DiRT

One method i used was to change the accelrate and reverse controls to the thumbstick so you can setup rubber band and get easy miles by just reversing from the starting point until it spawns again to the start line. This method proved to be quite reliable for me and useful if you want to get this achievement without playing, provided you can keep your controller from disconnecting.
You get about 1mile per minute on Windy Point hill climb in tier 1 with EVO IX using this method.
And make sure to finish the race as well.

Easily the most frustrating achievement in the game. You could race each event 2 or 3 times on your first play through career and you should be close. The easiest way to boost mileage if you need to is to race the first event in tier 10 (Pikes Peak Full) and pick the Celica GT. You can do 12miles in about 8mins without any loading. Rinse and repeat.

Right there is a 3rd way to do this, I'm not saying its better than rubber banding or Pikes Peak but it is another option for how to do it.

Career >
Tier 1 >
7th event along (Windy Point Short A) >
Rookie >
Evo IX >
Race >

Right the first half of this course is as good as straight, with only one or two basic corners that you can take at full speed. To cut a long guide short heres what you do;

Drive until you get to the second time board (about half of the stage), spin around and drive back to start. Rinse and repeat as many times as needed.

This has the advantage of knocking off the miles as quick (if not quicker) than the Pikes Peak method with no loading times and is almost as simple as the rubber band method.....so its a nice in the middle 3rd option for you to try.

I was getting 15 miles every 10 minutes doing it this way and the journey to the second time board and back is about 2.7 miles.

After you have accumulated all the miles required to unlock all the cars, you will likely be left with several hundred miles to fill, and not much desire to do so. However, by using elastic bands, and the Pike Peaks hill climb track in tier 8, you can do it while you sleep (provided you have a wired controller). It took me about 11 hours to do just over 450 miles this way, without any of my effort.

On that track, about 30% of the way up, there is a hairpin right turn, with a gate, and railing. If you crash through the gate, you will fly through the air and then respawn. Simply by holding down RT, this process will continually repeat. Make sure that you have damaged your front left wheel so that the car tends to drift slightly to the left to ensure that you do not make any progress.

You can then just leave your Xbox to do its work. If you lack a wired controller, just watch TV and fiddle with the right analogue stick every now and again.

STAFF EDIT: Unavailable video removed.

start career, go to the first race, rubber band both sticks together and cellotape the trigger down and just leave it (either go watch tv or leave it on over night). when you start, you are off the track so you should be fine.

Well, I don't have Guitar or wirelles controller, so had to try all other possible ways that i can.

I tried fall of corners at Pikes Peak and rubber band but i came over one thing - my controller switches off all the time after few mins [about 15mins] so i cant left it over a night.

Tried making circles - miles coming way to long.
And i understood, there is only one way for me and its:

Do it Legit way!
Tier 10, first race:
Pikes Peak Full [12.4mi.]
Aprox time: about 8mins+.

No loading time after 2mi. and track isn't so hard after few times.

Easy math:
12,4mi. x 10times = 124mi. in 1,30h.
Not so bad.

GL & HF!

P.S.
Dont forget to watch some good movie at the sime time.
Thats how time runs faster. :)