What's the gravities effect on the speed of a sattelite circling the earth?

No change. The satellite is in a state of balance, equilibrium, in an ideal world. What WILL change is air friction in the real world. Even at many hundreds of miles, there IS a small number of air molecules in space, which will have a cumulative effect from DRAG. So, in the real world, all satellites slow down and descend because of air friction. What changes is the RATE of descent. The closer to the earth, the more air molecules per cubic foot, the more air resistance. It is this drag in space which is why communications satellites at 32,000 miles need thrusters so they can do "station keeping" to keep the orbit stable to hold the position over the land below.
 
If it's orbiting the earth, gravity is holding the satellite in place. Without gravity, the satellite would just fly off into space.

However, if the satellite is spinning downward toward the earth (ultimately to crash), then the stronger the gravity, the faster and at greater speed the satellite will descend.
 
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