I have a 2020 GMC 2500 AT4 Duramax Diesel. Fuel economy is normally 18-20 mpg local and 21-25 mpg highway. I've noticed multiple times that if I completely fill the tank up, that my instantaneous fuel economy and average drop significantly. Instantaneously runs about 8-12 mpg and my average over the last 50 miles drops down to 11-13 mpg. Then suddenly after about 25-50 miles of driving, it will jump right back up to good fuel mileage. If I never completely fill the tank, this doesn't happen. Dealer says they've never heard of this, but I don't trust dealership feedback. Previously I had reported multiple times that DEF fluid levels were completely wrong and erratic and they said there were no reports about it. Then miraculously there was a software recall that fixed it.
I have a 2020 GMC 2500 AT4 Duramax Diesel. Fuel economy is normally 18-20 mpg local and 21-25 mpg highway. I've noticed multiple times that if I completely fill the tank up, that my instantaneous fuel economy and average drop significantly. Instantaneously runs about 8-12 mpg and my average over the last 50 miles drops down to 11-13 mpg. Then suddenly after about 25-50 miles of driving, it will jump right back up to good fuel mileage. If I never completely fill the tank, this doesn't happen. Dealer says they've never heard of this, but I don't trust dealership feedback. Previously I had reported multiple times that DEF fluid levels were completely wrong and erratic and they said there were no reports about it. Then miraculously there was a software recall that fixed it.