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Bugno
gbj_tester
Dérailleurs took over from hub gears in TTs for a reason.
Fixed = up to ~98.5% efficiency
Dérailleurs = up to ~97% efficiency
Hub gears (oil lubrication) = up to about ~95% efficiency in direct drive, falling by about 2% for each stage of gearing.
Some tests put the hub gears at an even worse disadvantage than that.
If you use the overdrive as your down hill gear, so you ride most of the course throwing away 5-10W compared with an identical fixed gear, you can't make up the time by missing out on the spin-out losses we suffer on fixed, as you're only doing those high speeds for a short time and you're losing some more power in the transmission which brings you back closer to those on fixed whose power has dropped off at high cadence.