calculated vs. computer MPG - Please post your results

Another measure I have that is a good indication of short/long trip activity is the number of miles per day. I calculate this in my spreadsheet by having the odometer delta divided by the days delta. Very easy unless I get two fills in one day!

 

4th Fill-Up:
Date.....Gallons...Miles...Trip MPG....Calc MPG....MPH
9/13/09.. 7.03.....298.....43.7........42.32....... 15

It's still going down! This is very frustrating. I think this is the reason though...look at my average MPH ... it's only 15mph! Obviously, I don't drive at 15mph. However, I seem to be a red light magnet, especially during the last tankful! If I have to drive through 20 intersections with traffic lights, I'll have a red light at 18 of them. That, coupled with hilly terrain, is making it really difficult to get decent mileage. On average trip, Looking at the 1-minute consumption display, you'll see it literally goes up and down between about 20mpg and 100mpg every minute or two.

 

BTW, the displayed mpg vs. calculated mpg has been pretty consistent - much more consistent than on my Gen II. I don't consider a variation of 1-2mpg to be significant. The calculated mpg is generally the lower though.

 

mine varies - calculated 52-57, actual 49 and change. Here in the sound area around Seattle, I don't think I will ever see the big numbers other people reported - too many hills :-(

 

I'm in IL and driving on flat roads. My tire pressure is good, my technique isn't bad... for some reason I have only been getting 45 mpg. I just can't seem to do better. Not sure what to do. Perhaps I need to try harder. /shrug

 

It looks like you might be able to take "E St Hwy 33" out to N 1750th St, and take a right, follow it around to E 1200 th Ave, and a right at N 1600th St, back to E St Hwy 33. Cross over to E 1400th St, another right and then a right at N 1750th St. That looks to be about 4 miles or relatively flatland. Try holding the speed at 30 mph on that route for 10 miles and let us know what your MPG comes in at. Also, let us know how many stops.

The ideal test track is one that you can maintain a constant, cruise control speed w/o stops or a minimum of stops. Just run about 10 miles at say 30 mph and let us know your MPG.

The other option is to get on Hwy 33 and holding 60 mph with the cruise control, run over to Dietench. Record the numbers and do the return trip. Again, at 60 mph, your MPG should be at about 60 MPG at a cruise control held 60 mph.

GOOD LUCK!
Bob Wilson

 

The only problem is that the winds are from Dieterich when you head SE on 33 and from Effingham when you go NW on 33.

Seriously, winds can be a bit of a nuisance and just mangle your MPGs.

I think the recommendations for both 30mph and 60mph are great suggestions. He may have to adjust 60 to 55 or 50 in order to get consistant speeds.

Another good option may be to use US 45 (south of the Airport and past Watson as a starting point) for a good 10-mile, fairly uninterrupted trip. The RT would tell a lot about the car.

 

Believe it or not, the roads you suggested are somewhat traveled now. I think on those particular roads though, I would just do my thing and let people go around. I am really surprised you guys are so familiar with my roads, (unless you just looked at a map.) I know apriori is close, but alabama? haha. Thanks I will definitely go for a few tests. I drove home from Terre Haute Indiana last night, and I achieved 55mpg! I had to slow it down from 60-62 to a steady 55. I had to sacrifice some speed going uphill, and then glide down and slowly creep back up to speed. I had several people pass, but it was worth it.

I had the 2nd generation all down, I was trained by john's website.. but this one is a little different. I will get back to you guys on this. (I will probably just take a local country road and also record how many stops I take.) I can drive the Montrose blacktop road into Dieterich north to south, then I could drive between dieterich and effingham, then circle back. That would give me a north-south, east-west, west-east, then a south-north run. heheh.

 

Filled up in Astoria Oregon at Safeway. 423.8 miles and it took 8.983 gallons when the pump stopped at low speed at the first shut off and no more gas was pumped in. MFD read 50.2 calculated was 47.17 mpg.

We than drove to Longview / Kelso Washington to the Three Rivers Mall area and filled up there at another Safeway.

Pumped the gas at slow speed the same way as before for consistency and first click off recorded 1.052 gallons for 56.2 miles / MFD was 53.4, calculated was 53.422 mpg.

This is the first time ever in nearly 11,000 miles the MFD was right on with calculated mpg. The trip was mostly highway with speeds 45 to 73 mph on interstate 5.

My wife was driving and she normally gets 46-48 mpg, I was suprised.

Maybe the MFD is more accurate the first 50-100 miles or so than begins to have fluctuations that cause inaccuracy.


alfon

 

I'd never suggest that Bob didn't have a way of actually getting to Effingham and driving the route himself before suggesting it to you!

