I installed these the wrong way, but truthfully wouldn’t know the difference. Been on all sorts of gravel, road, single track with these bad boys. I guess I’ll find out when I need to replace sealant, but it was such a pain to install, there was no turning back.
That was one advantage of rim brakes over disks. With a rim braked front wheel you could of just rotated it 
The real reason is that there’s an overlapping joint in the casing. You want it to get tighter, not looser. It doesn’t really make a difference though. The direction of the tread is pretty meaningless - the tire presses down and comes up, it’d need to be slipping to be pumping anything.
Someone mentioned kart/racecar tires, these tend to get a set shape to them as you heat can cool them repeatedly. Anything you’re doing this to won’t be making competitive traction past 5-10 heat cycles, so those would be practice tires you’re trying to extend the life of.
"The real reason is that there’s an overlapping joint in the casing. You want it to get tighter, not looser. " - do you have a source for that as it relates to bike tires? I can see a similar issue with that type of slip being an issue with the direction that one installs tubeless tape on a rim for example, but in a tire it all seems locked in by the actual rubber compound that is coating everything and calendered in.
FFS I literally posted on this very thread explaining which way round these go earlier this week 
I’ll swap it round when it’s stretched a bit - they’re a right pain fitting when they’re new.
Don’t bother. It’s fine.
Until I get to a coffee shop 
Learned from my dad a long time ago: Tractor tires have the chevrons pointing forward on the top, so that they are pointing backward where they contact the ground. If the wheel spins in mud, it pushes the mud to the side so that it can contact firmer ground. I’ve always thought that MTB tires use the same logic, but if my MTB wheel spins, I usually need to unclip. Also note that the front tire is used for braking, so you may want it reversed (but I don’t). I can’t say anything about road bike tires.
I just changed my 4 year old GP5000 with new ones today and notice that I had both mounted with the wrong direction 
can’t wait to test drive them. I’m hoping for atleast 10% speed increase 
Josh Poertner of Silca spoke about this on a show a little while back. He said that back in the fairly early days when he was with Zipp and experimenting with wind tunnel analysis, they found the Continental tires had some never-seen-before aero “magic” going on. He said they came to realize it was because of the tread pattern. So, they reached out to Continental for some insight. Apparently Continental basically didn’t know what they on about and said that any aero benefit from their tread pattern was purely by accident. This was before the GP5000, so maybe the GP4000.
Sorta like that, Trek was doing testing on their road tires and accidentally discovered some watt level differences in rolling resistance for the same tire run both directions. Turns out the casing they used had better performance in one direction. That’s why they added markings on their road tires even thought the tread pattern (or pure slick) makes no difference itself.
I used to orient my tires by putting the label on the chainring side with the label centered on the valve stem.
I found those arrows on the Grand Prix by accident after riding one in the wrong direction. They weren’t on the GP3000 and they weren’t on the first batches of GP4000s. Then they showed up.
