I used Joe Friel's 30 min LTHR test about 10 years ago. It's pretty solid for my LTHR, based on observations while I'm riding, and hasn't really changed (I think it went up 1 bpm (and some days maybe 2 bpm) in '24 when I made a big fitness jump from riding a lot of z2, or maybe my fitness has me handling lactate clearance better, but I left my zones alone and keep that in the back of my mind).

When I started using power, I realized that Friel's HR zones (particularly the top of z2) wasn't as well aligned with my power zones (Coggan), so I switched my HR zones to the Coggan zones, and it's pretty consistent (I spend most time on mtb, with no power meter, so it's important for me to be consistent across bikes). That being said, I spend a lot of time riding in high and low z3, and I can talk fine way up into z3, so maybe that says Friel's zones are more accurate for me, but if so, I'm ok with being a little low rather than high for top of z2.