I'm a computer science major at UCSB. I can tell you that GT3 internally represents numbers for speed and mileage as 32-bit signed integers. This means that there are a total possible (2^32) / 2 combinations of speed possible. Since a speed of zero counts as a combination, the maximum speed is 2147483647. When the game tries to increment this number, it gets and overflow error and crashes. Since mileage is constantly increasing at a very fast rate (at high speeds), the mileage and mph should reach a similiar limit.