Old thread by yet interesting discussion...
I don't think there is any "standard" speeds at all... at least not in the games that I have played when playing multi-faction games. Speed has been more of a strategic or tactical tool where each faction tried to get some advantage in some form, be that speed or mission tonnage.
Usually what I have seen in my games as different factions tries to outmanoeuvre each other in the area if logistics, production, technology and tactics then speed of their ships is only one factor among many. I also found that most factions had many different speeds on their ships based on their function. There are things like research to take into account, that is... multiple types of especially large engines can be very research intensive. Large engines might be effective and efficient by leave very little room for different design optimization and ship roles.
A faction with a unified speed is also allot easier to design a more optimised counter in respect to missiles design, beam weapon targeting systems and even your own ship designs. If you know that all enemy ships have a 5000km/s speed you can design some ships to be faster and some slower depending on their role, which give the opponent problem when engaging on a strategic level. Perhaps most of the scouting elements and front line ships are faster while the heavy hitter is slower but carry more weapons rather than engines, this more dynamic force set up will give you advantages that is difficult to put numbers on.
In my experience, so far, is that factions generally base their engine designs on the fuel efficiency of the drives and expected operational range of the ships. Therefore most capital ships end up in about 0.6 to 0.8 in fuel efficiency for combat ranges of about 15-30 billion km. Ships with lower operational range can have much less fuel efficient engines. This seem to be about what the logistical system can expect to support in general.
If you make your engine extremely fuel efficient you just get very large engines for no good reasons as ships really don't need hundreds of billions of operational range without the use of tankers or refuelling stations inside your own borders. It is better to offload the engine and fuel mass to the logistical side rather than the military side.
This means that the larger the ship and engines you use (or better technology) the higher the power factor usually become to keep that efficiency within roughly 0.6 to 0.8 fuel efficiency of the ships. This means either more speed or more missions tonnage and a relatively linear logistical burden on your fleet with the same mass fleet over time. You still need roughly the same amount of tankers for the fleet now as you do in the future. This also make long term planning allot easier... both in terms of tankers but also in the whole logistical chain.
The actual speed of your ships should in general be based on the strategic needs. You might have some ships flagged as a tactical manoeuvre or scout force where higher speed is more important. They need to be able to both engage and disengage more frequently than your main fleet would. The main combat ships, such as carriers, might not need that much speed. Once you commit them you should be very confident that you can bring overwhelming force or stay hidden from the enemy to retreat if necessary, they probably also will be screened by faster ships that find and fix the enemy before you commit the heavy hitters. Less speed means more weapons you can unleash at the enemy.
In my campaigns it also have never been so simple, as factions rarely have the option to have only ONE type of main combat ships in a specific point in time. Fleets will always be in a state of flux and newer ships will be mixed with older ones all the time. Even different systems on ships might be in different state of upgrade status. In such complex environments you don't have time to keep everything universal, that is neither efficient nor practical.
