I have really hard time believing you can attain 18-19 mpg hwy or not on 35s. I'd be surprised of 15 actually. Well maybe if you drive 55 mph with tailwind.
How many miles did you drive, what conditions. How many tanks of gas? (Anything less than 2 tanks has very little meaning)
19.7 is very good for stock truck as well. Prob not real either. Yeah I've calculated something like that occasionally on fill up but it's really just a fluke. On real hwy trip something like say from Missouri to Maine and back totaling 4500 miles it averages to about 17 - 17.5.