This is just to document our experience, and not in any suggest towing with an vehicle rated capacity less than the load.
I just towed my son's C Dory "cruise ship 25" from Bakersfield to San Clemente with my Yukon XL (heavy duty tow package). Because I have the 3.08 rear end, the tow capacity is 6300 lbs. We didn't get a chance too weigh the boat, but suspect is close to 8,000 lbs on the galvanized trailer (which weighs 1500 lbs) The same Yukon chassis with the 3.42 gear ration is rated at 8300 lbs tow capacity. So transmission was the limiting factor. Of course we used the "tow haul" mode.
The tow was in 110 degree ambient temp, the Grapevine is the worst part of the tow, and although formally listed at 6% there are sections which are 7%. I drove watching the transmission temp, which was my major concern. I have both the dash temp digital gauge, and a Scan G, which will measure not only transmission temp, but all other OBD perimeters. The two transmission temps agree. We got up to 260 degrees for about 10 minute at the max, and over 250 for about 20 minutes. We stopped near the top, and let the engine fast idle while we checked the tires, and hub temps, with a laser thermometer. The transmission temp was already down below 230 by that time. We ran mostly in second gear at 20 mph and RPM at about 2500 (the auto transmission can be locked in a gear with manual shifting). Conventional wisdom is that there is harm to run the transmission at these temps for any length of time. [230°F = 25,000 miles (Valves Stick) 250°F = 17,000 miles (Varnish Forms)] We were probably no more than 10 miles at temps over 230. When we stopped for the day, I checked the fluid level, and it was fine, no "burnt" smell. But we will change it when we do our next oil change.
The truck handled the load well, other wise. On smaller and shorter grades the trans temp stayed below 210 degrees. On the level the Transmission temp was in the 180 range. The water temp never went over 220, and the oil pressure remained at normal levels. There were two codes on the OBD reader when we were idling the engine at the rest stop. I did not record them manually, but will be stored in the computer. No codes as we proceeded.
Brakes--the trailer (Road Runner 6500--rated to carry 8,000 lbs, total), has surge drum brakes, which appeared to be in good condition. I always allow "adequate" stopping room when freeway driving--but we were slowed down by at least 5 rear end collisions we passed along the way , in Los Angeles / Orange County traffic. We had to make one "panic stop". The rig stopped in predictable distance, without locking up the brakes. (The usual idiot changing lanes in front of you, and then slamming on his brakes...)
Conclusion--although the 1500 SUV GMC series can tow the 25, I would not recommend it, without the low ratio rear end and a upgraded, or additional high capacity transmission cooler.
My son needs to buy a truck, and has found a 2014 Tundra "1794" edition (Crew cab, short bed), which has the 10,000 lb tow capacity, with all options available. Any comments on that specific truck for towing the 25? We will put the Scan G gauge on his truck also.
