When reading Yewdall's Practical Art of Motion Picture Sound, you can read him talking about varying road textures in the movie Christine... I can't quite represent myself how one would go about doing this since I canct figure out how to record road textures alone... is there something I'm missing?
What he's talking about is how the tires sound over different surfaces, ie. smooth pavement, rough pavement, dirt shoulder, cobblestones, gravel over pavement, etc. You can focus on those textures by aiming your mics mostly at the tires for passbys, or mount mics in the wheelwells for onboards and steadies. You can also use this technique to make passbys, aways, and in & stops more interesting by telling the driver to vary his position on these surfaces as the car approaches the microphones. Example: Car passby, medium speed. Instead of staying on the pavement, the driver can swerve over to the shoulder slightly and then back to the road again.
One technique is, as Justin mentioned, to get the car moving with out the engine on. I recently had to get some gravel road tire noise without the engine and was able to find a nice gravel road with a hill, so I got to the top put the car in neutral and killed the engine and I was able to coast down the hill and get some pretty usable gravel textures. The scary thing was with the engine off it killed the powered steering and after about 30 seconds the breaks basically stopped working (anyone know why that would be? I am not much of a car car guy?). Luckily it was a long road and the car just rolled until it lost momentum. But I would like to highly recommend doing tests on small straight hills before getting any real speed just to see how the car reacts.
On another note I was getting a lot of low end rumble, not wind (I has a full zeppelin and furry on the mic), it was easy to roll off later but I was surprised at how much low end the gravel produced.