So, I installed OpenWRT on my router because it was having problems with it proprietary firmware.
Anyway, it shows the signal strength, noise levels, and transmission rate.
It is a 802.11n router that I bought two or three years ago.
Three of the devices, two on 2.4 GHz, one on 5 GHz, which are using 20 MHz channels, are using MCS7 according to the router. The one device on 5 GHz is using 40 MHz channels and for some reason is using MCS5 (and I think two streams).
The signal level and noise is roughly the same for all devices and all the devices are in the same general area.
Is there any reason that a device on a large channel width would have to use a lower MCS?
Similar to higher modulations, wider bandwidth signals also require a better SNR to work reliably, Depending on the actual losses you are experiencing, I would expect at least one MCS (rate) difference even if you were in the same frequency band. 5 GHz signals also have shorter range, so a difference of two MCS rates doesn't seem outrageous.
Try playing with the the SGI on your AP, you may actually get a slightly higher data rate , at a higher MCS rate, with the long guard interval.
Even with the lower MCS rate at 40 MHz, you are still getting at least 75% better throughput on 5 GHz.
That makes sense. Thanks again!