This is a paid asset, but now you can download Complete Vehicle Pack Free.
Detail this asset from Unity Store: Original Link
Complete Vehicle Pack v1.4 (Latest version)
This package contains 27 different vehicles, varying from ordinary town cars, to police cars and firetrucks.
It’s ideal for racing or delivery simulators, or top-down games.
The package contains 74 prefabs, each vehicle has a mobile friendly version, and for those who want to customise their cars, some separate wheels are added to the mix.
All models share the same material and texture, a 4096*4096 atlas, for optimum performance on mobile devices. The triangle count for each model is around the 1k mark and each vehicle has separated wheel meshes
Check out the second volume of this package Complete Vehicle Pack V2