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High speed mileage

Mileage goes down with higher speeds. My own experience is that mileage goes down 5-10 mpg between 55 mph and 75+, though I think this may be partially attributable to throttle adjustments at higher speeds.

The mileage can be maximized by driving a little slower than traffic and keeping the cruise control on. Speeding up to pull out and pass a slower vehicle uses a considerable amount of gasoline at 75+, and this severely affects mileage. Finding a slightly slower speed that matches traffic, or results in only a few cars passing you, will allow you to keep the cruise control on and improve mileage significantly.

On a recent 125 mile trip on a relatively flat stretch of highway, I averaged approximately 40 mpg at 75+ mph, while repeatedly slowing down and speeding up to overtake slower vehicles, in moderate traffic. The return trip was at 73 mph in the late evening. Since traffic was very light, most cars passed me; but I was able to keep the cruise control on for almost the entire trip. MPG jumped to 52 mpg, and the difference in travel time was only about 10 minutes.

An additional plus, the drive was much more pleasant without the stress of constantly fighting traffic.
 

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Highway mileage

Just a few more mileage calculations. A recent trip from Phoenix to San Francisco resulted in a round trip mileage of 40 mpg @ 80 mph.

Also, the Prius goes great in the mountains. We have a couple of grades where the semi's are going about 25-30 mph, and the Prius will hold 75 mph up the grade, easily. The electric motor torque is good at holding the speed, but acceleration is a little lacking. If you try to accelerate, like from 45 to 75 going up the grade, the ICE is at full throttle and you've got to watch the rear view mirror pretty close for other cars driving up your back.

Steep mountain grades at highway speeds give drastically different results between uphill an down. Uphill mileage can drop to the high 20's for short periods, but downhill is fun. On the trip down from Flagstaff to Phoenix (about 125 miles), the Prius got 75 mpg. One vacation last year, we drove from Ruidoso to White Sands NM (over 50 miles) with the graph showing 99.9 mpg. You just can't beat the mileage, when the ICE is not running!
 

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Highway Mileage

Just made that trip from Phoenix to San Francisco, again. Took a new shortcut though the mountains above San Bernardino over to I-5 to bypass LA. Prius runs great on the mountain grades and had no trouble holding any speed. Passing acceleration on hills was slow at highway speeds, but I passed a lot of other cars that couldn't keep up. Also, average 44 mpg for round trip with most of the interstate driving at 80 mph.
 
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