MPG Run the Fourth

[TominMO]

Today I took a run at 60 MPH with the rear windows open. I know, I know, this introduces two variables, speed and wind drag. But I wanted to get away from 55 MPH to something more real-world; plus most of the road course I am using has a 60 MPH speed limit, with one small section being 55.

I decided to try to eliminate one variable simply by predicting what my mileage would be, based on experience with this car. Yesterday at 55 MPH it got 58.5 MPG, so I felt it was reasonable to predict that at 60 MPH it would go down a bit. I chose 55 MPG, a 3.5 MPG loss. This is of course tentative and arguable; I will do another run, windows closed at 60 MPH, in the future to give a baseline.

The car is as it was yesterday, and the wind conditions the same; virtually nonexistent. My road course is a big circle, so that should practically eliminate wind as an issue.

So the only variable left is open windows. I drove the exact same course I always do, which again came to 84.5 miles. Fuel consumption was 1.78 gallons, giving a figure of 47.97 MPG. This was much lower than I predicted. Another member claims better mileage with the rear windows open, due to lower turbulence at the rear of the car. I think this run calls that into serious question. First of all, I should have gotten the same or better mileage if this was true; i.e. 55 MPG (my predicted figure) or better. Instead I got 48 MPG. This is much too large a discrepancy to be accounted for by a 5 MPH change in speed, I feel.

Bear in mind that I use the same course all the time, in basically windless conditions, so two big variables are eliminated. Also I am now very careful to fill the tank to the same super-full level every time. I would suspect that the claim of better MPG with rear windows open is based on a less-rigorous testing methodology; the same course, wind conditions, attempts to keep consistent speed, and tank filling method must be used to have verifiable results. And also loaded vehicle weight, mainly meaning # of passengers. A full car w/windows closed will most likely get lower mileage than a driver-only car with windows open.

So from now on my MPG runs will be with windows closed.

[TominMO]

BTW, when I am done making mileage runs, I will post a summary of everything I did, for simpler reference.

[batstiva]

It appears the difference in speed should result, at worst, in a loss of MPGs in the neighborhood of 2 or 3, at most. Going by examples on Wiki. Still looks like, in general, between 45 and 50 mph is going to yield the best mpgs.

Actually went through and read the procedure the EPA uses to determine fuel economy. Kind of a joke. TominMO's mpg runs are far more meaningful than the way the EPA figures it. I would be curious if you went back and drove it changing only one variable (either speed or windows open) to see which of the two makes the most difference. Interesting about indexing the plugs, re gaping, and the higher voltage coil. Got me thinking....

EPA testing procedure through 2007

Two separate fuel economy tests simulate city driving and highway driving: the city driving program consists of starting with a cold engine and making 23 stops over a period of 31 minutes for an average speed of 20 mph (32 km/h) and with a top speed of 56 mph (90 km/h); the highway program uses a warmed-up engine and makes no stops, averaging 48 mph (77 km/h) with a top speed of 60 mph (97 km/h) over a 10 mile (16 km) distance. The measurements are then adjusted downward by 10% (city) and 22% (highway) to more accurately reflect real-world results. A weight average of city (55%) and highway (45%) fuel economies is used to determine the tax.[22]
In some cases, this tax may only apply to certain variants of a given model - for example, the 2004–2006 Pontiac GTO did incur the tax when ordered with the four-speed automatic transmission, but did not incur the tax when ordered with the six-speed manual transmission.
Because EPA figures had almost always indicated better efficiency than real-world fuel-efficiency, the EPA has modified the method starting with 2008. Updated estimates are available for vehicles back to the 1985 model year.[23]

[edit] EPA testing procedure: 2008 and beyond


New fuel economy label.


As a means of reflecting real world fuel economy more accurately, the EPA adds three new tests[24] that will combine with the current city and highway cycles to determine fuel economy of new vehicles, beginning with the 2008 model year. A high speed/quick acceleration loops lasts 10 minutes, covers 8 miles (13 km), averages 48 mph (77 km/h) and reaches a top speed of 80 mph (130 km/h). Four stops are included, and brisk acceleration maximizes at a rate of 8.46 mph (13.62 km/h) per second. The engine begins warm and air conditioning is not used. Ambient temperature varies between 68 to 86 °F (30 °C).
The air conditioning test raises ambient temperatures to 95 °F (35 °C), and the vehicle's climate control system is put to use. Lasting 9.9 minutes, the 3.6-mile (5.8 km) loop averages 22 mph (35 km/h) and maximizes at a rate of 54.8 mph (88.2 km/h). Five stops are included, idling occurs 19 percent of the time and acceleration of 5.1 mph/sec is achieved. Engine temperatures begin warm. Lastly, a cold temperature cycle uses the same parameters as the current city loop, except that ambient temperature is set to 20 °F (−7 °C).
EPA tests for fuel economy do not include electrical load tests beyond climate control which may account for some of the discrepancy between EPA and real world fuel-efficiency. A 200-W electrical load can produce a 0.4 km/l reduction in efficiency on the FTP 75 cycle test.[25]

[TominMO]

It appears the difference in speed should result, at worst, in a loss of MPGs in the neighborhood of 2 or 3, at most. Going by examples on Wiki. Still looks like, in general, between 45 and 50 mph is going to yield the best mpgs.

