2011 Pajero Sport 3.2 Fuel gauge inaccurate
We have the regular 70L tank and an aux tank above spare wheel (about 30L). Needle stays at FULL mark until aux tank is empty. I found that the range I got for aux tank was a good 120+km more than expected. Fuel gauge needle goes up and down with ignition on and off as per normal, just give incorrect readings. If I shake car then I can hear some fuel (quite low) but needle says 50% tank.

Everything else is in vehicle is working fine. Is the first place to look inside the fuel tank at the level / float sensor or should I be checking for something electrical? Many thanks for any tips...