How to Modify Dynamic Running Costs for Planes (vehicles)

[hyckor]

Hello,

I am using a plane set with very strictly defined running costs. The running cost value also changes when I modify the economy speed reduction factor, so it is clearly not just the value defined by Action 0 property 0E. I assume it is calculated or modified through callbacks.
However, there must be some base multiplier or factor value that determines the final running cost, and I would like to reduce it. Does anyone know where this value can be found?
I would like to edit the GRF file directly using HxD. The GRF is the new 32bpp CZTR Plane Set.

Thanks.