Forza Motorsport 6

You don't have to take a nascar to get this achievement.
You can go to free play, choose homestead miami speedway track and take an Indy Car. You will finish a round under 25 seconds.

Really easy and achievable in the first race of the expansion.
This must be done on the new Homestead track included in the Nascar expansion.
First, make your way to the first place, as this race has quite a few laps you should have a couple of clear laps without other racers ahead of you. When you have a clear track, start making some clean laps. Before you enter a corner, just release the throttle or tap the brakes to have a fluid line through the corners, no need for heavy braking. You should exit a corner with around 270/275 km/h and reach around 300km/h on the straight before you release the throttle
Most of you will get this without trying.

EDIT

As multiple comments state , it is even easier to get it than mentioned in the solution above. It seems to unlock even if you do not finish the lap under 30,5 seconds. Even a time of 31,3 seconds unlocked the achievement. So if you want the achievement very easy , go get it now before it gets patched !!