1200 km/h travel speed, pretty good

300 feet per 6 seconds = 3000 feet per minute = 180000 feet per hour = 54.864 km/h

I am not a math guy but that seems more like it.

1200 km/h would instantly sheer any surface it went near. The fastest wind speed recorded is 408km/h. Of course, this is magic, so...

My math is really bad, but isn’t actually 600 feet per 6 seconds, since you can only take the dash action. Or for a rogue 900 feet per 6 seconds. 

I know isn’t near 1200km/h but is a significant change.

so my math would be doubled or tripled. So max out at 150k/h.

Good catch on the dash action. I missed it.

I did a quick googlle search to see where you could get on Earth in the spell's duration. New York to Atlanta, Or, give or take the entire length of the U.K.. Not a bad transportation spell.

 Actually the spell states the only action you can take is dash, besides converting back from or into gaseuos form. That would make the 300 ft per turn the actual dash speed, so basically 272.72 miles that can be covered in an 8 hour time period. But that is still excellent movement speed.

Movement isnt an action in 5e

300 flying speed, dashing every turn for another 300. 600 ft per 6 seconds. 6,000 ft per minute, that's 68ish mph. 8 hours at that speed, and you're going over 500 miles.

Not a bad thing to keep in mind for a wish spell.

if an wizard cast expendious retreat on themselves before getting wind walk cast on them, can they dash both as an action and as an bonus action? becuase if so that be hella cool to see somebody go 1200 feet per round.

and what of a third caster cast haste on the wizard before they cast expendious retreat on themselves and the druid cast wind walk? I can still take the dash action, but can i take the dash action three times per turn with all these spells? and will haste double the flying speed if he did not have it before, or does the order in whom these spells are cast matter? and can i cast a spell with a range of touch on an gas cloud, as long as my hand intersects the could itself? 

please, answer soon, i wanna go fast

there is not "movement action" in 5e, but there is such a thing as an dash action that doubles your movement speed for the turn

Can travel 545.45 miles per 8 hour casting time

If you give a bunch of skeletons with this you could make a plane by having them push a metal triangle with a cockpit in it. 

i think its up to the dm but
this + bonus action dash=900feet not 1200feet (300wind walk +300 action dash+300 bonus action dash=900)
this+haste = (300*2=600 600base+600dash+600hasted dash=1800)
this + haste +bonus action dash= (300*2=600 600base+600dash+600hasted dash+600bonus action dash=2400feet in 6s ... wich would be 438.91200 kilometers per hour for everyone in the world who isnt from central america in the country USA)

thank you sir, i thinkt this should be sufficient fast for my purposes

Weird. Useful for high-speed travel, and  when that's no longer required, you can just un-prepare it, I guess, because druid.

According to the Dungeon Masters Guide in the section on Exploration subsection "Special travel pace"

Special Travel Pace

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.

A character bestride a phantom steed, soaring through the air on a carpet of flying, or riding a sailboat or a steam-powered gnomish contraption doesn’t travel at a normal rate, since the magic, engine, or wind doesn’t tire the way a creature does and the air doesn’t contain the types of obstructions found on land. When a creature is traveling with a flying speed or with a speed granted by magic, an engine, or a natural force (such as wind or a water current), translate that speed into travel rates using the following rules:

• In 1 minute, you can move a number of feet equal to your speed times 10.
• In 1 hour, you can move a number of miles equal to your speed divided by 10.
• For daily travel, multiply your hourly rate of travel by the number of hours traveled (typically 8 hours).
• For a fast pace, increase the rate of travel by one-third.
• For a slow pace, multiply the rate by two-thirds.

For example, a character under the effect of a wind walk spell gains a flying speed of 300 feet. In 1 minute, the character can move 3,000 feet at a normal pace, 4,000 feet at a fast pace, or 2,000 feet at a slow pace. The character can also cover 20, 30, or 40 miles in an hour. The spell lasts for 8 hours, allowing the character to travel 160, 240, or 320 miles in a day.

Also, one turn could be 3000ft via action surge. A level 20 PC to do all this solo could be

2 Rogue (cunning action dash)

6 Fighter (action surge + extra ASI)

12 Circle of Grassland Druid (Haste + Wind Walk)

Ultimate Royal Courier?

EDIT: If you were to RAW Rules Lawyer it, you could even stack the Tabaxi Race for their Feline Agility ability to double the speed again, for a single round 6000ft speed. 

ok i was 100% sure your math was off .... (that happens alot with people on this site)
but windwalk 300feet 
haste doubles speed to 600
haste dash 1200
rogue bonus action dash 1800
action dash 2400
action surge action dash 3000
... en the tabaxi thing .... 
im pretty sure you at that point arrive in a sonic boom killing everyone


*edit

at that point you are going 1828,8m per second ... sound is going 343m per second ... jeah you are breaking it

That's 6,000'/1828.8m in a round, which is 6 seconds.  So only 1000'/304.8m per second, a little under the speed of sound.

Negate travel completely with this spell

My math is not good but take it up with your dm