nice job , good tips are given here
i think you also need to reconsider the technique you use for the side refs , they line is too sharp
also the whole body needs more difference in light and dark area's

like on top > roof and hood there would be highlighted area's as well as on the top of the side
the lower you get some area's are darker , things that stick out like spoilers and skirts have dark areá's and the parts catching light (sticking out) will have lighted area's

you can do without any for of real reflection in a chop but not without highlighted and shade stuff

thats where you can gain the most i think

basic brush is good now try to work on that