A couple of weeks ago I decided to see how much fluid I'd lose over the course of an average ride in these higher temperatures. The temp was hovering around 100 when I left and I ended up losing a full 6 lbs in 2 hours/42 miles (I'm 6'4 215 lbs)
Since 1 liter of water weighs 1 kg, 6 lbs comes out to 2.72 liters of water. Add the 1 liter of water I drank along the way and I lost just under 1 U.S. gallon of fluids in two hours.....

Needless to say, I gained the weight back pretty quickly