i hit the brakes the speedometer shows the decrease a little faster. this is my first mod. is there something i need to modify?? if so what is it??

i have a 2008 dodge charger r/t