Based on my observations, when the ICE is providing between 20 to 35 kW of power, efficiency is around 35%. Of the 33.705 kWh of energy released from the combustion of a gallon of gas, 35% of this energy is converted to mechanical energy used to power the car and charge the HVB. When the power provided by the ICE is less than 20 kW, efficiency starts to fall. For example, when providing 10 kW of power, the efficiency might drop to 25%. (It requires about 20 kW of power to go 65 mph and about 10 kW of power to go 50 mph.) The efficiency the motor/generator in generating electricity is probably around 90%, i.e. 90% of the mechanical energy provided to the motor/generator is converted to electrical energy. The efficiency of the motor/generator in providing mechanical power is about 80%, i.e. 80% of the electrical energy provided to the motor/generator is converted to mechanical energy. This means about 90%*80% = 72% of the mechanical power provided by the ICE to charge the HVB is actually recovered in EV mode. There is little advantage in having the ICE charge the HVB when it is providing 20 kW of power (around 65 mph) or more to propel the car. You are only going to recover 72% of the energy produced by the ICE when in EV mode. So 35% of the energy released from the combustion of gas is converted to mechanical power to power the motor/generator, and then 72% of the resulting electrical energy is converted back to mechanical power to propel the car later in EV mode. The overall utilization efficiency of this portion of the gas that was used to charge the HVB and later power the car later in EV mode is then 35%*72% = 25%. If you now run the car in EV mode at 65 mph using the energy from this gas, you are now effectively only getting 25% efficiency from the combustion of that gas and have lowered the overall mileage (mpg) of the car. If you had not used the gas to charge the HVB, and instead used it to propel the car, you would be getting 35% efficiency. Charging the HVB and running later in EV mode reduces mileage (mpg) at 65 mph. However, if you could utilize the energy stored in the HVB at a later time when the power required drops below 10 kW and efficiency of the ICE drops below 25%, then it is advantageous to charge the HVB at 65 mph. With the gas used to charge the HVB, you are effectively getting 25% efficiency. If you had instead used the gas to power the ICE and propel the car (without charging the HVB), you would have gotten less than 25% efficiency since ICE efficiency has dropped below 25% at 10 kW of power. This can happen during downgrades on the freeway. So you might want to charge the HVB while driving 65+ mph so you can power the car later in EV mode while driving on a downgrade. (There are additional considerations, such as having the HVB assist the ICE to reduce the load and drive it into a more efficient operating region.) Better yet, is to charge the HVB when the power required to propel the car is less than 20 kW. The additional power required to charge the HVB increases the overall ICE efficiency from maybe 25% to 35%--the ICE is providing 20 kW of power rather than 10 kW of power. So now you are getting 35% efficiency from the gas that is used to propel the car (rather than 25%). In addition, you are going to get at least 25% efficiency from the portion of the gas that was used to charge the HVB. So charging the HVB and running later in EV mode increases mileage (mpg) at slower speeds (around 50 mph or less). The goal is to achieve maximum efficiency for all gas consumed. Don't run in EV mode when high power is required (high speeds or up hill), and don't charge the HVB. Don't run the ICE when low power is required (at low speeds or down hill), unless you can also charge the HVB to increase the load on the ICE and can run later in EV mode at low speeds. There are additional considerations when deciding to charge the HVB. The HVB of the Fusion Energi is composed of 84 cells (less for the Fusion Hybrid). The cells need to all be kept balanced, i.e. maintained at the same SOC and voltage. Using power from the HVB tends to disrupt this balance--the greater the power drawn from the HVB and the lower the SOC, the greater the disruption. Charging the HVB restores the balance. The weaker cells experience greater stress than the stronger cells. If the cells are not rebalanced, the weaker cells will continue to experience more stress until they fail. See the following and subsequent posts: "http://www.fordfusionenergiforum.com/topic/1683-obd-ii-data-for-hvb/?p=14915"