Meteorites typically hit the Earth's atmosphere at velocities ranging from 10 km/s up to 70 km/s.

In more familar units, that's from 22,000 mph to 157,000 mph.

Small meteorites are however slowed down to "almost nothing" by our atmosphere, and they end up hitting the ground with the same velocity as an object dropped from a plane say. (So called "terminal velocity", about 220 mph.)

Large meteorites are not slowed nearly as much by the atmosphere, and can reach the ground with high velocity. The one that caused the "Tunguska event" in Siberia in 1908 is thought to have been travelling at 20 km/s as it exploded in the air 8 km above ground level. That's about 45,000 mph. The energy of the blast was similar to a hydrogen bomb.

As for the contribution of the Earth's graviity to meteorite velocity, the maximum contribution from that is the same as the escape velocity from the surface of the Earth. That is 11.2 km/s or about 25,000 mph.

So depending on the meteorite's speed on entry, we could say that Earth's gravity contributed anything from less than 15% up to nearly 100% to its velocity,

http://csep10.phys.utk.edu/astr161/lect/meteors/impacts.html

http://www.newgeology.us/presentation26.html

http://www.bautforum.com/space-astronomy-questions-answers/85892-gravitys-contribution-meteorite-impact-speeds-earth-moon.html

http://www38.wolframalpha.com/input/?i=escape+velocity
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