[aerohead]

A BSFC map for your engine would be a requirement for the necessary calculus.
The OEM gearing was likely, already optimized for all the transient loads the SAE/ ISO automotive engineers must take into consideration for every conceivable scenario the vehicle will be exposed to.
A 'flat' torque curve, as is more common than ever before, would provide a lot of 'wiggle room' with respect to engine rpm range which would maintain a minimum, 'best-case' BSFC island of operation.
I know of no way to simulate different gearing, without actually changing the gearing.
Since BEVs are 'immune' to BSFC-e variability, and always get 95%-97% efficiency, it's a hard sell, making expensive modifications to an ICE vehicle, which could never compete.