You'll never be able to get true 1:1 with only 270°. You will need to use wheel turn compensation, as VicaNo says...
If you set the in-game wheel lock to 270°, the game will automatically stretch the in-game car's steering to turn from lock to lock, but if you set the in-game wheel lock to the lock of the in-game car, you will have 1:1, but only 270° of it - it won't be able to reach the steering lock at all.