As the OP has mentioned losing significant water weight over the course of a short-ish ride, they really need to watch for signs of overheating. It can come on relatively suddenly, and it's no fun.
It just takes a couple of times of overheating your core and you'll know the signs-- I get black dots in my peripheral vision (I call 'em black sparkles,) shaky hands, and elevated heart rate that will not recover. The HR is easy to spot-- when you have to stop at a light or intersection, heart rate recovery should drop by 10+ beats (unless you're barely working.) So if you're ticking along at say 150bpm, stop for 30 seconds, and it's still 150bpm, more water, more shade, less effort.
I've never lost more than 7lbs over the course of a ride... but on that same ride I drank over 11lbs of water, so the total production of sweat would have been 17-18lbs. This was in about 4.5 hours of moving time, ~175oz of water and/or Gatorade.
I didn't weigh myself after finishing my most recent descent into near-heat exhaustion, but I drank 2.5 gallons over the course of 9.5 hours, all but 3 quarts of it straight water. But I was still sweating (heavily, I might add) so I kept going.
At least in my case, when I get to where it's ~1oz/minute intake, it's just about dialing back intensity and trying to make it home as thoughtfully as possible, because my electrolyte balance is pretty much out the window, and I'm going to feel worse the following day than I do at that moment. The last 11 miles of the ride I mentioned last, I took in two full 25oz bottles-- one water, one full-strength Gatorade.
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