Battery saving optimal speed for a quadcopter

I don’t have a working telemetry and I’m not sure that it is possible to monitor current on this FC: Eachine MatekF405 clone (is it?)…

I was able to fly over 120km/h with my drone but I doubt that this is optimal speed for longest range. So I would like to hear from your experience what approx. optimal speed should such a drone have… Doesn’t have to be precise… just to get a feeling… Is it more in 30-40km/h or 80-100km/h range?.. Some technical details: 280mm, 1kg, motors: 2507 1800KV, 4s, props: T7056C.