17 Express 2.8 Diesel

I just posted regarding excessive wallowing and had one other concern with this van. Typical weight is about 7500 lbs. The tow/haul mode is not engaged and at that weight is not even close to being needed. The 8 speed Allison transmission holds low gears for a long time before up shifting. Using the manual shift still won't make it upshift sooner. Two typical problem times are at about 30 to 35 mph and at highway speeds. The transmission will not upshift until 37 mph, so you end up running at high rpms for long periods of time and then if you accelerate to 37 mph it will shift and drop to about 1400 rpm, then you can slow down and easily maintain speed. At highway speeds the transmission will hold 7th gear (2100 rpm) unless you accelerate to 67 MPH and then will finally shift to 8 gear and rpms drop to about 1500 RPM, easily holding speed at 70 mph. I can see the transmission holding higher rpms to assist in braking in the city, though it really should upshift when you are holding steady speed. On the highway holding that low gear until you reach speeds that are illegal in most states seems rediculous. In addition, the mileage takes a significant hit... the dash reports about 27 MPG at a steady 60 MPH, but jumps to 30.3 at 70 MPH. There is plenty of power to keep the vehicle moving when the tranny finally upshifts ( the engine is already making 100 HP at 1500 rpm according to the GM dyno charts).