Quote from 6thLyranGuard >>

And then the character takes 20d10 fire damage from atmospheric friction and 20d10 necrotic damage and gains six levels of exhaustion as their body fails due to having been pushed ludicrously beyond its limits.

 Actually, it would be thunder damage from the friction. You need to be going much faster to burn up.

And since you were being sped up by magic, I don't see why additional drawbacks would be expected. If you drive a car at 60 mph, you don't experience the fatigue of running that fast.

In a car it's not your body that's moving that fast.  That's not what all of this is doing, this is about supercharging your own body to make it move several orders of magnitude faster than the average car.  Aircraft have to be specially built to withstand the forces affecting them at that velocity (which includes high temperatures).  Tabaxi don't have that king of resiliency.  So while each individual effect might not be dangerous, the cumulative multiplier effect of using them all to go several times faster than was ever intended does not automatically get to be free of consequences.

Though if we really wanted to get technical, the friction would cause slashing and bludgeoning damage before anything else.

when you're late to the paladin's wedding

Alternatively, the character could spontaneously turn plaid.

You guys are forgetting falling. If you use falling you can add terminal velocity to your speed and even use this speed as a devastating attack. I speak from experience.

You can add terminal velocity to this speed if you fall 1000+ ft. Now we can convert this speed into damage.

18evels monk 2 fighter and mobile feat tabaxi 

With haste and boots of speed and boon of speed that gets you 60+30+10*2*2*2*2*2*2 (dash,dash as a ba action surge dash again, boots of speed,feline agility) giving you a grand total of 6,400ft in one round or 6 seconds equating to 727.273mph or 0.947Mach speed. Putting you almost at the speed of sound

Quote from maxwelllcarroll >>

18evels monk 2 fighter and mobile feat tabaxi 

With haste and boots of speed and boon of speed that gets you 60+30+10*2*2*2*2*2*2 (dash,dash as a ba action surge dash again, boots of speed,feline agility) giving you a grand total of 6,400ft in one round or 6 seconds equating to 727.273mph or 0.947Mach speed. Putting you almost at the speed of sound

5 levels of Totem (Elk) Barbarian and 10 Monk levels are much better than 18 Monk levels. 5th level Elk Totem Barbarian gets +10 speed by default (offsetting the loss of 10 ft from Monk) - but they also get an extra 15 when raging.

With the extra 3 levels you now have to play with - you can take 2 in Bladesinging Wizard for another 10 while in Bladesong. Having Wizard also grants access to Longstrider.

Basically there are many more optimisations you could make if you want to go pure speed.

If you add in Expeditious Retreat, where would that bring it?

Note: Nevermind, it just adds the dash action as a bonus action, which I believe was already included in the former builds.

Quote from Greighdoc >>

If you add in Expeditious Retreat, where would that bring it?

It wouldn't add anything extra. Monks can already Dash with their Bonus Action.

Expeditious Retreat is also concentration - which a raging Barbarian cannot do.

Now to ask a slightly different question, what would be a good practical speed, not silly?  You could make a case for 300 so with a dash you could appear beside a very surprised longbowman, but do you really need that?

Quote from WulfRanger >>

Now to ask a slightly different question, what would be a good practical speed, not silly?  You could make a case for 300 so with a dash you could appear beside a very surprised longbowman, but do you really need that?

Depends - how well can you use the speed?  Can you fly?  If you can fly, there's no real limit.  The faster you can go, the more useful you'll be - not only does a fly speed free you from the travel pace rules, you can do other things, like grapple an enemy, fly up, and let go.  Other factors that matter are your senses (if you have Darkvision, speed above your Darkvision range is less useful than speed that lets you catch up to your Darkvision range) and your practical attack/spell range (the lower the range of your attacks and spells, the more relatively useful speed is, because you can treat it as additional range).

what would the thunder damage be on a tabaxi monk doing this

To nearby things or to themselves?

