Do magical items that allow creatures to fly prevent exhaustion from dashing?

Question

Scenario: a PC is equipped with a magical item that allows it to fly (either a specific speed or same as walking speed, not relevant here). If the creature decides to dash, is it subjected to the dash limit in the DMG of three plus constitution modifier before having to make saving throws to prevent exhaustion?

Answer 1

There are no hidden rules

If you are using the optional chase rules, then a creature is limited to a fixed number of Dash actions before checking for exhaustion. So, if the creature takes the Dash action, it’s subject to the rule. It is not affected by the mode of movement or even how far the creature moves (you can take the Dash action and not move at all if you want).

Unless the particular magic item has a rule that overrides that, or it is the item that is moving - like a Broom or Carpet of Flying (but they can’t Dash so it’s moot anyway), then the creature is subject to exhaustion if it Dashes too much.

Answer 2

Exhaustion from Dash is an optional rule for chases

The core movement rules do not have any exhaustion for dashing. You can dash as often in a row as you like, without having to make any saving throw. So by the core rules, there is no need to prevent exhaustion, because there is no exhaustion.

The rule you refer to is an optional rule from the DMG, p. 252, for chases. If the DM uses that rule, than it does not matter what mode of movement you use, the rule says

A chase participant can freely use the Dash action a number of times equal to 3 + its Constitution modifier. Each additional Dash action it takes during the chase requires the creature to succeed on a DC 10 Constitution check at the end of its turn or gain 1 level of exhaustion.

There is no reference to any kind of movement mode - the only thing that matters is if you take the dash action, or not. So a magic item that gives you a fly speed changes nothing about having to make the save.

Observe that these Chase rules as written apply only to chases:

Strict application of the movement rules can turn a potentially exciting chase into a dull, predictable affair. Faster creatures always catch up to slower ones, while creatures with the same speed never close the distance between each other. This set of rules can make chases more exciting by introducing random elements.

So strictly speaking, even when you use this optional rule, it would not apply to limiting movement speed outside of a chase situation.

Why use this rule outside of chases?

It looks like your DM is using this rule also outside of chases, when it is a question of how fast a PC can get somewhere. Why? Ask your DM -- they have modified the rules here.

We do it too in our group. For us, we do it because this makes travel speeds a bit more consistent. Otherwise, there is a big discrepancy between overland travel speed, and the sum of tactical movement speed over the same time frame:

Fast overland travel speed for a creature is 30 miles in 8 hours (PHB, p. 181), independent of their individual speed. The DMG justifies this as follows (p. 242)

The rules on travel pace in the Player's Handbook assume that a group of travelers adopts a pace that, over time, is unaffected by the individual members' walking speeds. The difference between walking speeds can be significant during combat, but during an overland journey, the difference vanishes as travelers pause to catch their breath, the faster ones wait for the slower ones, and one traveler's quickness is matched by another traveler's endurance

However, if there is no limit on dashing, there is really no reason why you would need to "catch your breath" or need "endurance". With a speed of 30 you can dash to move 60 feet in a round, 600 feet in a minute, or 360,000 feet in an hour -- about 6.8 miles per hour. Keep it up for the 8 hours of travel, and you would move more than 54 miles in 8 hours, not 30.

But if you really cannot dash for extended periods because of the chase rule, then you cannot keep this up. You can basically travel your normal speed over this time frame (about 27 miles, a little more than the normal 24 miles in the PHB), plus a little extra from spurts of dashing you can do. The chase rule does not match up perfectly, and also do not contain anything that would allow you to walk at normal speed to recover your breath for more dashing or maybe brisk walking (as that does not make sense in a chase situation), but to us at least it is better than the huge gap you otherwise would have.