I saw a video explaining that put the car in higher gear where possible to avoid high revs thus maximising mileage, this lead a thought in my mind about what happens if the revs are low and gear is high (not so low as to lead to stall). Does this still consume same amount of fuel or does additional load in low revs lead to worse mileage?
what happens if the revs are low and gear is high (not low to lead to stall). Does this still consume same amount of fuel or additional load in low revs leads to worse mileage?
Short answer is: you can reduce fuel consumption slightly, operating more efficiently, by using higher gear and lower RPM for a given load condition. (Make sure you are not confusing load, which is the actual amount of work your car's engine has to do, with throttle, which is a particular control component of the engine that affects specific fuel consumption.)
To explain why…
To a first approximation, fuel consumption is directly related to load, only. The more work the engine has to do, the more fuel it burns.
At constant speed, there are two primary factors for load: rolling resistance, and air resistance. Rolling resistance is, for the most part, constant (irregularities in the road surface could affect resistance vs speed, but this effect is small), while air resistance increases exponentially with speed. So for the purposes of a rough analysis, your speed is the primary variable for load.
Another factor is, of course, weight vs incline. At a given speed, it requires more power to go uphill than to travel over level ground or to go downhill.
There are a bunch of other factors that all come into play, and will cause smaller variations in fuel economy. But primarily, the work being done and thus the fuel consumption is determined entirely by your vehicle's speed (working against air resistance) and any incline (working against gravity).
So far, so good. No mention of RPM or throttle yet. But it's not quite that simple, because while all that explains the amount of work that ultimately is required by your movement, there are inefficiencies in the engine that waste work, before "the rubber meets the road" (so to speak).
Among these "bunch of other factors that all come into play" is the work required for the engine to breathe. Part of the engine's function is to draw air into it. Mixed with fuel, this is what provides the power for the engine. But it takes work to move the air. And at a given RPM, it takes more work when the throttle is open less than when the throttle is open more (the throttle is essentially an air valve that restricts to a variable degree how much air goes into the engine).
The final piece of the puzzle is to understand that (again, to a first approximation), the power from the engine is a function of two variables: RPM, which determines how often a single charge of fuel/air mixture is burned in a cylinder; and how much fuel is actually in that charge. For a given amount of power, you need to burn a certain amount of fuel per time period; if you are burning fuel more often, then you need to burn less of it each time you burn it.
Or put another way, at lower RPM you need the throttle to be more open, so you can burn more fuel per cycle, and at higher RPM you need the throttle to be less open, burning less fuel per cycle because your cycles come more frequently.
Ignoring all the other (arguably important) factors, we can see then that you can get the same amount of work from the engine regardless of engine RPM. It's just a matter of choosing the right combination of RPM and throttle setting.
Given that the total amount of work the engine is doing has to be the work required to maintain your speed, plus any work wasted by the inherent operation of the engine, the fact that it takes more work to draw air into the engine when the throttle is open less means that all else equal (including the exact load on the engine), you'll burn more fuel at higher RPM than at lower RPM, because you waste more work with the less-open throttle associated with the higher RPM.
Caveats: lots of other things affect engine efficiency, including various accessories attached to the engine, weight of components, etc. Also, many diesel engines don't have an actual throttle plate per se, and so none of this discussion would apply to those.
It is possible one would consider this question already answered on this site. Consider these other very similar questions and their answers:
fuel consumption - gear - rpm relation
Fuel efficiency and Shifting/RPM
Does flooring the throttle while traveling at lower speeds increase fuel consumption?