Normal vehicles vans and cars 100 at full size. Some people go down to 72dpi. Some people up to 150dpi.
Buses and big trucks you can drop this down to keep your file sizes in check.
I think those massive billboards are at like 30dpi.
If you have a current image open it up in photoshop. Check the image resolution (http://bit.ly/9ID9lm). If say the image is 10cm by 10cm at 300dpi then you can resize it to 30cm by 30cm at 100dpi and you’ll be fine. You won’t be resampling the image.
If you need the image a lot larger you’ll start to loose quality.