Forza Motorsport 5

You can buy the F355 Berlinetta (currently the cheapest Ferrrari) ten times and do 1 lap in a free play race of your choice (do not have to win) and pop the achievement. Change the colour on each car to help you keep track of which ones you have already driven.

Those fortunate enough to have been playing the game during the U.S. Thanksgiving weekend will have gotten the cars at half price, but the first step is to obviously purchase the Ferraris. Here are the ten I purchased (was going for cheapest, but already had the F12berlinetta):

2012 F12berlinetta
2010 599 GTO
2009 458 Italia
2008 California
2007 430 Scuderia
2003 Challenge Stradale
1994 F355 Berlinetta
1991 512 TR
1987 F40
1984 GTO

Go to Track Select and pick the Top Gear West Circuit track.

Start Free Play
1. Set the Max Drivatars to your preference (I set to 15 - Shadow XBL confirms you need at least one other racer)
2. The the Drivatar Difficulty to New Racer
3. Set Laps to 1
4. Set Collision Mode to Always Off

Run each race, then just cycle through your cars. The achievement will pop at the end of the tenth race. You do not have to win the races.

None of these solutions worked for me on free play so I did timed races in a private multiplayer lobby.

-Go in to multiplayer from the main menu
-Press Y and create your own private lobby
-Press A and change your game options

These are the settings I used :-
Game Type - Timed Race
Environment - Any
Max Players - 1
Max AI Players - 15
Race length - 00.30 seconds

-Once you have done that Press A again and select car
-Press X to start race

I'm not sure if this is necessary but once the race started instead of just leaving the timer run out I would drive my car forward just so that it's crossed the starting line and then I would leave the timer run out. Rinse and repeat this method with 9 other Ferrari's that you have, they don't have to be different models.