To edit you need to DL the Workbook for Panthers in the Fog, you open it in a spreadsheet program like Excel.
Paste in the data from GtC in the relevant sheets. Make edits and save as tab de-limited .txt files.
Put the text files in your base/data folder.
See this thread for vehicle speed modding and limitations :
tm.asp?m=3993035
Vehicle speed is very dependent on the team experience and morale values. Vehicle acceleration and Max speed values found in vehicles.txt are the base values for each vehicle, but also Vehicle Move coloumns in Elements.txt will influence speed for all vehicles in a given element.
Bog and imobilize coloumn values also seems to influence speed in a given element.
Attached is a zip with some of my element and vehicle speed modding. Speed and acceleration is set to 250 and 40, I think you will see even higher speeds by going to 259 for both, but then you meet the cap on vehicle speed as discussed in the link above.
So in addition to this you want to edit TEAM morale and experience to see the full effect.
The speed and acceleration edits is the single most important mod I have done to the game. It makes it more realistic and fun when tanks are more agile.