The jump from Cruisers to Capitals in fuel usage is pretty steep right now, though it's been that way for awhile. I believe the intent is keep Capitals "at home" until needed rather than flying around willy-nilly with them. After all, you don't take a modern carrier, with its massive logistical cost, just out for a spin. That said, the jump from Cruiser to Capital is disproportionately high relative to the other jumps, on average over 3x higher. Frigates and Destroyers are fine, IMO, but Cruisers feel disproportionately cheap relative to Frigates/Destroyers and Capitals seem disproportionately expensive relative to everything else.
My suggestion is to have a flat +1 Fuel/ly added to Cruisers, with a few exceptions, and have a -2 Fuel/ly deducted from Capitals, with a few exceptions. Low-tech ships already have a fuel premium on them, so these would remain.
I believe the Falcon and Apogee should retain their 3 fuel/ly rating as one is a light cruiser and the other is an exploration ship likely geared for efficiency. For the Capitals, the two battlecruisers (Conquest and Odyssey) would go down to 7 fuel/ly while the average capital is reduced to 8 fuel/ly and the two fuel hogs (Onslaught and Legion) would go down to 12 fuel/ly (remaining 50% less fuel efficient compared to other capitals). This makes the "average" cruiser twice that of a destroyer and the "average" capital twice that of a cruiser. Because of a few low-tech fuel hogs, the curve for fuel usage would be much smoother, rather than the extreme jump on the high end.
The new curve would look something like (current fuel costs in parenthesis):
Mercury/Hermes: 0.5 (0.5)
Most Frigates: 1 (1)
Most Destroyers: 2 (2)
Enforcer: 3 (3)
Falcon/Apogee: 3 (3)
Most Cruisers: 4 (3)
Mora: 5 (4)
Dominator: 6 (5)
Battlecruisers: 7 (10)
Most Capitals: 8 (10)
Tug: 10 (10)
Onslaught/Legion: 12 (15)
Overall, I would estimate that fuel usage would go up slightly in the mid-game, due to cruisers costing more, but even out by end-game. Carrying a capital around with you would sting a little less but you still wouldn't want to go on a survey mission with one. Of course, capitals having such high fuel costs may be an intentional money sink in the end-game, which there are too few of.