Is a MPH "Sweet Spot" a myth?

Bob Wilson made this graph, there are certainly dips and plateaus.

 

On long commutes I find 60mph and lower provides much better results than 65mph+. It seems after 60mph the loss starts to really dive below what I find acceptable. The difference in time between 60mph and 65mph is negligible on trips under 50 miles.

 

Even on a drive with no altitude change at all the "best" mpg will vary with the wind velocity. Your "algorithm" - set the cruise control at the speed limit and wait - in my opinion is pretty much optimum as long as there are enough lanes so you can be passed without making all the other drivers cranky. Sure, you'll save a little gas if you go 50 or 55, and you'll probably survive, too. But if your considerations also include consideration of the other drivers, the value of your time, and the fact that the optimum speed varies with wind, temperature, and altitude, along with not having to watch for the traffic police, you've pretty much nailed it.

Richard

 

...I was doing <40 MPG last weekend on PA highways (up/down mountains in 15F cold air and snow).

 

Below are two graphs illustrating the effect of speed versus fuel efficiency.

 

Mathematically speaking, the MPG vs speed curve is almost certain to have a maximum. It is going to depend on a lot of details of course. Cruise is unlikely to be getting you maximum MPG on inclines and declines, as it works exactly wrong for that (constant speed rather than constant load).

 

You can see in my sig the difference between 55 and 50 mph on the wifes 2011 Prius. Those mpgs were done without any shifting into neutral. I am waiting for warmer weather and the proper trips to try it while coasting where available. My 95 neon has a nearly 3 full mpg gain in economy when coasting. Around this near flat area of Indiana my trips usually have a good 10% coasting mileage, even on longer trips.

 

Good, because you do not want to in a Prius.

 

According to Jim Palmer graph the slower the speed the better the mileage.

Also good video.

 

It is Bob Wilson's graph, I just shared it. Anecdotaly, that is true for me, the slower I go (down to 25 MPH) the better the mileage. I just dislike slow.

 

There is nothing keeping you from:

• record temperature, wind direction, and speed before each commute
• document altitude change between each direction (aka. Google Earth)
• reset trip meter after setting cruise control

You can add to our Prius knowlege base by doing the experiment and recording the raw data and sharing it with us. There is no rush, you'll have this car for a little while so record the data and share it with the community.

Bob Wilson

 

Thank Jimbopalmer for posting the spread sheet and Bob Wilson. I have been getting an average of 46mpg, but im always on the freeway @65-75mph. I will try driving streets only on a full tank and report back my stats.

 

Simply try to learn how to drive at 60-65mph on the freeway and you should be well into the 55mpg range once summer hits. Also learn how to glide to stops. If there is no one behind you on the freeway then lift off the throttle and enter a glide well before you hit the off-ramp.

 

First of all MPH is not used to determine the sweet spot on a conventional gas or diesel powered vehicle. The "Sweet Spot" of a conventional gasoline engine is based on the highest power output per volume of gasoline/fuel used. In a conventional gasoline engine which using an Otto cycle - the *Sweet Spot* is determine by narrow RPM range and the highest PSI as found on a compression cycle/vacuum gauge, e.g. 1400-1600 RPM. Hybrids like the Prius uses a Atkinson cycle gasoline engine that has an extended compression cycle that makes it more fuel efficient than a Otto Cycle at the expense of power and acceleration response. The results of an extended Atkinson compression cycle is an extended compression cycle from 990 rpm to about 1900 rpm where there is a very high power output given only a minumum fuel consumption.

For most cars, 35-55 mph is when aerodynamic drag forces overtake frictional/rolling resistance forces as the the main dampening force of a vehicle's momentum.

For most trucks, vans, and Sports Utility vehicles, 15-30 mph is when aerodynamic drag forces overtake frictional/ rolling resistance forces as the the main dampening force of a vehicle's momentum.

Listen to Bob Wilson - he's done the homework on this stuff to the nth degree.

hope this helps

Walter

 

Best hwy tip is to stay directly behind a car in front of you and not run in the open road! Instant 20% increase in mpg!