Rally Wrc Monte Carlo 2017

[denkimi]

Actually you can. Normally You don't run longer gearbox with weaker engine (unless it's completely different design, i.e. has completely different characteristics). See R5, S2000 or even gr.N where cars with stronger engines have also higher top speed.

In case of 2017 I would say that the main unknown factor is drag force from the aero package.

you run gearbox ratios based on the event, not based on the engine power. that's why they use different gearboxes throughout the season.

there's no point in having a gearbox that makes your car go 220km/h, if you know you will never exceed 190 on the event. and vice versa, if you know you will be at very high speeds often, there's no point in being on the limiter all the time.

and as you said, without knowing the drag coefficient there's absolutely no way you can determine the difference in engine power based on top speed.

only partially related, but still interesting:
https://www.youtube.com/watch?v=Yktl2-lVm_s
he has more speed gun measurements on his youtube channel.