The hand painted rear fenders are really obvious. Instead of trying to match the color and just paint it, consider using the clone brush/matching tool so it keeps the 'texture' of the paint.
The windows you added look nice, good use of gradient to simulate glossy window. Same with the new side mirror.
The two lens flares are really silly. Even one lense flare it considered cliched now, fyi.
There is no shadow beneath the car. You can do a shadow using a 'darken' tool. Don't try to draw the shadow using a paint color, it won't work with the ground you have it on.
For all the speed holes you put on the car (behind the front fender, on the running board, etc), consider using a darken tool instead of a solid color. The solid color really stands out as painted on.
If you have to have those black speedholes on the front bumper, consider softening them slightly so the edges aren't as sharp.