I don't think it is possible to average 7.8 miles/kWh, weather you drive on the highway or not. If you are really efficient around town, you could probably get 6 mi/kWh, but even that tough to achieve. What are the efficiency units you chose? mi/kWh, km/kWh, or kWh/100 km? If the units you are reporting are km/kWh, then 7.3 to 7.8 km/kWh makes sense and you are doing a decent job driving efficiently.