Morning, merry crimbo all
Fuel burns at your consumption rate, which is the number 2 below (0.27/h), as measured in Tons per hour:
http://guides.gamepressure.com/elitedangerous/gfx/word/447221154.jpg
Deactivating modules reduces the consumption rate (or building for lower power consumption in Outfitting initially), which reduces your fuel use.
The fat bar (number 3) is your main fuel tank, and is consumed when making FSD jumps.
The thin bar directly below Fuel (between 2 and 3 as numbered in pic above) is your reserve, and refreshes from the main tank. You consume this in normal space, and i'm pretty sure speed has no effect (or using boost). Super cruise also uses the reserve, but "speed" vs fuel consumption seems to be strictly related to your consumption rate.
Not 100% positive on normal space stuff, but pretty sure, as most Fuel Rat clients are stationary once they need us, so would require testing with identical ships for a definitive answer.
As for looping vs travelling straight, sounds like a simple race-off to test. Line up a few CMDRs at a star, half go straight and half loop, see who gets to within 7.5km of the target station first (to ignore differences in docking speed).