What would the speed be of a wavelength of 8cm

Maybe there are some situations where speeds depend on wavelengths.
I don't know, and I can't say for sure that there aren't any at all. 

But at least in the cases of electromagnetic waves and sound in air,
speeds don't depend on wavelength.  All wavelengths of sound have
the same speed in air, and all electromagnetic wavelengths ... like heat,
radio, visible light, X-rays etc. ... have the same speed in vacuum.

If your wave is sound in air at sea level, then its speed is about  340 m/s .

If it's a radio wave, then its speed is about  300 million m/s .