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Because they change ratios every run to optimize their performance ---lot of it depends where in the run they feel an improvement needs to be made--- they tweak these things to gain thousandths if not hundredths of a second--- makes a big difference between qualifying 1st or 12th - or for that fact even being in the show-- they probably have 10-15 ratios for each gear that they might use depending on what track there are racing at ---altitude(Denver) they might be down 300 horsepower so they put the biggest 1st gear in there they can get away with --sea level they put a different ratio in because the engine is making gobs of horsepower and they don't need that steep of a 1st gear