As with the suggestion earlier try a more powerful flash, use "rear" sync setting on the camera with the flash which will nicely integrate any blur you will have (because of your shutter speed / aperture combination in the ambient light) with a sharp image at the correct end of the resulting "direction of motion" blur. You may have a mismatch between white balance between the cool flash and the warm ambient lighting but the vehicle (which is the focus of the image) is perhaps more acceptably rendered in the "correct" balance of the flash. Also, again as above, crop tighter, or get closer. There is an old adage: Get in as close as you think is OK . . . . then get half as close again (if possible)!! Getting closer will make your flash even more effective. Looks like it has a lot of potential! Experiment with slower shutter speeds to accentuate the blur = motion aspect. You may have to try the "rear slow" sync setting to achieve dramatic effects. Again it looks like a lot of fun to be had here.