If you think of it the other way around its simpler, ie a cold charge is less efficient from an economy point of view.
The reason being that the whole point of the chemical reaction (which is a burn not an explosion as people commonly think) is to turn a load of liquid molecules into gaseous ones, and critically to change the number of molecules of gas present to cause an expansion which pushes the piston down. If the gas starts hot, it ends hot, and hotter means more expansion.
At full throttle you are struggling to fill the cylinder as much as you can to just allow you to get as many molecules of air in as possible to allow you to burn more fuel so a cold charge is useful, but at part throttle you arent trying to achieve that at all so a hotter charge that contains more energy to begin with is preferable.
Dont expect the effects to be noticeable just by trying to work out your own MPG though, we're only talking a very small effect here. But its just an example of where things arent always as obvious with engines as people think, ie that assumption its very natural to make that colder air would always be better.
Finally, and I know I can google this, but whilst you're here () what even is the point in whatever "RON" or "octane" is? From energy principles it's all about Qlhv & from a stoichiometric AFR point of view it's all about the correct mass of air for a certain chemical composition of fuel. Neither 113 nor these E numbers seem to feature joules/gram or the number of oxygen atoms?
The key thing the RON number tells you is the detonation resistance of the fuel, so its telling you how hard you can work it without encountering problems. Like you say though, it tells you nothing about how much energy you are actually buying per litre.
Yet its very common for people to misunderstand this, look back in this thread and look at the number of people wrongly making the assumption that 95 or 98 is somehow telling you that the fuel is more or less diluted and that you'll need a bit less 98 etc and you can see how commonplace the misonception is, even among people who are quite involved with engines.
It really all is a bit "smoke and mirrors" with the way that fuel is marketted, we all know very little about what we are actually putting in our tanks!
And thats before you even consider fuel degredation and look at your tank a couple of months after its filled it up, when you are talking highly strung race engines fuel that is a couple of months old can have changed enough to now be putting your engine massively at risk, and the scarey thing is that a lot of that ageing can have happened before you even put it in your tank, while its still underground at the petrol station if its a garage that isnt heavily used and so ends up with the fuel being there for a lot longer.
Its one of those examples of where road cars really do need a conservative map to take all this into account.
