A given speed in miles per hour will have a specific RPM. Those two numbers are constant no matter what other conditions are at hand.
similarly, if you're in a lower gear for the same miles per hour, you will have a higher RPM and that will be constant for that same speed in miles per hour. A given RPM will always produce the same miles per hour in a particular gear.
The direction of the wind and road grade (going uphill or downhill) will affect the amount of throttle that it takes to maintain a given RPM/mph.
If you're riding into the wind or going uphill, it will take more throttle to keep the RPM and may even prevent you from achieving high-end RPM.
Conversely with the wind at your back or going down hill, it will take less throttle and you will notice an increase in miles per gallon efficiency.
You are more likely to achieve possible top speed or even surpass it momentarily with the wind at your back (or going downhill). (Provided an RPM limiter does not activate and impede your speed/RPM)
I'm sorry if I was not clear enough in my prior post
Wrigacona clearly understands the concept. And it can be boiled down to 1 word . . . Load or load factors which is what the wind resistance and road grade affect/produce.
Sometimes I don't explain myself as clearly as I would like.