Let's move on to the fuel issue. In C# Aurora, fighters will have considerably reduced range, while larger ships will see increased range. A 250 ton fighter can take a 100 ton drive, which, given C# Aurora's rules, will have a fuel consumption modifier of x2.23. The 5,000 ton escort, mounting a 2,000 ton drive, will have a modifier of x0.50. In my fleet, that'll be a 1,500 ton drive with power modifier of x2.0 and a fuel modifier of x1.41, or a 100 ton drive with a power modifier of x3.0 and a fuel modifier of x34.86. The fighter is only twice as fast, but has maybe one twentieth the range. With a bit of insanity and 50% engine ratios, you can actually get an escort up to fighter speeds with only a x1.55 fuel modifier. I don't see how that can be realistically countered, especially since speed is everything in missile combat. You can also scout effectively with these larger ships.
I prefer to start building a serious fleet only around mid-game, when I start expanding, so I tend to aim for around 10,000 km/s speed and 40 billion km range for the slowest ships at Magnetic Confinement Era. I don't generally have issues with fuel at that point - sorium harvesters are really good. Running the entire fleet of a hundred ships at full throttle nonstop for a year will cost me around a million tons of fuel, but at that point, my production is normally in the low hundreds of thousands of tons per year, with a multi-million ton strategic reserve, and a logistical train of tens of tankers and tugboats. I also fail to see why I'd need my ships to cover 400 billion kilometres per year?
Sorry for a late more detailed reply on this part but it sort of was bypassed by me by accident...
First I must say that this tech level are pretty much end game tech levels for me in general playing in multi-faction games... tech progress are so much slower in those games for me.
With all that said I looked at a normal 15000t multi-purpose destroyer that one faction had in one of my game at Ion Drive tech level. These ships used x1 engines for a .45 fuel efficiency. They used 750.000 liters of fuel for a combat range of 20bkm which tend to be a relatively common combat range for most large capital escorts in these types if games. Just for kicks I inserted a x1.5 engine of the same size and ended up with a fuel efficiency of 1.42 roughly what you have. I had to increase the fuel tanks from 750.000 liters to 2.75m liters to propel the ship 20bkm and forced to increase the size to slightly more than 17.000 ton. This meant that I went from using 750t of fuel to 2750t. This for a total increase in speed of roughly 38%, cost increase of 21%, size increase of 13% and fuel efficiency reduction of three times.
I probably could have used a slight power multiplier on that ship if I wanted a perfect speed versus total size of engine and fuel ration, but the same engines are used in multiple ships with different speed and range requirements so that is not always that easy to do. General fuel efficiency is also somewhat important overall as well, most of the time.
A fighter might have much lower effective max range but they ride in the hangar of a carrier with much more efficient engines so they only need range to strike the target. In my experience a range of 500-1000mkm is generally enough at the tech levels most of my games revolve around. These ranges will probably be lowered in C# Aurora as will general missile engagement ranges as well.
If you use roughly 40bkm as normal combat operational radius at roughly 1.45 fuel efficiency you need to use 32% of the ship tonnage as fuel, that is ALLOT of fuel or roughly 3.5m liters of fuel for a 10000t ship.
I might have misunderstood the ratio of fuel usage of the engines you used. But this
http://aurora2.pentarch.org/index.php?topic=9146.0 thread is good for building ships of decent range and what ratio/power multiplier to use on your ships.
I might also disagree that speed is everything in missile combat.. in my opinion it is logistics and intelligence which is what makes or breaks ANY combat.