That's a weird thing about GT7.
It seems like what they do is scale up both the actual fuel tank capacity of a car, and the depletion rate, so that the player gets a nice consistent round 100L to work with while maintaining an accurate rate of depletion for the real tank size.
e.g. Car has 80L real-world fuel tank:
80L tank = 1.0x depletion rate.
Scale it up.
100L tank = 1.25x depletion rate.
For a car that already has a 100L real-world fuel tank:
100L tank = 1.0x depletion rate.
No need to scale it up.
If the depletion rate is tied to the engine, this could explain why race car engine swaps improve road car fuel economy, as the depletion rate didn't have to be scaled up to meet the 100L tank size.
It's the equivalent of doing a fuel tank swap alongside the engine swap.
Do I have evidence that this is how it works? Of course not. This is just a theory trying to rationalise why things might function the way they do.
I'm also aware that there are other factors that go into overall fuel depletion, such as engine displacement & revs.
True, even if road cars could have similar fuel depletion to race cars, it doesn't mean Polyphony have to use fuel depletion for those events.
All that the road/race car depletion differences do is add one more reason why they wouldn't use it.
Though there doesn't seem to be any events featuring race cars (besides Gr.B) that don't use fuel & tyre depletion in the World Circuits part of career mode, making the Weekly Challenge special events an anomaly of sorts.
Even if my [admittedly] mad rambling about a Racing Fuel Tank isn't a solution to the road car vs race car fuel economy topic, I do still think there should be something that we can do to achieve a similar effect, that isn't an engine swap.
I know ECU's deal with fuel/air mixtures and economy so maybe that's one potential alternative, but I'm open to suggestions and insights on the matter.
