The first thing to consider is the earth’s rotation. The earth is 24,900 miles in circumference at the equator, or 40,000 kilometers. The earth takes 24 hours to make one rotation. So:
24,900 / 24 = 1,037 MPH or 1,666 KPH
As you move toward the poles that number decreases. At the north pole the speed is zero and you are simply rotating in place at one rotation every 24 hours. So let’s assume you are sitting somewhere in South Florida moving at about 1,000 miles per hour or 1,610 KPH.
The Earth is also making one orbit around the sun every year. That sounds like a long time, but the orbit is huge. The Earth is roughly 93 million miles (150 million km) away from the sun, giving its orbit a circumference of 584 million miles (942 million km). That works out to 66,666 MPH or 107,000 KPH.
If you are on the side of the planet where the planet’s rotation is moving in the same direction as the orbital direction, these two speeds add together. If you are on the opposite side, they subtract. We are trying to calculate a maximum speed, so we will be adding.
Our solar system itself is also moving in an orbit around the galactic core. The solar system is something like 25,000 light years away from the center of the galaxy, and the galaxy makes one rotation every 250 million years or so. That gives the solar system a speed of something like 420,000 MPH or 675,000 KPH.
So there is speculation that the galaxy is moving through the universe at a speed of 1,000 km/s, which means 3,600,000 KPH or 2,237,000 MPH.
Adding it all up, you get:
1000 + 66,666 + 420,000 + 2,237,000 = 2,724,666 MPH
Or
1,610 + 107,000 + 675,000 + 3,600,000 = 4,383,610 KPH
In other words, you are hurling through space at 2.7 million MPH (4.4 million KPH) even though it feels like you are sitting still.