Fan speeds are controlled by the Motherboard and the temp of the computer,
If it is not going at full speed most likely its that the computer has no need for it to run at that speed,
Now if its getting way to hot and not spinning up more then it could be a issue with the bios or a sensor on the motherboard (hardware) since its a new fan you know most likely its not the fan.
if you want to update your bios go to support.dell.com and go to the drivers section.
As far as i personally know on all the time i have spent with many, almost all Dell Copmputers out there in the world right now there is no way you can manually change the fan speed.
