Gran Turismo 5 is realistic. How can you tell? Well, a tiny little Fiat 500 has trouble getting up a mountain in the real world, so it makes sense it can't get up a mountain in GT5, either.
Note we're not talking about the rather shiny new Fiat 500. That's fine. This is the original, which could produce a whopping... 13 brake horsepower. Which is not much more than a lawnmower.
If you're lazy, the race starts around 2:00 in, with 4:00 the spot you'll really start willing the little guy up the mountain.