What is the most fuel efficient freeway speed at which to drive a gen 3 Prius? Is it 55 mph or can it be driven faster say 60 or 65 and still get optimal fuel efficiency?
You asked optimal, so Bisco's answer is correct with any car, including the Prius. Fuel economy is still excellent at 65. Just not optimal
For our usual run into town we have a choice of freeway, limit is 90 kmh, or secondary highway with 80 kmh limit (around 50 mph). Haven't been on the freeway in years: get in the right lane on the slower highway, set the cruse at 78~80. If the limit was 70 would do better still. Like the graphs above attest: there's pretty much a direct relation between speed and mpg, all the way down.
The most efficient cruising speed is the lowest speed you can otherwise accept, down to a limit not shown on the chart Jimbo posted. That chart used the U.S.-market dashboard display which is capped at 99.9 mpg, even when the computer holds higher numbers internally. Other threads have found that limit, through both purely empirical and through some semi-analytical methods, to be very approximately 15 mph, when all accessory loads are turned off. Accessory loads both lower the curve, and shift the peak to the right, i.e. higher speeds. It is all a tradeoff. The Prius can push the effectiveness of this tradeoff down to far lower speeds than can traditional drivetrains.
My one tank, stunt a couple of years ago: At one time, there was a Japanese 1000 mile tank club and they did even better. BTW, these are GPS confirmed numbers. I drove almost totally on cruise control which limits the low speed to above the 23 mph cut-off speed. Bob Wilson
I agree with fuzzy1 that it's all a tradeoff. The slowest speed that I am willing to endure is cruise control at 60MPH and I got 60MPG (computer), which I thought was fantastic. For my purposes, I consider that to be optimum. Driving 55MPH is too slow for me. And by the way, try driving 55MPH on a Los Angeles Freeway at night (which is about the only time you can drive at any consistent speed). You will get run over by some little old lady from Pasadena. Mike
OP received excellent answers. I have looked at the graphs posted above many times and I still am surprised how almost linear they look in parts despite our well known rule that air friction increases exponentially as speed increases linearly. The underlying reason is that tyre and road friction remain significant contributors to overall fuel economy until traveling something over 70 mph. So one answer to OP might be: if your roads are crappy, you might as well go faster The less tongue-in-cheek answer is: go check the tyre pressures of your car.
The simplest answer is driving the posted speed limit on the highway, not above it. However in rush hour traffic, I prefer to drive 5 mph over the speed limit in the slow lane. Otherwise, I've found it generates a dangerous amount of heavy tailgating. If you're talking about cruising at a constant speed on a flat surface, the optimal speed for the Prius gearing is roughly 45 mph (72 km/h), which is too slow for the highway. If you're varying your speed, such as with pulse-and-glide, the optimal speed is even lower.
