So, unfortunately, this game ended around a month ago, but the formula remains!

So, this has some prerequisites, and here they are:

Lv 15 Artificer, Armorer, and three 3rd level casters.

Boots of Flying - 65ft/perturn (Infiltrator/ Reactive Armor/ Propulsion Armor/ Longstrider) (Reactive Armor only gives a +10 to movement speed)

Cloak of the Avariel - 60ft/perturn (homebrew, Cloak of Elvenkind + Wings of Flying)

Fly Spell - 60ft/perturn

Now, a regular person is also capable of these same spells, and magic item attunement, but only an artificer can get the extra 20ft. This is also avoiding the fact that Monk and Tabaxi exist because my character is neither of those.

Total:

185f/base movement speed

185f/Base movement speed

+185f/Dash (Base Action)

+185f/Dash (Hasted Action)

+185f/Expeditious Retreat

x2 (Haste)

1480ft/perturn

+60/ Range of the Burning Storm

+200ft/Terminal Velocity (only applicable in a descending value)

+500ft/Shadow Walk

2240ft/final value = 220d6(Bludgeoning) + 1d6+5 (Piercing) + 6d8(Lightning/Enchantment)

However, there is one more thing to take into account: Mass x Velocity = Acceleration. 256w x 224v = 57,344mph

With this new calculation in mind, the actual damage dealt when falling at such an extreme acceleration, or at least, what would be dealt with a specific target, is actually 5,734d6.

Whether this is already calculated into movement and whatnot is up for debate, but these numbers are to be feared never the less.

Now, I'm here to determine whether or not the later equation is applicable, in the hope that I'm not completely breaking this formula. This was once something that resembled D&d but with the climbing scale of power, our DM was going to move into Spell Jammer. As such, I was trying to determine how much damage could be done in a single plunging attack, he was able to fade into the Plane of Shadow (as per the 3.5 rules) before impact and negate any damage dealt. But if you're curious I believe that the estimation used to determine the actual mph is used when someone was trying to do the same for regular movement of 30ft with only a slight variation of 0.4. 

Not entirely sure where you're getting mass x velocity = acceleration...

Acceleration = change in velocity over time. Momentum = mass x velocity.

at 2240ft per 6 seconds, you're doing 254.5 miles per hour - 22400ft per minute, 1344000ft per hour. The speed of sound is 767mph, so by your original calculations you're going Mach 75, which quite rightly seems a bit excessive!

As for the "fall damage", the physics will become more sketchy and the limitations of an RPG simulation will start to show. The force imparted on impact (the fall damage, or more specifically, landing damage) is derived by F=MA (Force = Mass x Acceleration). To work out acceleration, you need to work out how fast you were going and how quickly you stop. Let's assume that the impact time is 0.5s, for simplicity.

To establish the comparative "fall" damage, you need to work out how far you would have to fall to get to the same speed you're running at. The calculation for this (assuming the character is a single point of mass travelling in a vacuum) isv^2 = 2as (u, initial velocity, is 0). So velocity is square root of 2 x acceleration due to gravity (32ft per second squared) x distance fallen. Our goal for V is 2240/6, which is 373ft per second.

rearrange to determine the distance fallen (s) gets you s = (v^2)/2a, which equals a 2178ft fall.

a creature takes 1d6 bludgeoning damage for every 10ft fallen - so if you ran into a solid wall at this speed, you would take 217d6 bludgeoning damage.

But wait - there's more!

Running into another creature is a different kettle of fish entirely. If you run into someone, you knock them backwards, and that reduces the force of the impact (like bending your knees when landing). To work out the forces involved here, you need to treat this as an elastic collision (one where energy is conserved).

Momentum is always conserved - and momentum, as said above, is mass x velocity. so your momentum (mass x velocity, so 256*373 = 102948lbft/s (oh how I hate imperial...)) plus the targets momentum (0, assuming they stand still) will always add up to the same total. It's important to note that as velocity has a direction, if they run towards you, their velocity is negative and as such so it their momentum. we'll assume they are standing still, unaware of this hurtling doom approaching.

Let's assume you strike a carbon copy of yourself - the same weight. The momentum stays the same after the collision, but the mass has doubled. As such, the speed halves, to keep total momentum constant.

This means that the target has been accelerated to half your speed - 186.5ft per second. They will take half as much damage as expected, and so will you - so "only" 109d6 bludgeoning damage, and they are also flying backwards - probably to hit something and take more damage.

Now let's assume you hit a giant 9 x your mass. when you collide, your combined masses are 10 x that of just yours (9+1) and so the combined speed is 1/10th of what it was - 37.3ft per second, or 223ft per turn, so still very respectable. The giant has accelerated from 0 to 37.3ft per second in half a second, so has an acceleration of 74.6ft/s/s. You, on the other hand, have decelerated from 373ft/s to 37.3ft/s in 0.5s, so have an acceleration of 671.4ft/s/s. You'll notice these add up to 746, so we know momentum has been conserved. We also know that the total damage will be the same, but you would get 9/10ths of it and the giant only 1/10th. So you will take 196d6 bludgeoning damage and the giant will take 22d6 bludgeoning damage.

Meanwhile, the roles are reversed if you hit something 1/10th your mass - like a chicken. If you kick a chicken whilst travelling at that speed, you deal 196d6 damage to the chicken and take 22d6 damage to your foot.

This was fun - I love physics!