var YourVehicleClass Car;
var float CarsSpeed;
// (im not sure what function uses in ut3's hud)
// write new hud content
simulated event PostRender( canvas Canvas )
{
CarsSpeed = VSize(Car.Velocity);
Super.PostRender(Canvas);
Canvas.Style = ERenderStyle.STY_Alpha;
Canvas.SetDrawColor(255,255,255,255);
Canvas.Font = Canvas.MedFont;
Canvas.SetPos(0.703*Canvas.ClipX, 0.056*Canvas.ClipY); // 0.703 and 0.056 are coefficients of screen location
Canvas.DrawText("Speed: "$CarsSpeed);
}