Diff swapping and percived performance

[GGray657@xxxxxxx]

   Having swapped numerous diffs(final drive ratio's, ring pinion gears) in a
variety of cars from VW to P cars ( not easy ) you do change the shift point.
But it will also effect how the car performs. Take a  E28 535 with a 3.25 diff
max speed is around 134-136, last time I checked in fourth gear, car will
probably not be able to pull to the redline because of drag/hp(this is  with a
chip). Take the same car put a 3.73 diff in and fourth gear will now pull
clean to the limiter@125mph 6,600rpm. And it will  pull in fifth. And it does
accelerate a bit faster. 
    Now take the cars to a road/ drag, course/strip and run the donkey out of
both taking timed laps/runs....swap the diffs and see which one is
faster...The 3.73 will be several seconds faster because it can put the power
to the ground more efficiently. The car will be able to accelerate out of the
corners better because of the gearing change.  Races are won by seconds
sometimes tenths of a second. On the street it will feel like a new car "like
instant HP".  
  Most Manufacturers use tall gearing to meet the MPG  laws put out by the
government. Don't meet the laws set guideline... get a BIG mother of a
fine.....Take the corvette  six speed that goes from second to fourth, last
time I checked that didn't make for real good performance...

  Been there done it a few times. Its why most racers and hot rod junkies swap
out there final drive gear sets(diffs in a BMW)...goes for Mustangs, BMW's,
Honda's, etc....

Ggray 
87 535is turbo, 3.73 diff 
88 M3 Club racer, soon to have a 4.45 gear set. 

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