I have a 82,k-10 with a 454,t-400,373 rear gears. On 12.50x33x15 B.F. Goodrich tires.
I towed a 28' travel trailer all over the country,including the Smoky Mountains, east side of the Rockies and over the continental divide.
Got about 8 miles to the gallon average.
And,yes I towed at 70 mph.
Mike.