If you are driving 75 mph and getting 3.3 miles per kWh for 200 miles. In 2.66 hours you would use 60,006 Wh for the 200 miles. Running a seat heater is about 40 watts. 2 seat heaters for 2.66 hours is about 213 Wh. Total use goes from 60,006 to 60,219. You have reduced your range by 0.7 miles...