Forza Motorsport 5

Update: The April game update makes all of the initial 200 cars that launched with Forza 5 available as rental cars so it's no longer necessary to use the "rental glitch" if you don't own the car that is required for this achievement.

Go to Free Play
Car: 1962 Ferrari GTO
Track: Sebring - Full Circuit
Rounds: 1

If you don't own the car select "Rent Car" and choose any car (the Ferrari isn't available there). Then select "Buy Car" and choose the Ferrari. Before selecting a design press "B" until you're back at the main menu of Free Play. The Ferrari will be your car now.
Finish the race to unlock the achievement.

This achievement is super easy to get and although worth 0GS, you have to get this achievement if you want to unlock another one (I forget the name but it will be worth 25GS)

Go to free play.
Click Select Car, then click on Rent car.
From there you will want to choose Ferrari 250 GTO 1962.
Click Select Track, you'll choose Sebring International Raceway. Then click the first track called Full Circuit.

It's completely up to yourself how many laps you do but it only requires one to unlock the achievement. If you want to change the number of laps, do this after you've picked your map.

Go to Game Options, Laps and then choose how many you want to do.

Very easy to get online. Just set up Private Match, Timed Race (30 seconds) on Sebring, car class can be "Any" or "C" (this is the class that the GTO is). Start the race (you can just let it sit for the 30 seconds or you can drive). Achievement will pop after the race is over.