Tiers separate tank classes, organizing them by the characteristics of their barrel(s).
These tiers are represented in the Diepio Class Tree, shown to the right, which is a series of concentric rings containing every class in the game. It can be displayed in-game by pressing the Y key. In the direct center of the Class Tree is Tier 1, or Level 1. The first ring contains Tier 2 (Level 15) tanks, the second ring contains Tier 3 (Level 30) tanks, and the outermost ring contains Tier 4 (Level 45) tanks. Areas shaded black on the tree indicate an unavailable upgrade in that tier.
The official class tree. (Click to enlarge)
Interestingly, there is a recurring pattern of colors in the upgrade selection menu and the Class Tree. The first tank will always have a light bluebackground. The second will be green. The third will be red. The fourth will be yellow, and the fifth will be dark blue. Regardless of speculation, these colors do not seem to represent anything other than a simple order pattern to more easily discern tanks apart when quickly selecting an upgrade.
All players begin their journey as a Level 1, Tier 1 Tank, a balanced tank with no base stat buffs or nerfs.
Tank
Upon reaching Level 15, the player may choose to upgrade to one of these Tier 2 specializations:
Twin
Sniper
Machine Gun
Flank Guard
Upon reaching Level 30, the player may choose to upgrade to one of these Tier 3 specializations:
Triple Shot
Quad Tank
Twin Flank
Assassin
Overseer
Hunter
Trapper
Destroyer
Gunner
Tri-Angle
Quad Tank
Twin Flank
Auto 3
Note: This requires players to stay as a Basic Tank until at least Level 30.
Smasher
Upon reaching Level 45, the player may select one of these Tier 4 specializations. This is the final tier that most tanks are able to legitimately reach.
Triple Twin
Battleship
Octo Tank
Auto 5
Triplet
Penta Shot
Spread Shot
Overlord
Necromancer
Manager
Overtrapper
Battleship
Ranger
Stalker
Booster
Fighter
Hybrid
Annihilator
Predator
Streamliner
Tri-Trapper
Gunner Trapper
Overtrapper
Mega Trapper
Auto Trapper
Landmine
Auto Smasher
Spike
Note: This requires players to stay as a Machine Gun until Level 45.
Sprayer
Auto Gunner
Gunner Trapper
Streamliner
Auto 5
Auto Gunner
These classes existed at one point in the game, but have since been replaced or removed altogether.
Mega Smasher
X Hunter
Machine Gun II
"Mega Destroyer"
(upgrade from Machine Gun II)
Auto 4
("Auto Tank" in-game)
Sprayer
(Tier 3 upgrade)
Predator
(before merge with X Hunter)
All Tier 4 tanks are at least 126 pixels wide, and at most 175.
The old Predator and X Hunter are still able to play on mobile.
There is a running theory that the background color of the upgrade icon corresponds to the base stat debuff it gets, as it typically matches the color of the respective stat bar.
There are several issues with this:
If the background color depended on the tank's stat debuff, there wouldn't be color variants, as seen in the Overtrapper, Auto Gunner, Streamliner, and Battleship, for example.
The stat debuffs aren't necessarily true; for instance, the Battleship may have a dark blue background, but with its notoriously fast drones, it certainly doesn't get a debuff to bullet speed.
In general, the theory can easily be debunked with simple Sandbox testing.
Acquiring level 45 requires you to get 23,536 experience.
In Free For All, Team DM and Maze, this is equates to:
2354 Squares
942 Triangles
182 Pentagons
24 Green Squares
10 Green Triangles
8 Alpha Pentagons
2 Green Pentagons
1 Boss