Mercury is the planet closest to the sun and has the greatest velocity on average 107,00 mph. This speed was found by taking the perihelion speed of 127,000 mph and it's apogee speed of 87,000 mph.
The fact that surprises most people is that Mercury is not the hottest planet. Mercury rotates once every 58.65 days. That is two thirds of the orbital period. So it would be logical to say a year on Mercury is three days long. (each of those days would correlate to 58.65 earth day lengths.). This is because Mercury lost it's atmosphere billions of years ago. Because there is no atmosphere to contain heat, it is lost to outer space. Only a thin envelope of helium remains. The surface of Mercury resembles the moon so closely that some times even experts are fooled.
Unfortunately the Sun will become a red giant, five billion years from now, Mercury will be engulfed by the Sun's expanding outer layer, so will Venus and Earth.