https://www.quora.com/How-does-iRacing- ... an-Turismo
Sorry, I was right. GT may be heaps of fun and may very well be the most realistic simulator experience available on a console. I never disagreed with any of that. I even agreed that GT's main thing is doing high fidelity copies of real cars, as far as interior, exterior, sound, and the way they drive. These PC racing simulators don't even attempt to do that. So, where I suspected (and claimed with no evidence) it wouldn't stand up to is what is available on a PC, in terms of simulation/physics and that kind of stuff. Looks like I was right. Which is comforting.
Without getting into the specifics (because I didn't do the research), I basically summarized the general tone of this response. You shouldn't have said Gran Turismo was "the best, most realistic driving simulator available anywhere." That's what I disagreed with. You should have said "it's fun, you should try it."
Read on if you still want to argue about this.
This is a pretty broad question, and a little akin to asking how an apple compares to lettuce and spinach. iRacing's direct competitors are simulations like rFactor and NetKar. This is kind of like asking how Mario Kart compares to Forza. But here goes:
Forza and GT are easily-approachable console-based racing entertainment games. They allow for the casual person to pick up a d-pad and go racing within moments, and probably win their first race. Both games readily sacrifice realism and accuracy in order to achieve maximum immersion and enjoyment.
iRacing is a PC-based racing simulation service. Though a d-pad controller can be used, it's not recommended; a steering wheel (such as a Logitech G27, Thrustmaster T500RS, or Fanatec CSR) with separate pedals is essentially a requirement. iRacing's overriding priorities are realism and accuracy.
All three advertise their accuracy with track modeling. Both Forza and Gran Turismo visit a track, walk it with a GPS, take thousands of reference pictures, and can often finish scanning a standard track within a day, or even half of a day; they claim accuracy in all three dimensions to an inch. iRacing takes several days to use their laser-scanning equipment to measure all three dimensions to the millimeter. (Laguna Seca never never never felt right to me in Gran Turismo or Forza--one lap in iRacing and I was a convert.)
iRacing is a subscription-based model with some free content, and most available by purchase. With Forza and GT, you receive most of the content as part of the purchase, and then some extra content is available by purchase.
With iRacing, you can race only against other people. There is no competition against the computer. Your driving view is only in-cockpit (though there are essentially an unlimited number of views in replay).
But the biggest difference between the two is in the physics. If you aren't knowledgeable about vehicle dynamics or the difference between understeer and oversteer you can still easily succeed in Forza or GT, but in iRacing, you will probably find it more difficult.
If a car has a natural handling imbalance (like, say, the Skippy, and it's propensity for oversteer), you have to learn how to drive around it, or tune it out with the setup (while not adversely affecting the car over a race's distance), but the best iRacers understand why that imbalance exists and how to use it to their advantage. Forza or GT would design the car with less oversteer so that it's more easily driven by the masses.
iRacing is a realistic simulator, and is the one with the most professional racers in its ranks: I've seen Dale Earnhardt, Jr., Scott Speed, Shane van Gisbergen, Alex Gurney, and many others in races. Tony Stewart and Lewis Hamilton both used it extensively prior to their seat swap this year. There are literally thousands of amateur racers on iRacing that use it as a training tool.
Grand-Am, IndyCar, and V8 Supercars consider iRacing their official series. NASCAR sanctions it: yes, the NASCAR iRacing Drivers World Championship is an official NASCAR series. Ray Alfalla, the most recent winner, received $10,000.00, a championship ring from NASCAR (the same style ring as Tony Stewart just got for his Sprint Cup championship), and his very own press conference.
Forza and Gran Turismo are great games, but they're just that: games. Their primary purpose is entertainment for as many as possible. They're easily approachable and entertaining for a very broad audience.
iRacing has a very steep and very long learning curve (just like racing in real life), and you have to set your own goals, but it has the most satisfying and entertaining racing I've ever encountered.
This is the other one from that question:
To add to Kevin Browne's good answer, the difference in iRacing and Gran Turismo and (I have never played Forza) is the racing.
If you have ever spent time on the weekend doing track days or High-Performance Driver Education, then you will feel right at home with iRacing.
Everything about iRacing is about the racing. You can't approach it anything like an arcade style driving game.
You get penalized for going off the track, wrecking, and especially, for hitting other drivers. The auto-clutch is speed penalized. In an actual race, tensions are pretty high. You want to go fast, but you can't wreck or hit other drivers, so, it adds a check and balance that I just don't see in something like Gran Turismo.
I own Gran Turismo 5 and like it. I have amassed a fleet of megahorsepower black cars, but... I really it feels more arcade to me, and the physics are decent, but don't really compare to iRacing I feel.
They just released a controversial new Tire model, but so far I seem to like it better, especially in regards to getting a wheel in grass or driving over a gator. With the old tire model you would just spin out. That was not realistic. Now assuming you are not too out of control or sideways you can run off a little bit or attack gators and it's fine. My favorite example is T5 at Road Atlanta. In real life you drive all over that gator as you run out and charge up the hill. It works correctly now.
Mostly though it's the respect factor which I find that makes iRacing so compelling. If you are outclassed by the other drivers (and the game is international, there are A LOT of really good drivers) you will defer to them and let them pass and not wreck them out. There are a lot of fights about people who act like fools and generally they don't last too long in the iRacing world.
The iRacing mindset does a great job of approximating the real driving experience in that way.
iRacing does not seem to look as good as GT5 or Forza, but the game play is beyond gripping. You will see the sun come up on numerous occasions.