Forza Motorsport 3

Okay to get this Achievement you'll need to complete the fifth year of that season.
The championship class that you are going to race is going to depend on how much you have achieved in level and how far you have come in terms of cars in your garage.
Based on what I have seen, most players in the fifth year can end up playing in the following categories: Class S, Class R3, Class R2 or Class R1.

Here are the details corresponding to each Championship.

Class S (700) World championship which consists of 8 races. For this class, I suggest any of these cars: Saleen 2004 S7, Cadillac 2008 #8 Remington Shaving CTS-V, Dodge 2008 Viper SRT10 ACR, Lamborghini 2008 Reventon, and of course the popular Bugatti Veyron 16.6, which is not my personal favorite.

Class R3 (800) World championship which consists of 9 races. I have tried several cars and the ones that I found more effective for this R3 800 Class was the Ferrari 2005 FXX (R3 789). This car is fast, with a good braking system and very effective on large tracks; the Panoz 2006 #81 Team LNT Esperante GTLM (R3 776) has very good handling and is ideal for small curvier tracks.
Note: You can Upgrade your Nissan Datsun F(102) to an R3 (791) which I recommend because it can reach about 232 mph.

Class R2 (900) Championship which consists of 11 races. For this class, I suggest any of these cars: Koenigsegg CCGT 2008, Ferrari F50 GT 1996, Porsche #26 AG 911 GT1-98, Nissan #23 XANAVI NISMO GT-R 2008, Chevrolet #4 Corvette Racing C6.R 2007.

Class R1 (998) Championship which consists of 13 races. For this class I suggest the following list in this particular order: Peugeot #8 Sport 908 2007, Peugeot #9 Sport Total 908, Acura #66 de Ferran Motorsport Arx-02a, Audi #8 Sport Team Joest Audi R10 TDI 2006, Porsche #7 Penske Racing RS Spyder Evo.

After you finish each championship, you have a break and one event for you to pick. If you want to move fast and just Complete your season year, I advise that you just pick a short event with only 3 laps, but if you are interested in increasing your Driver Level, which requires more XP, you should pick an event with more laps, so they can pay you more. If you have followed this long method you should reach Driver Level 50 by the end of the 5th year.

When they give you the option to pick one of the three events, you will most likely find at least one of Class A. For this matter, I will recommend that you use the Lamborghini Gallardo 2005 (A 588) or the Audi 2010 (R8 5.2 FSI quattro) Class(A 588).

Don't forget to change your difficulty to Easy, this will make a huge difference on the process.

Best of Luck.