Specific Range (SR) is the “miles-per-gallon” performance of the aircraft and engine. SR is used to determine which altitude and power settings are most economical for a given atmospheric condition. There are two different types of SR that can be calculated—specific air range (SAR) and specific ground range (SGR).
SAR is the no-wind specific range that is calculated by dividing the TAS by fuel flow, and is expressed in NM/pound of fuel. For example, if you are flying a Canadair Regional Jet at 400 KTAS and fuel flow of 3000 PPH, the SAR is 0.13 NM/LB.
SAR = TAS / FUEL FLOW
In other words, each pound of fuel will allow the aircraft to fly .13 NM. There is really no reason why you could not use gallons instead of pounds for light aircraft, but fuel density becomes a consideration for large aircraft that carry several thousand pounds of fuel.
SGR is just like SAR, except ground speed us used in order to take wind into consideration.