Just add this snippet in fpsgame/client.cpp at line ~15
XIDENT(IDF_SWLACC, VARP, radarspeedfov, 0, 1, 5); /** multiply the speed-output with this (0 = off) **/
float calcradarscale()
{
fpsent *a = autohudplayer(); /** Get the data from the hudplayer (which is following or self) **/
float curspeed = a->state != CS_DEAD ? a->vel.magnitude2() : NULL; /** current speed: either vel.magnitude2() or 0 if dead **/
return clamp(max(minimapradius.x, minimapradius.y)/3, float(minradarscale+(min(int(curspeed), 150)*radarspeedfov)), float(maxradarscale)); /** Just add curspeed to the minradarscale and don't let it be too much **/
}