Right. I figured a loss of 3.5 MPG going from 55 to 60 MPH is on the high end, with the rest of the loss, 7 MPG, being due to open windows and the turbulence they create.

[batstiva]

Hard to believe those windows being flared out, what do they stick out an inch or so, makes that much difference. Very interesting.

[TominMO]

Hard to believe those windows being flared out, what do they stick out an inch or so, makes that much difference. Very interesting.

Yeah, I was quite surprised too. I suspect it's more about the turbulence than the amount the windows stick out though.

A good example of learn by doing rather than theorizing. I feel like the Festy MPG Mythbuster.

So far, the most dramatic results were from:
(1) going 45 MPH--lower wind resistance and RPMs give 62.5 MPG, before I did the...
(2) ignition upgrades--45,000-volt coil and proper gapping/indexing of a good plug gave about a 10% improvement. Then I tried...
(3) open rear windows--a negative result but good to know.

No more runs for a couple of weeks. I can only do them on weekends due to rush hour traffic Mon - Fri. Next weekend I am picking up Pimpstiva!!!

[TominMO]

From Wikipedia:
Speed and fuel economy studies

Steady speed fuel economy. Fuel economy at steady speeds with selected vehicles was studied in 1973, 1984, and 1997. The most recent study indicates greater fuel efficiency at higher speeds than earlier studies; for example, some vehicles achieve better mileage at 65 than at 45 mph (72 km/h), although not their best economy, such as the 1994 Oldsmobile Cutlass, which has its best economy at 55 mph (29.1 mpg), and gets 2 mpg better economy at 65 than at 45 (25 vs 23 mpg). All cars demonstrated decreasing fuel economy beyond 65 mph (105 km/h), with wind resistance the dominant factor, and may save up to 25% by slowing from 70 mph (110 km/h) to 55 mph (89 km/h). However, the proportion of driving on high speed roadways varies from 4% in Ireland to 41% in Netherlands.

There were complaints when the U.S. National 55 mph (89 km/h) speed limit was mandated that it could lower, instead of increase fuel economy. The 1997 Toyota Celica, got 1 mpg better fuel-efficiency at 65 than it did at 55 (43.5 vs 42.5), although almost 5 mpg better at 60 than at 65 (48.4 vs 43.5), and its best economy (52.6 mpg) at only 25 mph (40 km/h). Other vehicles tested had from 1.4 to 20.2% better fuel-efficiency at 55 mph (89 km/h) vs. 65 mph (105 km/h). Their best economy was reached at speeds of 25 to 55 mph (see graph).

[batstiva]

I suspect you're right about the turbulence being the issue. Guess it doesn't take much change in the vehicle profile to significantly change the turbulence created, eh?

[TominMO]

I suspect you're right about the turbulence being the issue. Guess it doesn't take much change in the vehicle profile to significantly change the turbulence created, eh?

Yeah, every car is different. That's why the only way to really know is practical experimentation.

[eurotiva]

^ good work Tom. Keep it up! Soon you'll be testing a b3 turbo fuel economy. I bet we can make it get even better!

More power=less pedal

[TominMO]

^ good work Tom. Keep it up! Soon you'll be testing a b3 turbo fuel economy. I bet we can make it get even better!

More power=less pedal

Mebbe so, mebbe so...I'm planning on test Pimptiva and Red (1.6 DOHC non-turbo) as well. If I get Whitey up and running, I'll do tests with and without A/C at various speeds, and try to answer the age-old question: at what speed does it become better for MPG to close the windows and use the A/C?

[batstiva]

Mebbe so, mebbe so...I'm planning on test Pimptiva and Red (1.6 DOHC non-turbo) as well. If I get Whitey up and running, I'll do tests with and without A/C at various speeds, and try to answer the age-old question: at what speed does it become better for MPG to close the windows and use the A/C?

I don't know specifically with the Festiva, but I have read studies with other cars. Apparently running the A/C versus having the windows down a couple of inches is about equivalent, but windows all the way down was much worse as far as mpg's go.

[eurotiva]

Tom you need to do my car. I can't keep my foot out of it anymore!

[TominMO]

I don't know specifically with the Festiva, but I have read studies with other cars. Apparently running the A/C versus having the windows down a couple of inches is about equivalent, but windows all the way down was much worse as far as mpg's go.

It depends on the speed. There is a break-even point I want to find; at some speed, it becomes more efficient to use the A/C; and below that speed, no A/C and windows cracked open. Of course at city-driving speeds it's more efficient to have the windows all the way open than use A/C, since you're sitting at lights, driving 35 MPH, etc.

[FestYboy]

hmmm, that's rather odd. the only thing i can ask is did you crack a front window open for equalization? you know you've got it right when the wind noise gets really quiet.

i can't say i've tested so thuroughly(sp), but i've always seen better milage when i can keep the rear windows open (and usually i'm hovering around 70 while doing so). seeing results with the windows closed should prove noteworthy.