Kazunori Yamauchi: “We had maybe 300 polygons in a car in the first game. Now we have about 500,000 polygons in each car. Back then, pieces of the car were more like symbols. Now they are real and reflect light.”


This is a huge and massive Leap! Considering the fact that uncharted 2 was 80,000 Polygons per character (Best game of the generation pending the release of GOW III, )
       Forza3 - 150k polygons on varying vehicles.
   from GT5P 300k Polygons Per Vehicles,
    But half a million polygons per cars on GT5 is more than impressive,  anything greater than the perfectionist should be attributed to the legendary Kazunori Yamauchi,
   Don't forget GT5 will have 16 cars per track compared to 8 cars on Forza 16X 500,000 =  8,000,000 Polygons for all cars, excluding trees and other effect, , This game is just using only 80% of the PS3, 


http://www.examiner.com/examiner/x-12218-Video-Game-Examiner~y2010m1d14-GT5-500k-polygons-in-a-car