FLT CREW OF B737-300 ENTER FAHRENHEIT VICE CELSIUS TEMP IN TKOF THRUST COMPUTER. MUCH LOWER THAN REQUIRED THRUST RESULTS.

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WE HAD PLANNED FOR A RWY 19R DEP GOING TO LAS. WE CALLED FOR TAXI AND WERE GIVEN RWY 19L. WE HAD AN FAA INSPECTOR ON THE JUMPSEAT. WHILE I TAXIED; THE FO CALCULATED THE TKOF DATA FOR RWY 19L. WITH THE NEW NUMBERS; HE ENTERED INTO THE FMC A REDUCTION FOR 57 DEGS WHICH THE COMPUTER PROVIDED. UNFORTUNATELY; THAT WAS SIMPLY THE FAHRENHEIT FOR 14 DEGS CELSIUS. I DIDN'T CATCH THAT ERROR UNTIL WE ADVANCED THE THROTTLES FOR TKOF TO AN N1 SETTING OF 83.9. THAT'S NOT A NORMAL THROTTLE POS OR ENG NOISE LEVEL FOR TKOF. I SUSPECTED SOMETHING WASN'T RIGHT AND PUSHED THE THROTTLES UP TO A MORE REASONABLE SETTING (IN HINDSIGHT I SHOULD HAVE PUSHED THEM TO MAX) AND THE FO CONTINUED WITH HIS TKOF. WE ROTATED AND LIFTED OF AT THE CORRECT V SPDS BUT CONSIDERABLY DOWN THE RWY. AT CLB THRUST; THE THROTTLES WERE ADVANCED TO THE APPROPRIATE CLB PWR SETTING AND WE CONTINUED WITH A NORMAL DEP TO LAS. THE 'FED' REMARKED ABOUT THE LONG TKOF ROLL. AFTER CLRING 10000 FT; WE RECHKED THE COMPUTER AND IT BECAME CLR WHAT OUR ERROR HAD BEEN. WE HAD INPUT 57 DEGS CELSIUS INTO THE FMC. WHY DOES THE COMPUTER GENERATE FAHRENHEIT NUMBERS? WE SCREWED UP; BUT WERE 'MISLED' BY THE COMPUTER. WE HAVE NO ONE TO BLAME BUT OURSELVES SINCE WE INPUT THE BAD DATA; NONETHELESS IF THAT 57 DEGS HAD NOT BEEN ON THE TKOF OUTPUT SCREEN; IT NEVER WOULD HAVE BEEN INPUT TO THE FMC.