It really matters on time and location. I've been going on two way lanes in the night where the average speed on the left lane was 180km/h and on the right lane

It really matters on time and location. I've been going on two way lanes in the night where the average speed on the left lane was 180km/h and on the right lane probably 120km/h. This is probably the most secure and comfortable situation for speeding. One thing which bothers me when people talk about going fast with their cars is that it really depends on your car. I've seen high-end cars with 6 gears where the engine is absolutely relaxed when going 150km/h, especially with a Diesel. One popular example are Mercedes Sprinter, who really need very few fuel even for large speeds. Other cars (not neccessarily smaller, but for instance with petrol engine and also 6 instead of 5 gears) are already quite expensive in fuel need at this speed.