My guesses, from most likely to least likely:
I can assure you 2,3,4 have all been ruled out by me multiple times
have you measured you top speed independently, aka GPS? same road conditions? Have you measured the speed of the old setup independently with GPS? Maybe you had the wrong settings before, who knows.
Did you take apart a skate that worked?
Sounds like motor Kv differences. Motor Kv is typically +/-10% from the labeling due to manufacturing variances.
This was my best guess. Do you know how to check motor kv via vesc tool or variESC tool? Seems like a good first step for the man to try out.
An example:
Typing “kv” in the vesc tool console while full throttling may or may not do the calculation for you
Sounds like that top speed drop roughly equates to the difference between a 190kv and 170kv motor. So, based on that, I would most likely go with what everyone else is saying. Definitely ghosts. 
Calcs to ~163kv assuming 7 pole pairs and my quick math isnt trash and based on 49k erpm
Well that might be my mystery solved, they state 190kv
I will hook my 6355 up later and do the same thing to see where they hit.
How did you determine that from what #'s were given?
49000 erpm / 7 pole pairs = 7k rpm
7000rpm / 45v = ~156
156 / 0.95 to account for duty cycle cap gets you to ~163kv
Bookmarked and screen shot taken, thank you very much! That’s pretty far off 190, is that normal?
Definitely not, that would be fine for a motor sold as 170kv, not for 190kv.
I mean… It’s not that far from normal… Assuming the numbers we used / calced are correct. A ‘typical’ tolerance might be +/- 10% which puts a 190kv motor in the 170-210kv range… Given that i only used erpm to the nearest 1k and voltage as measured by vesc (which can be dodgy), our estimated kv could also be +/- call it 5%…which puts us pretty well within a normal tolerance
I’ll be curious to see what the others hit at, looks like they might be closer to the 190 mark given how they preform on the top end.
So here is the same gear box, same stated kV, same everything except the smaller motors 6355’s & my battery is fully charged instead of half way. Big difference, also unloaded.
So the 6355’s are 180kv at a full charge and the 6380’s are 147 also if I did the same math at full charge. My mystery has been solved. @Venom121212 and everyone else who chimed in, thanks!!
Do you notice a decent torque difference between setups?
Since you noticed the speed drop, hopefully you notice more torque as well.
That’s the part that got me, I noticed no tourque difference even when it came to the biggest hill I climb around here. Maybe it’s in my head but I think the smaller ones were the same as the larger ones.
Maybe the 6380 that I have just suck, IDK
