LJ35 FLC MISINTERP GPS DME READINGS A TURN PRIOR TO VISUAL APCH CLRNC AT LAS.

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WE WERE CLRED FOR AND FLYING THE CRESO 3 ARR. BEFORE REACHING THE BOULDER VOR; APCH STARTED VECTORING US FOR SEQUENCING. APCH KEPT DSNDING US UNTIL WE WERE AT 6100 FT AND WE WERE FLYING NE TO N; AROUND THE S SIDE OF LAS ARPT. I DID NOT PUSH THE GPS/NAV BUTTON AS I USUALLY DO; WHEN APCH BEGINS VECTORING US. I THOUGHT APCH MIGHT HAVE US RESUME THE ARR. I HAD BEEN FLYING THE ARR WITH THE GPS AND THEREFORE MY HSI WAS READING GPS DISTANCE; NOT VOR DISTANCE. WE WERE ON ABOUT A 360 DEG HDG AT OUR ASSIGNED ALT OF 6100 FT WHEN WE CALLED THE ARPT. THE CTLR CLRED US FOR A VISUAL TO RWY 25L AND TOLD US TO TURN A 7 DME FINAL. I ASKED HIM TO CLARIFY A TURN TO FINAL AT 7 MI; SINCE THEY USUALLY SAY 'NO CLOSER THAN' OR 'NO FURTHER THAN' A SPECIFIED DISTANCE; WHEN A CTLR DOES THIS. BY THIS TIME I HAD NOTICED ABOUT 9 DME ON THE HSI SO I IMMEDIATELY TURNED A BASE THAT WOULD BRING ME IN CLOSE. WE WERE BUSY RUNNING THE CHKLIST AND GETTING DOWN SINCE WE WERE ALMOST 4000 FT ABOVE THE ARPT ELEVATION. THE CTLR MUST NOT HAVE HEARD US; BECAUSE HE DID NOT REPLY AND HE HAD ALREADY TOLD US TO CONTACT TWR. AS WE APCHED FINAL; I BECAME CONFUSED WHEN I SAW THE DME WAS INCREASING NOT DECREASING. I QUICKLY CHKED THAT WE HAD LAS VOR TUNED AND WE DID. THIS DID NOT MAKE SENSE; SO I ASKED MY COPLT FOR THE ILS FREQ AS I TURNED FINAL. WE THEN REALIZED IN THE CONFUSION WE HAD NOT CONTACTED TWR. WE DID AND WERE CLRED TO LAND ON RWY 25L. ONCE ON THE GND; I REALIZED I HAD NOT SELECTED NAV SO MY DME WOULD READ OFF THE LAS VOR. MY DME WAS STILL FROM THE GPS; WHICH WAS READING OFF THE BOULDER VOR; WHICH IS THE LAST FIX ON THE ARR. AS A RESULT; I PROBABLY TURNED A 3-4 MI FINAL RATHER THAN A 7 MI FINAL; AS INSTRUCTED. THE CTLRS NEVER SAID A WORD; EXCEPT THE TWR CTLR SEEMED IRRITATED; POSSIBLY WITH OUR LATE CALL. PART OF THE PROB IS THE FACT THAT THE GPS/NAV SELECT BUTTON IS INSTALLED LOW ON THE L SIDE OF THE PANEL WHERE IT CANNOT BE SEEN WITHOUT LEANING FORWARD AND LOOKING OVER THE YOKE. AS A RESULT; IT IS VERY EASY TO MISS. WE ARE VERY CAREFUL ABOUT THIS; AND THIS IS THE FIRST TIME IT HAS HAPPENED. IN THE FUTURE I WILL GO BACK TO SELECTING NAV ANYTIME WE ARE BEING VECTORED; SO I CANNOT FORGET. MY FLYING PARTNER AND I DISCUSSED THIS THOROUGHLY. ALSO; I WAS INITIALLY CONFUSED AT HOW THE CTLR WANTED US TO DETERMINE A 7 DME FINAL. HE SIMPLY WANTED US TO ARC AROUND THE LAS VOR TO ARRIVE AT A 7 DME FINAL. I ASKED THE CTLR ABOUT THIS WHEN I QUERIED HIM; BUT I ANSWERED MY OWN QUESTION AS I QUERIED HIM. HE NEVER REPLIED ANYWAY. IF I HAD ASKED FOR THE LOC FREQ SOONER; IT WOULD HAVE PREVENTED THE CONFUSION OVER MY DME READOUT; SINCE IT AUTOMATICALLY PUTS THE SWITCH IN NAV; UNLIKE A VOR FREQ SELECTION. AS A RESULT; WE TURNED ABOUT A 4-5 MI FINAL AND CONTACTED TWR AT ABOUT 3 MI OUT. WE BOTH LEARNED FROM THIS. IT COULD HAVE BEEN VERY DANGEROUS ON AN INST APCH.