Question

when the sun is overhead then what should be our speed to be with sun when it is overhead

Answer 1

The sun can be overhead anywhere between the tropics, depending on the time of year.

IF you were on a tropic on a solstice date then you would need to travel around the earth at the tropic in 24 hrs to keep up with the sun
the length of the circumference at the parallel of the Tropics is about 36 768 km

If however you were on the equator during an equinox, then you would need to travel around the world at the equator in 24 hours to keep up with the sun
Earth's equator is about 40,075 kilometres

So you can see that there is a range of answers to your question, depending on time of year....