If you see the % marker as red is because the Crew is not trained for the specific Tank.

The effects of having a crew non specialized on the Tank they are handling affects in multiple ways:

• If the whole crew was driving a tank different from the one you transferred them on, the general performance of the tank will be poor; lets assume you are transferring your crew from a medium tank to a heavy tank, the driving skills of the driver will be lower and the capacity to detect enemy tanks by the commander will be lower as well, same applies to all the other roles of the crew members. (Imagine transferring the crew members of a private Jet into an Airbus 380!)

• There are other scenarios that can happen, for example, mixing crew members non-trained for the tank and others who are, in this case it will depend on the role; so for instance if the commander is trained for the tank you are using, but for instance the loader is not, you will spot enemies normally, but your re-loading performance will be poor. Take into account that the Tank Commander influences the whole crew, so a more specialized Commander will result on a better performing tank.

You can always re-train your crew from tank to tank, by right clicking your crew member, and selecting the Training tab of their profile, there you select to which tank you wish him to be re-trained; remember there are different types of training (50%-free, 75%-20000 igc,100% 200Gold) and as you might have noticed they also greatly influence the performance of the tank. Note however you can not transfer crew from Tank Destroyers into Light-Medium-Heavy Tanks, nor from SPG's to any type of tanks and vice-versa.

It is generally recommended to have your crew at least trained to 75%, if this is not possible try to train your commander and driver to 75%, they influence the general performance of the tank more than any other member.

If you want further info on crew members, their roles and effects on your tank read: Crew- World of Tanks

Premium tanks do get a credit bonus; it's typically 130% or more of what a similar non-premium tank would get (I arrived at this number by comparing premium tank income to non-premium tanks that have a 30% or more credit bonus weekend).

The main money makers are the tier 8 premiums; a good round in one of those can clear 100,000 credits (gross; typically about 60-80,000 net, depending on ammo and repair costs) on a premium account.

You can see a great chart here that shows, over the last 30 days, the net income (after accounting for repair, reammunition, and replacement of consumables used in the match) of all tanks in the game (as if they were played on a standard account -- the chart is adjusted to account for premium bonuses etc). Premium account holders will generally double this net income due to tank costs being fixed.

If you are buying a premium tank, it's usually a good idea to buy one that shares a nationality and type with tanks you are actively working on; for you, if you're working the American heavy line, I would recommend the T34. Its playstyle will be familiar, but more importantly you can swap the M6/T29/T32/M103/T110E5 crews into the T34 and use them in that tank without retraining them in order to increase the amount of crew experience they earn.

One warning: if you have not progressed beyond tier 6 in any tank, be aware that your experience may be frustrating at first, simply because you are coming up against much more capable tanks (and much more experienced players), so it may take you a couple of hundred games in the tank to start regularly having decent matches. It may be worthwhile to buy a tier 5 or 6 premium first, use that to earn credits, and then once you have reached tier 8 "naturally", buy a tier 8 premium if you need/want to.

Another good way to earn credits is to run tier 5-6 tanks (on a standard account; premium accounts can make money at higher tiers as well), especially ones that currently have an income bonus. For these tiers I would suggest the M4 Sherman (use the 105mm with HE ammunition), the PzIV (likewise), the KV-1S (whose damage potential outweighs its ammunition costs), the Hellcat, or any other tank at tier 5 or 6 whose playstyle matches your own.

Note that tank costs are usually dominated by ammunition costs, so a tank with cheap but effective ammunition will often make more money than one that does more outright damage but has more expensive ammo.

Finally, if you are aiming to maximize credit income, do not fire premium rounds unless it may be the difference between a win and a loss (wins get 150% bonuses on XP and credits). They are very useful for maximizing experience, but will destroy your net income unless used very sparingly.

Spotting damage is also very efficient (since it costs you no ammunition), so running scouts (if you're good at them) can be very effective at bringing in credits.

Edit 9/4/2014: As of 9/12/2014, premium vehicles will receive additional bonuses. All premium vehicles will receive 1.5x normal crew XP (making them even more valuable for crew training), as well as a tier-relative bonus to all XP earned in the tank (which will stack with the crew XP bonus). Lower tier premium tanks will earn a higher percentage bonus, although this will probably not be enough to make them better on a per-game basis than higher tier (and more expensive) premium tanks (simply because higher tier tanks earn more XP on average). Prior to this update, the Churchill III (premium tier 5 Russian heavy tank) was the only premium tank in the game with an XP bonus (of 1.35) -- it will keep this additional bonus after the update, so it will still be the best crew trainer at tier 5.