Forza Motorsport 6

Make no mistake about it, the grind for this achievement is going to be boring and incredibly tedious. After playing single player exclusively from starting the game to work towards this achievement my hours played is tracked at 138 hours. This may vary between person to person but 100 hours will be the minimum completion time just for this achievement.

Now time to break it down a little.

Overall for this achievement you will have to complete a grand total of 585 races, Almost 100 of which are showcases.

The beginning of the volumes is where the game is going to feel slower but is actually quicker to complete, there are race series with either 4,5 or 6 races, all of which will have to be done in each variant of car.

Volumes

4 race series usually take between 20 and 50 minutes, factoring in loading screen times, there's only 2 of these so they should be the easiest to complete 6 times over. In total throughout the game this accounts for 48 races.

5 race series usually take anywhere between 30 and 60 minutes factoring in load times, there's also 3 of these and also shouldn't take too long. In total these account for 90 races.

6 race series are where the real grind begins to wear you down, on the earlier volumes (1-3) these series should take you anywhere between 30 and 60 minutes also, but during volume 4 and 5, these will take anywhere between 1 and 2 hours, although the cars are significantly faster here, the races are also significantly longer. In the grand scheme of things this is where more than 50% of the achievement will be spent, there are 10 of these series, with a grand total of 360 races.

Completed series will be marked with a blue tick in their corresponding volumes.

Showcases

A majority of the showcases are fairly simple and will take a matter of minutes per showcase, such showcases types are as follows

Top Gear Stig 1-1 [10]
Top Gear Showcase [11]
Track Day Shootout [7]
Passing Challenge [10]
High Speed Chase [5]
Bondurant Autocross [5]

These should approximately at most take around an hour each to complete, less for some, such as the Bondurant Autocross.

The real grind in the showcases comes once those are completed, There are also a few which will take upwards of an hour and take up to 2 or 3 hours to complete, these are

Factory Spec Racing [8]
Moments in Motorsport [9]
Race Driver Experience [13]

From playing these in the later stages of attaining the showstopper achievement it was taking roughly 15 minutes of race times plus a couple of minutes load time either side of each event.

Endurance Racing isn't as bad as some people are making it out to be, yes this will be the biggest grind during the showcases, but a majority of them will be completed fairly quickly bar 3 of the 9, the following can be all completed at in or around the 1 hour mark

Nurburgring 125
Catalunya 100
Road Atlanta 100
Spa 100
Bathurst 100
Indianapolis 250

The remaining races in the endurance take considerably longer, and is worth taking a break throughout by pausing to prevent yourself getting to bored, these are

Le Mans 250 - 2 hours
Sebring 300 - 3 hours
Daytona 300 - 3 hours

Endurance races take approximately 12-20 hours to complete purely based off of how good of a driver you are.This is approximately 10% of the time you will be spending earning this achievement

When you reach the end of this achievement you should be around the Driver level 200 mark, it doesn't really matter whether you do the career first or the showcases, during the showcases you won't have to buy the cars you use like in career mode.

Money also shouldn't be an issue in career mode for obtaining cars, quite a lot of the cars for this are either really cheap or will be earned through wheelspins, throughout the entire career I ended up buying 4 cars out of a possible 30 minimum (1 for each variant). If money is an issue the the Forza Hub is your best friend.

If there is anything anyone would like to add or ask questions about feel free to in the comments, constructive criticism is always welcome.

The way I have approached these grinds in the past Forza games has been to start from the lowest car class and work my way up from there. So, after completing the initial "introduction" of the career mode, I decided to create a list that breaks down each Volume and Showcase into tiers based on the car classes available in each. It might not be the most efficient approach, but I like it as I feel like I'm progressing from the slowest cars to the fastest without having to backtrack much to use slower cars. This is a grind regardless.

Anyway, without further ado, here is the list! I hope anyone who decides to use this approach finds it useful. I would recommend copying it into a word processor or something and marking off events as you go along.

Volumes


Showcases

Not really a solution, more an "add-on" to the best solution of this achievement:

For the very long endurance race, times given by TwistahXD can be reduce.
The best way to do fast laps is to use the mod which disables all collisions during the whole race. The "spectre" one.

I've used it during the 3 longest races and my times were:
Le Mans : 1h50min
Daytona : 2h12min (instead of 3h announced)
Sebring : 2h43min (15min saved!)