Quote from 6thLyranGuard >>

To nearby things or to themselves?

 that's a good point, wouldn't they die before they hit max speed? they would tear themselves apart from friction before they could reach max speed. Plus, if they somehow managed to make it to max speed then they would really be dead.

Quote from plopped123 >>

 that's a good point, wouldn't they die before they hit max speed? they would tear themselves apart from friction before they could reach max speed. Plus, if they somehow managed to make it to max speed then they would really be dead.

D&D doesn't simulate those kinds of things - even if what you're suggesting were true irl (see Kotath's reply). D&D doesn't even have a rule for momentum - starting and stopping movement is basically instantaneous. You can make up a rule for it but it would be homebrew.

Quote from Kotath >>

Quote from plopped123 >>

Quote from 6thLyranGuard >>

To nearby things or to themselves?

 that's a good point, wouldn't they die before they hit max speed? they would tear themselves apart from friction before they could reach max speed. Plus, if they somehow managed to make it to max speed then they would really be dead.

 

The falling rate (which is based on physics actually) maxes out at 500' per round and does so within the first 6 seconds. Does this mean anyone who falls six seconds burns up from friction? It is not actually that fast. 

An average major league baseball pitch travels at around 140' per second, so 840' per round equivalent. Baseballs do not burst into flame. 

 Check back a page, where there are means of breaking the sound barrier for a round.

there going mach 2 almost to other things

you would take zero thunder damage cus ur outrunning the sound barrior

Quote from the_great_and_powerful_og1212 >>

there going mach 2 almost to other things

 

you would take zero thunder damage cus ur outrunning the sound barrior

 That's not how it works.  The shockwave would be propagating through your body: you're moving faster than sound but you're still going to hear the noise generated by the air in front of you as you hit it.  There's a reason that super-sonic jet aircraft cockpits are incredibly loud without massive soundproofing.  And beyond that, you're only moving this fast for one round.  After that, the sonic boom hits you and you're suddenly deaf and airborne.

Quote from 6thLyranGuard >>

Quote from the_great_and_powerful_og1212 >>

there going mach 2 almost to other things

 

you would take zero thunder damage cus ur outrunning the sound barrior

 That's not how it works.  The shockwave would be propagating through your body: you're moving faster than sound but you're still going to hear the noise generated by the air in front of you as you hit it.  There's a reason that super-sonic jet aircraft cockpits are incredibly loud without massive soundproofing.  And beyond that, you're only moving this fast for one round.  After that, the sonic boom hits you and you're suddenly deaf and airborne.

Quote from Kotath >>

Quote from 6thLyranGuard >>

Quote from the_great_and_powerful_og1212 >>

there going mach 2 almost to other things

 

you would take zero thunder damage cus ur outrunning the sound barrior

 That's not how it works.  The shockwave would be propagating through your body: you're moving faster than sound but you're still going to hear the noise generated by the air in front of you as you hit it.  There's a reason that super-sonic jet aircraft cockpits are incredibly loud without massive soundproofing.  And beyond that, you're only moving this fast for one round.  After that, the sonic boom hits you and you're suddenly deaf and airborne.

 

There are two separate effects. There is the air compression in front and the constantly refilling vacuum behind. When you stop, the latter should not really be there, since it is only caused by that forward compression.

The reality is that the faster you go, the more 'solid' the air in front of you becomes to the point where it is like you are literally running through a solid wall if fast enough. So it really would be more like blunt damage than thunder, at least in clean air. Any particulate matter in the air would cause slashing damage. 

However that assumes that kind of air compression happens at all, which is not a given in a magical world.

The person could also have a "Flash-like' personal force field that specifically handles the negative effects of travelling so fast through the air. In Star Trek there is a similar technobabble handwavery. The starships in Trek have the main deflectors for combat but also have navigational deflectors, the purpose of which is specifically to handle the fact that space is not really a true vacuum. Same idea, even if tech instead of magic.

 ah ok