Sorry if this sounds completely stupid, but this just occured to me after doing some driving on GT5 and I recalled seeing some people talk about this so I'd thought I share it:
Now many have noted that some cars in GT5 are abit optimistic with their Speedometer numbers going up higher then the game's. For instance, this is noticable in the Lamborghini Aventador. Well after trying to do that out glitch at SSRX some time ago and forgetting to set the option back from Km to Miles, I did a few laps in said Lambo and thats when it hit me: This car and many others have their Speedometers modelled in Km and not the Mph setting thats default in the game. Once you change the Distance set to Km, it'll match those cars.
Again sorry if thats illrelevent, just something I thought I'd share for those that were long confused about that.