Hi, I am coming with a new question concerning the guel flow indication I have. I see numbers or indication if talking about an analogic instrument and I am not sure what is this indication. Is it litres, gallons and is it per minute, hour? Any help?

...But yet a 737-400 with a total 5400 gallons (in the fuel settings) with its 2 engines burning 20 (as indicated) gallons per hour, would need 5400/40 = 135 hours to empty its tanks which definitely not the case unless the 5400 in the fuel settings is something else like 54.00 or the indication of 20 in the FF gauge is 20*100 or something. Also the indication of 20 is one of the biggest numbers I have seen in the gauge.

Usually piston aircraft are in gallons/per hour. In jets, fuel WEIGHT is the primary indication. In the 737, I believe the indication you are seeing is (pph x 1,000). IE; 3.5= 3,500 pounds per hour. Usually on most modern jets, you will have a fuel flow gage(rate going to the engines at the fuel control), a fuel counter(rate being supplied to the engines by the pumping system) which is usually expressed in gallons or litres, and a fuel totalizer(gas gauge) which uses a capacitance system to check the wing(or tail) tank, fuel totals(in pounds or kilos).