A CE128Q FLYING IN ABQ; NM; RUNS LOW ON FUEL EVEN THOUGH FUEL TANK INDICATOR SHOWS MORE FUEL REMAINING THAN WHAT THE ACTUAL QUANTITY IS.

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ACFT EQUIPPED WITH STANDARD CESSNA L/R FUEL GAUGES; WORKING AND SDI/HOSKINS COMPUTERIZED FUEL MGMNT SYS. ACFT HAS LONG RANGE TANKS; 80 GALS TOTAL; 75 GALS USABLE. SDI INDICATED 66 GALS ON TKOF. TANKS WERE VISUALLY VERIFIED USING PIPETTE 'FUEL HAWK C182 LR' SHOWING ABOUT 60 GALS USABLE. CESSNA FUEL GAUGES INDICATING ABOUT 48 GALS. (PIC ALWAYS TOLD 'ACFT GAUGES ARE NOT RELIABLE.') ASSUMED FUEL ON BOARD 60 GALS; OR ABOUT 4.3 HRS AT 14 GPH. PLANNED FLT WAS ABOUT 3 HRS. 2 HRS INTO FLT; PIC MADE A DECISION TO RELY ON CESSNA FUEL GAUGES INDICTING APPROX 20 GALS AND LANDED TO VERIFY. SDI INDICATED MORE FUEL THAN CESSNA GAUGES. AFTER LNDG; PIPETTE SHOWED APPROX 18 GALS USABLE. NO SVCS AVAILABLE AT TIME OF DAY. PIC WAS ADVISED BY LCL MECH THAT A LARGER ARPT WITH 24 HR SVC WAS 20 MINS AWAY. CLOSE CALCULATIONS WITH PIPETTE; BURN RATE WOULD LEAVE MARGIN FOR 45 MINS RESERVE UPON LNDG. AFTER TRANSITION (15 MINS) FBO TOPPED OFF WITH 74 GALS. BY POH; 1 GAL WAS REMAINING. SDI ALWAYS RESETS TO 88.6 GALS REMAINING USING 'FILL UP OPTION.' LATER RESEARCH SHOWS THAT ANNUAL BATTERY REPLACEMENT IS NECESSARY. THE 13.6 GAL ERROR IS USING UP THE 'RESERVE.' THE INST WILL BE PLACARDED TO SHOW 'SET TO 75 GAL ON FILL UP.' THE PIPETTE WILL BE DISCARDED; AND SHOULD BE INVESTIGATED BY THE FAA.