Due to the recent additions of WWII tanks to Tanktastic, the leveling system was changed drastically. Tanks that were once lvl14 are now in the 30's and so on. Stats for each existing tank were boosted as well. It has been confirmed that more WWII tanks will be added, but not all will be purchaseable for now. In this page, The way levels are decided (based on upgrades) will be explained. With this knowledge, more informed leveling decisions can be made.
Base level tanks usually have a hard time getting into matches at their own level. The first upgrades made to any tank should be ammo upgrades, because they do not affect level. Even without great accuracy, the damage boost is enough to give an edge over other stock tanks. After ammo upgrades, start buying the upgrades the tank needs most. Transmission for slow tanks, Gun for inaccurate tanks and so on.
Upgrades that don't affect level
All upgrades that affect stats in any way affect a tank's level. Upgrades like Smoke Granades, Radio, Ammo(the only exception), Night vision, and Thermal do not affect level. Upgrades like Defence, however, change other stats as well. It's important to consider the speed decrease from armor and gun upgrades when upgrading a slower tank (though as of 20a, only the level 1 upgrades decrease speed). Also changed in 20a, smoke granades have more upgrades to decrease cooldown time, but this doesn't change any measured stats.
Upgrades that affect level
Tank level goes up by one for each upgrade that affects level. It's important to know this, because if you have a tank used for lvl40-51 matches, and it goes to 52 after you upgrade it, It's suddenly hard to find a match. If you do find a match, your tank might have to be the lowest level tank in the lobby, and you get zero kills.
Number of Upgrades
The roman numeral in the upgrade box indicates upgrade level. Depending on the tank, It may have only one or up to four levels. The number and types of upgrades varies for each tank, and would take forever to try and list. The price goes up exponentialy, and the base cost is related to the tank's purchase price. A Pershing has few, cheap upgrades compared to the Black Eagle's numerous 1000+ coin upgrades.
Some like to level their tank to fill a specific role. This usually means ignoring some types of upgrades to take advantage of a particular strength. This way, one can have a lower level tank that has a higher specific stat than any other tank at the same level. This is best done with non-MBT tanks like Artillery and Light tanks. Currently, Experimentation is difficult. Because upgrades cannot be undone or sold, there's no set guide on how to do this, for any tank.