Rekative Movement

I am using a Matrox box to show the outside view across three monitors. The speed over the ground as the airplane flies looks to be very slow. It was OK with just the one monitor. I had this problem once before and was told the fix, something in the FSX setup files, but I did not write this down so I cannot remember how to fix it. I would appreciate it if someone could tell me how to make the perceived speed match the aircraft speed when using a Matrox system to show the outside view, thanks.