Forza Motorsport 5

In order to unlock this achievement you are required to Drive 200 miles on Nurburgring in the 1976 Ferrari #1 Scuderia, or the 1976 McLaren #11 M23. This can be done on any of the 3 tracks as it is cumulative. It will take you roughly 13 laps on Nordschleife + GP, 16 laps on Nordschleife, or 63 laps on the GP Circuit. This can be done while doing rivals, racing in free play, or career as it is cumulative.

I hope this solution helped! Leave a comment letting me know if there is any room for improvement! Thanks.

if you like me already completed all the other Nürburgring achievements I suggest you go to Freeplay and load up Nürburgring Nordschleife (not the GP circuit) with 0 AI drivers.
As soon as the race starts, do a 180 and start driving on the long straight. When you're at the end of the long straight, do another 180 and drive back. Rinse and repeat and once in a while quit/finish the race so you can check your achievement progress.

Updated:
This could also work for the achievement "The German Connection" if you still got miles left there as well, provided that you change to a german car to go after that specific Achievement. :) (You can't use the Ferrari or McLaren for that achievement as those aren't German cars)