MPG Run the Fourth

[TominMO]

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.

Opened front pass. window about an inch or less. It was noisy; after 20 miles I closed it. But I had the dash vent open all the time, to let in outside air.

I'm not going to re-try this test; it gets too tedious to do multiple tests of the same thing. But if you want to try both a closed and open test run, with close attention to filling the tank, constant speed, same course, etc., go ahead, and report back.

I will do a 60 MPH windows-closed test in two weeks. Next weekend I am otherwise occupied.

[FestYboy]

i'll have to give it a try, i won't see your numbers though because i'm a carbed B6, but the differance is the important part.

[Festiva_tunner]

Tom I'm sure you've taken this into consideration but is time of day in your routine? different temps will have an effect on how well the fuel atomizes
Seriously good work from what I'm reading though

[TominMO]

Tom I'm sure you've taken this into consideration but is time of day in your routine? different temps will have an effect on how well the fuel atomizes
Seriously good work from what I'm reading though

All my runs are begun at about dawn on the weekends, to avoid traffic, and yes to try to keep some consistency in temp.

[batstiva]

i'll have to give it a try, i won't see your numbers though because i'm a carbed B6, but the differance is the important part.

Well, this is interesting. Just calculated my latest MPGs. Drove 261 miles. Again, about a 60-40 highway-city split. Only difference between this tank, and my first tank calculations, is my back windows were open. Driver's side window down all the way, pretty much all the time. 55-60 on the open road. Used exactly 6.0 miles to go 261 miles. Works out to 43.5 MPGs. Virtually the same as my first set of numbers.(ended up with avg. of 43.75 MPG on that one) And the back windows were closed on that one.

No idea why I am not seeing a difference in fuel economy, the way Tominmo does.

[FestYboy]

because you're using miles of fuel instead of gallons of fuel? :p

[batstiva]

:tard: :tard: :tard:

Okay, used 6 gallons of gas in 261 miles. 43.5 MPG, or is that MPM??:iconbiggrin:

[TominMO]

OK, I'm back from picking up Casper from SimonTuffGuy in PA! Next weekend I will do a baseline run on Sat @ 60 MPH; then on Sunday the same thing, but with the car waxed as Eurotiva suggested.

[batstiva]

Hmm. Really trying to figure out why Tom sees such a difference with his back windows flared, and I don't.

Last two fill ups. First one. 55-60 mph on the highway. About a 70/30 split, highway/city. Back windows closed. Driver's side window either cracked or down all the way. Results- 297 miles- used 6.91 gallons= 42.98 mpgs.

Second one. 55-60 on the highway. About a 70/30 split, highway/city. Back windows flared, driver's side window either cracked or down all the way.
Results- 295 miles- used 6.89 gallons = 42.81 mpgs.

Both times squeezed in all the gas it would take. Fairly similar road + weather conditions. Not seeing an improvement with the windows flared, but really not seeing any loss of fuel economy either.

Course, all these pretty mpg numbers go out the window when we start getting the crappy winter mix. :banghead:

[TominMO]

Hmm. Really trying to figure out why Tom sees such a difference with his back windows flared, and I don't.

Last two fill ups. First one. 55-60 mph on the highway. About a 70/30 split, highway/city. Back windows closed. Driver's side window either cracked or down all the way. Results- 297 miles- used 6.91 gallons= 42.98 mpgs.

Second one. 55-60 on the highway. About a 70/30 split, highway/city. Back windows flared, driver's side window either cracked or down all the way.
Results- 295 miles- used 6.89 gallons = 42.81 mpgs.

Both times squeezed in all the gas it would take. Fairly similar road + weather conditions. Not seeing an improvement with the windows flared, but really not seeing any loss of fuel economy either.

Course, all these pretty mpg numbers go out the window when we start getting the crappy winter mix. :banghead:

When I did my run with rear windows open, I also switched from 55 MPH to 60 MPH, so there were two variables. Bad science. It turned out that the bigger variable was the speed, not the windows. At 60 with windows closed, I got 50.5 MPG.

[blkfordsedan]

Windows flared out will create extra drag on the sides of the car. Think of it this way: with the trailing edge of the window sticking out, the full surface area of the window is exposed to the air flow. Turbulence is also created anytime air flows past a sharp drop-off. Remember the mid-80's Monte Carlo with the square back window? They got smoked all season by the '84 "aero" T-bird with the sloping back window. They could not "draft" behind the T-Birds because the smooth airflow made it like the 'Bird was never there. The 'Birds could draft all day in the void left in the airflow behind the Monte Carlos. Then NASCAR let Chevy run the special edition Monte with the sloping back window to even things out. Also the same reason that the '97+ Ford pickups are narrower at the tailgate then at the front of the box, the early Taurus had a sloping back end and rain drops are the perfect "airfoil" shape!

[batstiva]

I understand what you're saying. I remember when the Birds were kicking Chevy's butt. Just surprised I see no real loss of fuel economy with the flare windows opened. Would have thought there would have been a difference open versus closed.

[TominMO]

I understand what you're saying. I remember when the Birds were kicking Chevy's butt. Just surprised I see no real loss of fuel economy with the flare windows opened. Would have thought there would have been a difference open versus closed.

I recorded a 2.5 MPG decrease at 60 MPH; 48.0 with rear windows open, 50.5 with them closed. Not huge but noticeable; about 5%.

[blkfordsedan]

I understand what you're saying. I remember when the Birds were kicking Chevy's butt. Just surprised I see no real loss of fuel economy with the flare windows opened. Would have thought there would have been a difference open versus closed.

I would think the difference in drag is so small that it can be overwhelmed by any other of a number of variables. Air density (not just temp) and humidity seem to make the biggest difference on any fuel injected car I've ever owned. As a rule, cooler air = denser air, but high humidity = less air, etc..

It would be cool to be able to monitor the actual fuel rate in real time via the ECU. Some people also use a vacuum gauge as a relative indicator of fuel efficiency, but more as a tool to develop "fuel friendly" driving techniques.