Erring on the side of caution, the first batch of braking tests was done from 40 mph on a two-lane rural hill. With the Tow/Haul and exhaust brake off, we were moving up past 50 mph in just 10 seconds. With Tow/Haul engaged and the exhaust brake off, the truck still sped up, but only to 46 mph in the same 10 seconds. In Tow/Haul with the Pacbrake on, our speed dropped another couple of mph and stabilized within the 10 seconds.
We then tried a longer, straighter, 7 percent grade from 60 mph with a posted 15-mph exit ramp two miles down the hill. From 60 mph without Tow/Haul, the truck easily rolled to 65 mph, where we service-braked it to 55 mph and repeated this twice over two miles. After making the exit, we pulled off and measured a front brake rotor temperature of approximately 320 degrees.
In Tow/Haul mode without the brake on, the Allison's grade-braking feature was activated with a tap of the brakes, and it downshifted to 3,600 rpm and eventually slowed to 54 mph and maintained that speed. This time, the front rotor temp was 195 degrees.
Finally, we tried the hill in Tow/Haul with the Pacbrake activated. This slowed the truck to about 55 mph and held it, even without downshifting, and the tachometer registered around 2,400 rpm. It felt like we could make the exit without braking at all, and front rotor temperature was measured at an even lower 180 degrees. Manual downshifts would slow it more, as would a tap of the brake pedal to trigger the grade-braking logic.