there is a small design issue in your models:
whilst using lots of polygons for the tires (that will probably rotate most of the time and are black), the other parts and the main shape of the cars are very blocky.
It should be the other way roung. Just a few chamfered edges would do wonders.

Also you should rework the textures quite a bit. add some details and basic shading to the model to save tris. just a plain color map aint gonna work.

And - I know its old fashioned and some might not agree- close the mesh! You might run into unnecessary and ugly rendering issues in some applications. Also the designer might need to flip the model and so on.
its just a few faces and texels but in the end they can make the difference.

cheese and have a nice one