The root mean square speed, is the square root of  the square of the average speed
Root mean square speed : sum the squares of the speeds, divide by the number of particles and then square root the resulting number.
Average speed: divide the sum of the speed of all particles by the number of particles.
1, 2, 3, 4, 5, 6
Average = (1+2+3+4+5+6)/6 = 3.5
RMS value = ((1+4+9+16+25+36)/6)1/2 = 3.89
The rms value is always slightly higher than the average value.