I've been to the races around here(same as a high school football game) and a flash won't do it unless you were within 30' of the trucks. You'll have to get a faster lens, one with at least an F2.8 aperture. A prime lens with an F2.0 would be better, but you'd have a set focal. If you're within 30' then an SB900 flash would work. You'd also have to set the camera to aperture and raise the ISO until you get the shutter speed you're looking for.
Could use a little help here. My son just started shooting some action shots of some truck races. He is using a nikon D40 and there is some lighting about the same as you would fine at a football game. What type of flash could some one recommend???