MPH and MPG

I think if you can get enough numbers you will see a strong correlation. Whenever I see an extreme hyper-miler post a snap of their MPG readout as proof, I notice their MPH avg tends to be in the 30s or 40s.

 

I guess that there is simply too many ways to end up with the same average speed so that even if you may find some statistical tendency, don't expect any hard rule because there's just too many variable (two identical trips with the same average mph will not necessarily lead to the same average mpg).

We'll just see some new threads saying: My Prius is a lemon I'm only getting xx mpg with my yy avg mph trip while the chart says I should get zz mpg!!!

 

MPH gives a basic "profile" of your driving.

Higher values tend to indicate lots of highway driving. Lower is suburb & city.

Time when the system is running but the vehicle is not in motion is included. That plays a role in portraying the driving conditions using MPH too.
.

 

A search of PriusChat.com for "mph" returns 7,530 hits:

Google

It will probably be more by the time you click the link.

Tom

 

Use Google search, either from inside this site or externally. The built-in search software is pretty lame. A three letter search is too small for it.

For an external Google search, open up a tab with Google, and type in this string:

mph site: priuschat.com

Drop the space between site: and priuschat.com. I only included it to keep the forum software from turning it into a smiley face. The first part is the search string, the "site:" part tells Google to restrict the search to a single domain.

Tom

 

Regardless of how you search, I still think it is a valid thread. I have seen a couple posts about immediate correlation between mph and mpg, but not about tank mph vrs tank mpg. (granted I did not click on each of the 7,530 links that google gave me )

I would be curious to know what the avg tank mph is for those getting over 50mpg.

 

My guess is that the information you seek will be like a rice cake. It looks appetizing and filling, from a distance, but once you bite into it, you'll find it is mostly air and it doesn't satiate your hunger.

*bows*

Oh, and to answer your question

I'm averaging ~ 57 mpg (calculated) and my average MPH is usually 42 mph (+/- 2 mph).

BobWilson4web has a nifty chart with MPG vs MPH data. Now that information is like a extra large, vegetarian pizza with sausage. Lots of good stuff to go around and you won't leave hungry (although some might get indigestion just looking at it....).

 

Bob's chart is great but it doesn't give any contextual info over a tank of gas. Your numbers however do, and they also support the OP's hypothesis.

For somebody who does not get above 50 mpg, they might be wondering what they are doing wrong or if they have a lemon. In reality it could be that the odds are stacked against them because they are not able to average 30+mph in their environment. You can pulse and glide all you want to, but if you are averaging 13mph you would be hard pressed to break 50 mpg.

Avg mph seems pretty relevant to me.

Also, (when you come up from your bow) can you tell me how you can have a vegetarian pizza with sausage??

 

I am now averaging over 50 mpg per tank with the warmer weather and my average mph per tank is 39.

 

I agree with those who say that the average speed per trip (tank of gas) is not really useful. Here's why:
I might average 45 mph or 50 mph over a tank. But in fact, I do very little driving at 45 or 50. Most of my driving is at highway cruising speed, or at lower speeds on city streets or winding country roads.
So my 45 mph average might occur either because I cruised at a lower speed (good for my mpg) or did a lot of sitting around stuck in traffic (neutral in a Prius because the engine can go off; but terrible for mpg in a regular car). Similarly, the 50 mpg might occur because almost all of my trip was cruising at 65; or because half of it was cruising at 80 and the other half much slower--in this contrast, the 65 would probably be better for overall mpg. But the point is that you can't tell from the average speed whether I will have good or bad mpg.

Here are two things (other than the weather, which as you probably know is really important) which you can use to develop mpg strategies:

First, if you are on the highway a lot, average "cruising" speed (whether or not you use cruise control). I have to admit I cruise about 71 or 72 in a 70 zone--I would get significantly better mileage at 65; worse if I did 76 or so like I used to in my Corolla or do in my wife's Subaru. This is where Bob Wilson's chart is really useful.

Second, how much of your trip is the first ten minutes--particularly in the morning when it is cooler, and the car has sat overnight? When you look at the 5 minute average mpg chart on the screen, you will see that the first five minutes are usually quite low, the second five minutes are a bit better, but it's not until the third five minutes or so that you come close to reaching the good mpgs.
If you take short trips (under 15 minutes, say) and they are separated by several hours (so the engine cools down), you will not near the advertised mpg. If, on the other hand, you take longer trips, or shorter trips where the car doesn't have time to cool down, you will do much better.

 

The average mph is exactly that, an average. Can averages be skewed by extremes? Sure they can. I think we should all keep that in mind as well as all the other contributing factors to fuel economy.

However, avg mph is still relevant information. I am not saying it is a blueprint of exactly how you drove that tank, but it gives a much better sense of how a tank of gas was driven than mpg alone.

