GRID 2

Just keep driving and this should pop up after about 10-15 hours play. It's cumulative across online and career play as well*.

*According to ScuzzyBunny you should pop this during Season 4 of career mode so there's really no need to grind it.

However, should you feel the need to grind it - assuming you have access to it (pre-order/early DLC) - you can boost this in online/offline play by setting up a custom event on the Indianapolis Speedway (Oval Circuit) using Tier Four or Indy cars - based on their speed you could do it in about four hours.

Alright so both of these guides are not even close to being right. If you're on a harder difficulty and have to repeat races maybe, but if you're following the walkthrough for this game or are just smart and playing it on the easiest difficulty, then you should only be having to race each race in career mode once.

After I finished career mode, without losing a race and racing each race only once, I finished with about 1400 km, 210 km short of the achievement requirements. Now if you were to complete the achievements that you can obtain via custom races, (and you don't end up quitting out after you obtain the achievement such as me) That would add about another 50 ish km, STILL leaving you well short of the required amount for the achievement.

My advice would be to save this achievement for last, as you are very likely to unlock it while earning the multiplayer achievements. And if you are still somehow short, just grind them out in custom races.

There is no fast way of doing this, nor does there need to be. If you waste your time grinding it out for no reason that's your choice, but this should be achieved without having to go out of your way.

I got this early on in season 5 and can confirm it includes online and offline mileage.

I got it with about 916 miles done in career mode.