The second issue, which is actually not quite as debilitating (but still annoying), is that the game runs on a calendar system and on each day you can only buy vehicles from four manufacturers. Every two days these four choices change to offer up new cars to pick from. So, one day you might have Honda, Toyota, Mugen and Daihatsu, whereas two days later you might be able to pick from Ford, Lamborghini, RUF and Mitsubishi. Note that you have no idea what manufacturers will be available when, so you can't say, "Oh, Ferrari will be available in two days." It could be 100 days for all you know. Also, even if a manufacturer is available on a day, you will only have access to a limited number of its vehicles at any given time. By my count, you can purchase about a dozen at a time at most, though you might also only be able to choose from one or two cars. So when Lamborghini or Ferrari does eventually show up, don't count on being able to totally stock up as your selection may be very limited.
dass ist das wichtigste bei allen Gran Turismo teilen
...blödes, zeitaufwendiges Studium 
