[Piotrsko]

Not necessarily conflicting. It is however a case of not being able to look at all the data and compute a solution so you are resolved to only look at one aspect: a increase in efficiency giving a reduced fuel consumption, which you can measure. You can calculate drag, pretty sure it isn't measureable with the accuracy you could calculate.
Possibly a GPT analysis writing a program in a quantum computer could with a couple billion lines of code....

Your tire change has altered some sort efficiency either in the engine or in the aero flow, which may or may not be representative or duplicated in another vehicle, even the same as yours.