Pretty good choice on the shutter speeds, and good panning--the stabilized longer zooms are excellent lenses for slower-speed panning like this. Photos 1,4,and 5 are all very nice. I'm not a fan of the so-called Dutch tilt, but it's something a lot of younger people seem to find okay,so,whatever...
Photo 5 has the absolute cleanest,simplest background...the railing is a long, horizontal subject,that lends itself well to this panning technique, as does plain, clean,green grass. Shot 4, of the 97 car, for example is a nice "look" at the car, but the top of the frame is dirt, and it does not "read" nearly as well as a guardrail does...
Shot 3 of the #6 car, the red open wheeled car looks bad going "uphill" and also "slamming into the edge of the frame" as they say...the car needs more space to drive "into", and here, it looks like it has more space where it "was"--but not enough space to give the impression that it has vacated the empty space behind the car. Most people like to see a car with more space to drive into, rather than the other way around. So, Photo #3 to me is hurt by the crop and tilt, both about evenly--if there is more space to go back to, and re-crop, that could very easily make #3 a salvageable,successful photo.
On the speed of the cars...these are at the higher (faster speed) end of what I like to see...I kind of dig panning shots where there is a little bit more blur, a bit more flowing motion feeling on the car's body panels...which one can do with a stabilized lens, much more easily than with the older, non-stabilized lenses, where when you do 1/3 to 1.8 second pans, you tended to get up-and-down camera motion that would spoil the panning effect. On these, you have the wheels blurring, but still somewhat sharp--I think you could go slower on the shutter speeds, for an alternative look and feel that's a bit more impressionistic, more flowing,more gritty.
Of course, these are all just my opinions: sponsors wanna' see their decals, you know...wanna' read that oil filter's name,etc....but the creative more generic feeling of pure raw speed comes through a bit more,to me, when the car is a slight bit more blurred....it's like walking the ege of a razor though...too slow and it's a clunker, too fast and it's static, and that varies with focal length, car speed, car size within the frame, and how perfectly you keep up with the panning speed of the car, and how the car is going by you--quartering toward you and incoming, or perfectly parallel to your shooting spot,etc.
This kind of photography is gaining in popularity, and thanks for showing us your photos. Your results are better than the majority we see on the web, and I don't want to appear to be diminishing your choices or results--just talkin' about car pics and how to distill and convey RAW SPEED!!!! into one single still frame. No small feat, that!