Also it's harder to skew the mph data than you might think. Let's say you travel roughly a tank - 400 miles. To get 45 avg mph after going 80mph for 200 of those miles, you would have to go 10mph for the other 200. Most likely the 45 mph was not obtained by the 80/10 but somewhere in between which gives you a sense of how the tank was driven.

 

Correlate this!



Since I already have my fuel log in a spreadsheet, this was easy to generate. Graphed MPG is from the HSI display, not the pump.

Total distance, a bit over 13k miles, at 51.96 mpg (pump) or 54.9 (displayed). Distance-weighted average speed 34.8 mph (two tanks missing). Last summer and fall had a lot of road trip time, accounting for most of the higher average speeds. Now that I am employed again, commutes have created many low speed tanks with lots of stoplight and traffic jam time.

 

In some respects, our Prius have a 'Jekyll and Hyde' personality.

Dr. Jekyll

After warm-up and driven at constant and modest speeds, it is very gentle on the fuel. My mph vs MPG charts are done on cruise control on flat land with two runs in opposite directions after 20 minutes to throughly warm up not only the engine but the transmission and tires too.

Speeds above 70 mph are still fairly efficient up to 75 mph but there is a distinct roll-off by 80 mph. This is not negotiable. Mr. Newton, Mr. Watt, and M. Carnot explained what is happening but the Universe enforces it. Take 10 mph off climbing hills.

The following links point to the graphs:
http://hiwaay.net/~bzwilson/prius/pri_2010_800.jpg
http://hiwaay.net/~bzwilson/prius/pri_2010_810.jpg

Mr. Hyde (w/o morning coffee)

During warm-up, the Prius is mean on fuel. It really wants to get warmed up and doesn't mind burning big globs of fuel at every opportunity to get there. The trick is to drive as gently as possible in the first 5-10 minutes but it really takes 15-20 minutes to get fully warmed up. There is merit in letting the engine start and waiting for auto-shutdown before driving. My approach is to pull out gently to neighborhood speed, ~25 mph, and shifting into "N" as often as possible.

Ken@Japan has published the detailed warm-up cycle but the simple rule of thumb, slowest possible speeds for the first 1-2 miles. Modest speed for the next 10 minutes. After 15-20 minutes, you are driving the nice Dr. Jekyll.

Bob Wilson

 

Unfortunately, it's easier to skew the speed results than dogllama suggests. That's because you average speed over time, not distance.
So for ease of working with units that divide easily: Imagine I have a 30 mile commute that is all 60 mph. I average 60 mph.
Now, assume that the last mile is on city streets, and it takes six minutes at 10 mph (stop and go) to go the mile, including parking. What's my average speed? I'm already down to 51.4 mph (30 miles in 35 minutes), with only one mile of slow driving. If that last mile takes you 12 minutes (easy in NYC or Phila.), you are under 45 mph.
Now, in a car with a gas only engine, those 6 (or 12) minutes will kill milage because you will be idling all the time.
In the Prius, however, the ICE may be off much of the time (unless you are using so much electricity you have to recharge the battery), so your milage will not change nearly as much.
For those of us who live in a world of highways and city streets, this high speed/low speed is pretty usual. For those with different driving patterns, average speed may actually correlate to mpg much better.
The great variety is why the diagram fuzzy1 produced, showing a big blob, is so useful.

 

In addition, the time period is from the time the car is turned on/READY to the time it is turned off. If you stop at a red light, the clock is still counter and your are going at 0 mph. This will greatly reduce the average MPH. So the average MPH display from the MFD is not a true average MPH base on actual miles traveled.

 

Mmmmmm, pizza.....

Go to your favorite pizza place, the one that you rave to your friends and out of town guests that they'll get the best pizza they've ever tasted there (everyone has a place like this, it's uncanny). Order a Vegetarian pizza (usually it will have a bunch of toppings, mushrooms, olives, onions, peppers, maybe tomatoes or artichoke hearts) and then ask if you can have an additional topping added onto it (sausage or pick your favorite meat).

It is WONDERFUL (although, usually you will get some raised eyebrow look (even over the phone) "confirming" that you want a VEGETARIAN pizza with SAUSAGE?.

Try it, yum!

 

Start with Bob's steady state speed charts, factor in the following: warmup penalties, accelerations (number and pace), hills, winds, ambient temperature, surface conditions (snow, rain, road type), psi, cargo. Now you are getting somewhere in understanding results. I track average speed of my tanks, but as average speed drops, the reasons it drops vary too much (mainly time stopped). I could burn a tank at 20 mph and get 100 mpg or 40 mpg. At highway speeds where "idling" is less of a factor, the average speed follows Bob's chart, adjusted for other conditions listed above.