If you're interested in the amount of damage you'd be dealing from an impact, running at very high speeds, calculated from the fall damage, assuming earth gravity and all that jazz, then have a look on this thread I put together:

https://www.dndbeyond.com/forums/dungeons-dragons-discussion/dungeon-masters-only/97970-impact-rules-running-into-each-other-fast-maths#c1

For running at 110mph, that's 968ft. per turn. Fall damage caps at 200ft (20d6) and that is 680ft per turn, so you're at max falling damage dished out on the impact.

Assuming a medium creature hitting a medium creature, just crashing into them will deal 10d6 damage to both of you, plus your punch. As a DM, I would rule that the attack would have to hit to deal the damage, otherwise you'll skim past them and perhaps impact a wall, taking the 20d6 all to yourself!

Meanwhile, if you were to do this against a giant (huge) they would take 25% of the damage and you would take 75% of the damage, because they're that much bigger than you!

Quote from Mydianchlarus >>

I'll start this with the basic assumption that boots of speed stack with the Haste spell.

 you're also making the assumption on how they stack....it could be 30+30+30=90, or 30*2*2=120, or 30+(30*2)=90, or (30+30)*2=120. This gives you only 2 answers on four scenarios, but the more items/effects you add in, the more homebrew your math is. There's absolutely nothing that formalizes order of operations or how the math actually works.

Quote from HeathSmith >>

Quote from Mydianchlarus >>

I'll start this with the basic assumption that boots of speed stack with the Haste spell.

 you're also making the assumption on how they stack....it could be 30+30+30=90, or 30*2*2=120, or 30+(30*2)=90, or (30+30)*2=120. This gives you only 2 answers on four scenarios, but the more items/effects you add in, the more homebrew your math is. There's absolutely nothing that formalizes order of operations or how the math actually works.

I am pretty sure the math would be 30*2*2. The item and spell does not say that it adds speed; doubling means multplying by 2.

Quote from XXXGammaRay >>

I am pretty sure the math would be 30*2*2. The item and spell does not say that it adds speed; doubling means multplying by 2.

Keep going though and you'll see there's no definition of Base Speed or Adjusted Speed, you're first doubling your Base Speed, but then you're doubling your Adjusted Speed. With two items that are just doubling, its not obvious, but when people start taking things to the extreme, they do the math in their favor doing all the additions first, and THEN all the multiplying. That's homebrew math. Take something that adds 10 to your speed and another item that doubles your speed as a different example.  You can say (2*30)+10=70  or 2*(30+10)=80.  There's just no rules for it as everything talks just about 'speed'.  Once you have a ridiculous scenario with 10+ items, speed is just homebrew.

I’m not reading all of this to check if someone else mentioned any of this,, but if you go Monk 15/Barbarian 5, because of Barbarians’ Fast Movement feature, you net out 5 ft/turn faster.

I’m assuming some told you there is currently no sprinting this edition.

I don’t remember where or who, but about a year ago I saw someone with boots of speed, haste, Tabaxi Monk/Barbarian/Fighter/Rogue (Action Surge & Cunning Action), and i don’t remember what all else got one of those kitty’s up half way to Mach 1 I think. Because of the 4xdash + boots + Feline Agility, because of all the “doubling,” your movement so it was at 64x60=3,840 fr/turn= 640 fps = 436.36361176 mph = Mach 0.568723, yeah more than halfway to Mach 1. And I believe I have forgotten stuff.

Quote from HeathSmith >>

Quote from XXXGammaRay >>

I am pretty sure the math would be 30*2*2. The item and spell does not say that it adds speed; doubling means multplying by 2.

Keep going though and you'll see there's no definition of Base Speed or Adjusted Speed, you're first doubling your Base Speed, but then you're doubling your Adjusted Speed. With two items that are just doubling, its not obvious, but when people start taking things to the extreme, they do the math in their favor doing all the additions first, and THEN all the multiplying. That's homebrew math. Take something that adds 10 to your speed and another item that doubles your speed as a different example.  You can say (2*30)+10=70  or 2*(30+10)=80.  There's just no rules for it as everything talks just about 'speed'.  Once you have a ridiculous scenario with 10+ items, speed is just homebrew.

I do not see anything wrong with adding everything first and then do all the multiplying. It does not make sense for stuff that multiplies speed to pick and choose which added speed gets doubled and which does not. If I give a Boots of Speed to a monk, it makes no sense to say that it only doubles the monk's racial walking speed but not the extra walking speed from the class.

Adding everything and then multiplying everything does favor the player, but why would I even care about limiting the player in this case anyways? The player has a fun character concept revolving around speed and they have devoted a huge amount of resources into making it happen. Spellcasters can already bend reality with magic, break reality with Wish, teleport anywhere and travel the multiverse, bring people back from the dead, etc. A level 20 PC that just wants to break the sound barrier is pretty tame in comparison.