This was from a trip to Baltimore and back, 217.3 miles total. I was driving cruise control (CC) which, in the prius, is very efficient since transmission is variable and cruise control is optimized. I found the trick was to turn off CC and lightly press the gas pedal to run electric motors only whenever the car went into regenerative braking (i.e. going down a hill steep enough that the car would pick up speed if in neutral). Watching the energy monitor I could keep the car running on electric motors only with a light press of the gas pedal. Then I set the car back into CC when the speed began to drop. I figure at $3.60 per gallon the trip cost me about $12!
This is kind of like pulse and glide, except I left the “pulse” part to CC, and I “glided” only when going down hills. The whole trip I set CC around 55mph. The "to" part of the trip was even better going 109 miles at 69.4mpg!
I do a similar thing sometimes using cruise control but I use the pedal to keep the HSI bar near the middle during downhills (I'm actually looking for the iMPG to be between 75 and 100). Here is what I achieved one time using this technique. I had a small tailwind to help me as well.
In CC - when going downhill if the Prius goes over the set speed by the CC - the regenerative brakes are used to slow down the Prius but the regenerative brakes only capture about 20% to 30% of the energy so its not as efficient as turning off the CC going downhill and allowing the Prius speed to increase. As the Prius goes downhill, the friction/resistance against the wheel is much less than when the road is flat or going uphill - so it takes less energy to turn the tires/wheel and thus it is more fuel efficient to accelerate the ICE while going downhill. As long as the Prius isn't going too fast for too long aerodynamic drag loss(<60mph) will not be as bad as the regenerative braking loss. Near the middle of the HSI display to slightly left of the eco pill the ICE (when it is on ) is burning from .55 to 1.1 gallon s per hour at a very low RPM of 992rpm to 1300 Rpm - at that speed the % power coming from electric motor assist is significantly higher than when the ICE is at a higher RPM. Hence the Prius tends to burn less fuel at those low ICE speeds - unfortunately at those low ICE speeds the HP output is low - so unless the Prius has a running start (momentum of current speeed) or it has a gravitational advantage (going down a steep downhill ) the Prius will not be going very fast.
