I had no idea you could earn credits simply by linking your Xbox Live Account to the Forza web site. I linked it awhile back but totally missed that you earned credits.
I just redeemed 1 million credits here:
https://rewards.forzamotorsport.net/en-us/
That's a major chunk of credits for buying cars. If you've been playing Forza 5 (or had played 3 or 4 in the past) Head over to the site. Its the same place that your Forza Screenshots are stored when you take them in the game.
Animalor, you're eligible for 1 Million credits already (almost at 2 Million point level) and Fairweather, you have 1 million based on your previous Forza games if you haven't redeemed them yet from what I can tell on the friends list.
Not sure what the expire part is about so I'd probably redeem whatever points you get within the next week just in cast it does away.
edit:
Tier 3 is worth getting by the end of December for the extra 1 Million points.
December 2013
Tier 2: FM4 = 20,000 credits, FH = 20,000 credits, FM5 = 1,000,000 credits
Tier 3: FM4 = 40,000 credits, FH = 40,000 credits, FM5 = 2,000,000 credits
Tier 4: FM4 = 60,000 credits, FH = 60,000 credits, FM5 = 1,000,000 credits
Tier 5: FM4 = 80,000 credits, FH = 80,000 credits, FM5 = 1,000,000 credits
Tier 6: FM5 = 1,000,000 cred
Also compare it to the previous month's rewards confirms it's worth the effort this month in Forza 5
November 2013
Tier 2: FM4 = 10,000 credits, FH = 10,000 credits, FM5 = 10,000 credits & Ford Fiesta ST
Tier 3: FM4 = 20,000 credits, FH = 20,000 credits, FM5 = 20,000 credits & Audi RS7
Tier 4: FM4 = 30,000 credits, FH = 30,000 credits, FM5 = 40,000 credits & Ferrari F12 Berlinetta
Tier 5: FM4 = 40,000 credits, FH = 40,000 credits, FM5 = 80,000 credits & McLaren P1
Tier 6: FM5 = 160,000 credits