Interestingly enough, when I drive 70mph vs. 65mph on the freeway, I get better gas mileage. Most of my freeway driving is mostly flat with a few not-so-steep hills. I don't use cruise control as I did in the past. I think the additional speed is giving the vehicle more momentum so when I come to hill, I can decelerate while still keeping up with traffic and getting about 60MPG going up the hill believe it or not.

 

63 to 64 seems to yield the best results for me under those conditions. I have noticed that CC is too aggressive when the car travels up a hill.

 

Yes I agree. I was using CC in the past 500 miles. Now I feather touch the gas pedal and brought my MPG from 46 to 52.

I also noticed that fuel efficiency has a lot to do with the wind factor, outside temperature and aerodynamics. If I drive behind a large semi, about 50 yards behind even, I get better gas mileage.

My worst gas mileage is early morning, when the temperature drops. I can't wait for summer season to see how much I can get out of my Prius.

 

I expect that the best speed for high FE is below 60mph due to wind drag building to high levels above that speed.

It is for the HCH2 and probably is for most vehicles.

 

Aerodynamic drag is proportional to the square of the object's speed.

In basic terms, when speed doubles, wind resistance quadruples.

 

Traveling below 60 mph on interstates here would be very difficult to maintain without becoming a hood ornament. It would be nice if they would do a better job at enforcing the posted limits. Maybe they should bring the limit back to 55 to slow folks back down to 70 instead of 100+

 

It could be that at 70 MPH, you're less concerned about maintaining car speed - i.e. when the road turns uphill, you can maintain throttle position (and fuel consumption) and allow speed to drop reasonably until you reach the crest. At lower speeds, you might be more concerned at maintaining speed, for fear of becoming a road hazard, and consequently you increase pressure on the throttle to compensate, increasing fuel consumption.

I've noticed in my Passat that I can maintain higher MPGs at 42 mph than, say, at 37 mph, because I can only force it into fifth gear (Tiptronic) at speeds greater than 40. Sometimes I find myself accelerating more quickly just to reach 40 mph/5th gear.

 

If you're comparing steady-state 70 miles per hour to steady-state lower speeds, you're bound to get worse mileage, all things otherwise equal. (See 1stpik's post.) The Prius MPG Simulator verifies this. (It calculates the effect of speed and many other variables on MPG.)

On the other hand, your post suggested something other than steady-state. You may be (somewhat intuitively) keeping the vehicle's RPM in efficient operating ranges by allowing speed to increase on the downhills and to bleed off on uphills. Some advocate a specific technique called warp stealth on hilly highways to improve mileage at highway speeds. I have tested this on an interstate highway with gentle hills, comparing WS at an average speed of 60 MPH to allowing cruise control-maintained 60. (Results not posted .... One of these days I'll finally get the last edits completed!) With those identical average speeds, the results were nearly identical. WS might still be of some benefit on terrain with steeper hills. Hobbit (the author of the linked WS paper) certainly believes it.

I notice you just recently bought your Prius, so presumably your highway driving in it is still rather limited. You can't judge your results on such a limited experience. Many other factors affect fuel economy, including weather. Temperature and winds, in particular, can have a significant impact. A cross or head wind as little as 5 MPH, for example, can drop it by 8%.

 

But speed is not always proportional to MPG!

If that were the case, driving 10 MPH would give you better MPG than driving 20 MPH.

There are also "sweet spots" in the ICE's rpm / torque curve.
You may get higher bang for your buck if higher road speed gets you to an engine RPM that is in a "sweet spot".

Just thinking out-loud... I don't own a Prius.

The Best MPG for my Ford Escape is at 48 MPH.
The second best MPG is at 64 MPH.

It is NOT a linear function!
-John

 

Have you plotted MPG vs MPH? What is your methodology (aka., 30 minutes and display, 2 hours and tank fill-up?)

Thanks,
Bob Wilson