So if there is no way to retrieve this graph data now,
Yeah, definitely can't get that from a plugin. A plugin can pull the ratios from the current setup file as soon as you jump in the car, but then it would need to have you rolling along so it can check the clutch RPM vs speed ratio, then expand that out to max RPM and the max gear ratio to calculate the maximum speed. But by the time you let all that happen you might as well just grab a calculator and do it manually.
Like for exemple, real cars (street cars) don't have settable gear ratio and the theorical max speed could be find from the conjunction of gear setup/ratio and power (Edit:... and wheel base), so finaly, as it seems to be on the rf graph.
Well, no, that's actually a perfect example of why often what you're asking for isn't realistic. The graph just shows what the gears can do; the majority of cars never reach maximum RPM in top gear because aerodynamic drag prevents it. Street cars especially tend to be geared to run fairly low RPM at highway speeds, while F1 cars for example even before the hybrid era weren't normally set up to reach the rev limit in the highest gear - partly because maximum power is always at lower RPM, and partly because you need to allow extra room for slipstreaming.
To calculate the actual achievable top speed you'd need the gear ratios, engine curves (including ram effects, and turbo parameters if present), tyre size (including dynamic considerations), tyre rolling resistance (which is buried within the model, not a simple parameter like in rF1), aero drag parameters for the car itself and for the current aero setup, and even the level of downforce expected at 'top speed' so you can factor that in to the tyres' rolling resistance, radius, and grip.
In short, the game would need to calculate this and display/provide it.