I've been through many times, though only twice within the past year. The most recent trip I came up 33, so I didn't see 45 for myself (south of town, that is). I just recall that it was a good, open road -- maybe a couple of fun banked turns, but fairly easy to get some uninterrupted miles.

Give it a go. You'll need to learn the new things about the 2010. One of the biggies is that 45mph becomes a good place to be, not a bad place. Also, 55 to 60 mph is a VERY good zone for long-distance driving MPGs. Don't be afraid to get up to speed and stay there. Yes, I bleed off speed on uphills, but no so much that I can't get it all back (and more) on the backside of the hill. Good, strong acceleration helps on this car.

 

Ok, I'm a fan of maps:

• Google maps - has a terrain option that along with experience dealing with image data makes it fairly easy to find reasonably good, test tracks. It isn't perfect but often you can identify a to/from route and add way points until you have the complete route mapped out. It also has the little character 'Google photos' which I have used to read road signs for speed limits and check suspicious intersections for STOP or YIELD signs. <GRINS>
• Google earth - has the terrain altitude under the cursor. This allows me to 'fly' the route looking for hills and dips, the bane of our MPG existence.

These map resources don't replace going 'on site' but they provide a way to limit the on-site search for the ones most likely to provide stop-free, long distance, relatively low traffic runs.

Bob Wilson

 

I posted this in another thread but I will put it here as well:

I just finished a quick 6 day highway trip:

3353.1 miles, 49.7 MPG, as computed by the car

Using the odometer miles and gallons of fuel used from the gas pumps I calculate 47.58 MPG.

I tried to be careful and refill the tank to the same level at the end as it was when I started.

 

Prius III solar panel + NAV

After 6 fills I'm consistently registering 55-56 MPG displayed at fill-up and getting 52-53 MPG with the pump calculation. In this climate (Miami) the gas motor seems to be warm by 5 minutes after startup.

I also observe little or no difference between the ECO and PWR modes. Read my lips: no great advantage to ECO mode!

My style: I drive mostly in city traffic. I do accel+glide but don't skimp (over-glide) unless there's a traffic slowdown (red light or other) up ahead. On the freeway I just turn on the auto-pilot at 55-60 mph. If I were a nut case I might get ~2 MPG more.

Question: Is the pump calculation the more reliable? I've heard that pumps often overstate the amount delivered, due to thermal expansion or whatever.

 

In my mind there is no question that the pump calculation is more accurate (subject to the fact that you have to refill to the same level). IIRC, by law, pumps are supposed to be plus or minus 0.5% on a 5 or 10 gallon fillup. Of course it is possible for any given pump to be off by more than that by mistake or by a crooked station. But, by design, the pump makers must design and build equipment that is within that tolerance when built, installed and after many years of use...else (in theory) they would be discovered and out of business.

There is no such requirement on the in-car measurements. There is not any tolerance range or any repeatability requirement for gas consumption that I know of. All said, both calculations rely on the car's odometer for miles traveled (unless you use a GPS accounting which I doubt anyone does and it would be a pain to do). So the only question is how much gas was used.

To be the most fair, outside testers (like Consumer's Report) usually disconnect the gas tank and rig up independent, highly accurate small graduated-cyclinder type tanks that can be measured accurately (so they don't have to worry about refilling to the same tank level). This is the problem with using the pump calculation -- you don't really know if you refilled to the same level. But this inaccuracy goes towards zero with several to many fillups because it is a one-time error.

So if you want to, repeatably measure a short trip I'd use the car's display, because, while not as accurate it is probably much more repeatable for short comparison purposes...but the actual number is probably reading high, as we've seen on this thread by 4% or 5%.

3PriusMike

 

Not sure if this is the right place to post this but I thought you might be interested in a group we've created at our site, Moblu, where Prius drivers can engage in a little friendly fuel efficiency competition. Group comes complete with its own Leaderboard.

It's free and the group which was just created is open to any Prius driver.


We'd love to have some PriusChat members give it a try. I'd be interested to hear what people think about the group idea, whether the way we are doing it works and how we might improve it.


I'd also appreciate any guidance as to whether this post belongs somewhere else in the Forums.


Thanks.

 

Just to add my experience, after discarding the first two tanks (filled by the dealer and a gas station in Oregon where I could not pump my own gas), I have now completed five tanks for a total of 2072 miles.

Average display: 52.595
Average by gallons pumped: 50.264
Average Display excess: 2.331

Excess ranged from 1.857 to 2.616.

My results seem pretty consistent (and consistent with others), about 5% excess. Now when I see the display, I subtract 2 1/2 mpg and figure that I am pretty close. If I'm at 52.5, I'm making 50 mpg.

 

Been awhile since I've posted my mileage, but I was waiting to build up a list of figures to post. I've had my Prius for about four weeks now and have filled up three times. My drive is in the Los Angeles area with a commute one way to work of about 37 miles in varying degrees of traffic from light to very heavy and over a couple of hills.

