Very strange problem with my Sportrak

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I`ve had my Sportrak about 6 months now and in that time haven`t used it in 4wd much but when I have i`ve noticed a strange problem, today I have been able to narrow it down a bit. Whenever the manual wheel hubs are set to lock and the car is moving the fuel gauge goes down, when I come to a stop the fuel gauge goes back up again. I don`t think it matters if it`s in 2wd or 4wd (i`ll double check this tomorrow) it`s still the same.
Any ideas?

I think I might try just one wheel hub locked see if that makes a difference.

Bit of an update, locked

Bit of an update, locked passenger side hub and left drivers side free, set off to work and fuel gauge went down slower and stopped at around 1/4 full. Locked driver side and free`d passenger side and same thing. Locked both and gauge went down quicker and went all the way to empty.
Tried it in 4wd and it went down to empty even quicker.
Got to work by then, remembered I wanted to check something on front suspension so went to start engine to turn wheels and battery was dead, could locking hubs and using 4wd drain the battery and would this cause the fuel gauge to stop working?
I`m finding this very baffling, anyhow thanks for looking.
GPick