Like the contact patch, the footprint is, as its name suggests, the area of the tyre that is in contact with the road surface at any given time.
Where it differs from the contact patch is in the fact that the footprint refers to the actual tread contact area, taking into account the areas where there is a groove in the tread pattern, as opposed to the contact patch which is complete area covered. As with the contact patch the footprint will vary with tyre pressure and under braking / acceleration. One factor that can affect the footprint of a tyre is the type of tyre in question.
For example take two tyres of exactly the same size. If one is a winter tyre and one a summer tyre the summer tyre will have a greater footprint due to the amount of tread in contact with the road surface. This is due to the fact that winter tyres generally have what is referred to as a high “negative ratio”. Negative ratio is the ratio in a tyres contact patch of tread blocks to tread voids (no blocks – ie grooves). More information can be found under the heading Winter Tyres.