So, let's start with my tank calculations.
I didn't record my average based on the fuel computer with my first tank, so unfortunately I don't have that comparison.

Tank #1 (9/1/09) - 9.706 gallons pumped
Number of miles driven: 518
Manual MPG calculation: 53.3 MPG

Tank #2 (9/9/09) - 8.947 gallons pumped
Number of miles driven: 524 (total miles to date: 1042)
Manual MPG calculation: 58.5 MPG
Fuel computer calculation: 61.5 MPG
Average speed: 32 MPH

Tank #3 (9/17/09) - 9.494 gallons pumped
Number of miles driven: 545 (total miles to date: 1587)
Manual MPG calculation: 57.4 MPG
Fuel computer calculation: 61.9 MPG
Average speed: 28 MPH

Now for the daily calculations:

My best mileage was my second week of driving. I obtained (on the same commute) a best of 75.2 MPG based on the fuel computer. Interestingly, while I had several days that were over 70 MPG on the fuel computer, I have only managed to get to up to 69.6 MPG on the fuel computer since that second week. This log starts AFTER I was getting MPG over 70. For each log entry the trip computer was reset to 0 and the figures are the reading when I pulled into my parking space at work except for the odometer reading which was reading at the beginning of the trip.

Here's my daily morning commute log (sorry I didn't log the evening commute):

9/8/09
Odometer reading: 890
Miles driven: 36.9
Fuel Computer: 66.9 MPG
Ave. Speed: 28 MPH
Note: I stop for breakfast half way through my drive for about 1/2 hour. I have recorded the times, but roughly 35 minutes for the first part of my drive and 45 minutes for the second part. I recorded the exact times if that is of any use.

9/9/09
Odometer reading: 970
Miles driven: 37.5
Fuel Computer: 68.9 MPG
Ave. Speed: 31 MPH

9/10/09
Odometer reading: 1043
Miles driven: 38.3
Fuel Computer: 69.4 MPG
Ave. Speed: 27 MPH

9/11/09 - outside temp 72 degrees
Odometer reading: 1125
Miles driven: 38.3
Fuel Computer: 68.7 MPG
Ave. Speed: 29 MPH
I started recording the temperature to try to see if there was any correlation with not breaking 70 MPG any more. It was particularly hot the week I managed 75.2 MPG.

9/14/09 - outside temp 70 degrees
Odometer reading: 1287
Miles driven: 38.3
Fuel Computer: 66.6 MPG
Ave. Speed: 30 MPH

9/15/09 - outside temp 66 degrees
Odometer reading: 1361
Miles driven: 36.9
Fuel Computer: 69.3 MPG
Ave. Speed: 32 MPH

9/16/09 - outside temp 68 degrees
Odometer reading: 1433
Miles driven: 36.8
Fuel Computer: 66.6 MPG
Ave. Speed: 31 MPH

9/17/09 - outside temp 70 degrees
Odometer reading: 1514
Miles driven: 36.9
Fuel Computer: 69.6 MPG
Ave. Speed: 32 MPH

9/18/09 - outside temp 68 degrees
Odometer reading: 1587
Miles driven: 36.9
Fuel Computer: 66.1 MPG
Ave. Speed: 31 MPH

So there you have it... my logs so far.
If anyone would like to see the route I take, you can map it out. I'm starting out at the intersection of Baldwin and Huntington in Arcadia up to 210 fwy, to 134 fwy, to 101 fwy, to 405 fwy and exiting on Wilshire and ending up at 11900 block of San Vicente in Los Angeles. Traffic speeds up to the 405 are probably 50-65 MPH and then the 405 crawls.

Chris

 

I'm proud to report my best MPG so far. No, I don't consider myself a hypermiler yet, but I think I'm getting the hang of it.

Fill up #7 on 9/23/09

Gallons 6.239
$/Gallon 2.119
Trip miles 402.5

Calculated MPG 64.51

Computer Est MPG 67.0

The MPG on my last 3 tanks has gone up each time with zero changes to the car so I must be doing something right.

PS, this is my normal 40 mile per day round trip commute to work and home, plus going to lunch, etc. It's generally dark so my lights are on. I'm convinced I could do better in daylight hours.

 

On my latest tank I got 60.3 (63.3 displayed).

My first two fillups are already in Bob's data

miles gallons mpg(pump) mpg (disp) .. error
519 ... 9.20 ... 56.4 ... 59.1 ..... + 4.5%
532 ... 9.10 ... 58.5 ... 61.0 ..... + 4.0%
547 ... 9.07 ... 60.3 ... 63.3 ..... + 5.0%

3PriusMike