Diver

This last weekend I drove to Niagara Falls from Boston and back in my TL (2005 6TM, ~13K miles).
On my way to the falls the weather was reasonably cool and I only had my sunroof tilted open and
no AC. I averaged 32 mpg. On a way back, it was hotter and I had the sunroof shut and AC working
with temperature set at 72 with outside temperature in 80-85 range. To my amusement I averaged 34
mpg ! I don't know if it was combination of other factors (can't think of anything), but it looks like
the sunroof decreases mileage noticeably. I wonder what would I average if I had no AC going on and
drove slower. My speed was in 75-80 mph range and if I drove 65 or even 55 mph, I could've possibly
averaged 35 maybe even 36 mpg ?

Just an observation I wanted to share ..

Stan.

blktl1

Yeah, i saw this on that show called myth busters. they tested MPG with both AC on and windows open. It showed that the drag from your windows decreased your mileage more so than the AC did. Ac only hurts you when your city driving.

Diver

i think on that show they had no difference in millage ... i know about windows vs AC and you could imagine that open windows will create a lot of drag. I didn't expect that cracked open sunroof will make such a difference too ..

1995hoo

I could understand the drag if the sunroof were slid all the way open, but on tilt it doesn't seem that it should make too much difference. Did you compare your average speed and did you hit any traffic jams in one direction that you did not hit in the other?

Diver

there wasn't anything significantly different each way. going back (with AC and better millage) I actually hit some traffic for maybe 10 minutes, but I also rolled a bit on netrual comin down some hills not sure how much it attributed to the millage.

DocTL

The difference may be due to the quality of gasoline that you bought.

gurneyeagle

What about elevation changes? Were you driving "uphill" on the way over and "downhill" on the way home?

The aerodynamic differences between open and closed windows/sunroof at 70 mph is negligible.

gurney

Pure Adrenaline

iTrader: (2)

Too many variables to say anything for certain in this case.

But on Mythbusters, they did get a slightly better gas mileage when they used the AC with the windows up, compared to no AC and windows down.

Aerodynamics FTW

JERU

Depends on what value you put to negligible, tilting a sunroof on a car will result in a loss. You can only use the term negligible if you know the value of the loss. Engineers such as myself only use the term if the change does not affect the outcome when rounded off. 32 mpg versus 32.12 mpg. Thus you must know the change first. There are too many variables to say it does not make a major impact on his gas mileage difference. I can give you a formula based approximation once I get some free time. The model is not difficult to figure out with a free body diagram and couple of equations. There is a distinct reason cars lose gas mileage as speed increases, this will apply to the extra profile you are adding to the car's original aerodynamic profile as well. The faster he goes, the larger the loss that tilted sunroof will provide.

datmrman

I agree with gurneyeagle and others. There are many factors in play not just AC and sunroof: elevation, wind speed, wind direction, road conditions, etc...

gurneyeagle

I hear where you are coming from, but it ain't going to be 2 mpg. Titling the sunroof up is not going to increase the frontal area enough, and thus the coefficient of drag, to impact fuel economy at highway speeds.

An A-Spec or RonJon kit, 18" or 19" tires, or even a trunk wing might have more of a detrimental effect, but again, I doubt if it would be enough to make a material difference that could be measured by using the trip odometer and the gas pump's meter.

We can argue theoretical areodynamics all day, but the bottom line here is that there were way too many variables to identify, much less to factor into the equation. I grant you that tilting the sunroof changes the cd, and thus theoretically adversely affects gas mileage; it's to the degree of which I have much doubt.

gurney

HBaJ

Do u have a sunroof guard/reflector im sure in highway driving it makes a decent difference, i drove back from Philly (to CT) yesterday sunroof down- but i have a wind deflector and got pretty good gas mileage considering my avg. speed and all

leedogg

If I slap a shark fin on it, would my mileage decrease noticeably?


















just playin

nfnsquared

What about wind?

On a trip that long fighting a 10mph headwind one direction and having a 10mph tailwind the other direction could easily make a 2 mpg difference.

colossus

You and that shark fin of yours



Seriously though, I managed 37mpg on my 5AT this weekend doing 59mph with no AC and windows/sunroof closed. Today I also averaged the same 37mpg with the sunroof cracked open (not slid back).

My guess would be either dirtier gas, warmer road/ambient temperature, winds, or differences in elevation made the difference.

When I made the 37mpg test on my car, I ran for about 5 miles with the AC on Dual 65 fan setting of 2 bars recirculating - and got 35mpg.

My test were not labarotory controlled, just average runs of 5 miles on a 72 degree day down in Tustin.

According to that Road & Track article (see the one Michael Benz refernced about WOT in city) AC is supposed to eat up to 20% efficiency while open windows would eat about 3%. Obviously at high speeds (ie 80+ mph) open windows will create a lot of drag. But if they're open a tad at 60 I don't think they'de create too much drag. Then again recirculating AC on a low setting wouldn't eat too much gas either.

As a rule I turn on AC on the freeways and keep the windows open whenever my speed drops below 70 (so I can breath the wonderful LA smog )

RJANACONDA

iTrader: (1)

do you guys think that the moon roof wind deflector effects the milage significantly?

gurneyeagle

Absolutely not. Unless you strap a big luggage container to the roof, or deploy a parachute, there isn't a whole lot you could probably do to change the aerodynamics enough to affect your gas mileage.

I regularly strap my bike onto the back of the car and take it to work. I have noticed no change in my mileage. There is also no change in mileage between commuting in the winter (no a/c) and the spring (open sunroof and rear windows cracked).

With all due respect to JERU, I'm measuring my mileage based on the MID and the meter on the gas pump. As has been said, slight variations in either can affect the mileage calculation.

If I were to try and nail down the cause for Diver's mpg differences, I'd attribute them to wind, temperature, elevation changes, and most likely the inconsistency of his tank fillings. Mileage really needs to be calculated over several tanks to make any proper assessments.

As I stated above, my experience over several months of driving the same route is that there is no difference between sunroof/windows open and closed. Turn on the a/c, and I lose 1 - 2 mpg.

1995hoo

You might have trouble going much above 25 mph, if that matters to you. My brother used to strap a wooden shark fin on top of his 1974 Beetle, but because he had to drive so slowly he'd put it in the back seat until we were almost to the concert, then pull it out about 3 miles from the venue and strap it on then.

Unfortunately, one year he left it on top of the car when we went in to the show and when we came back it had been stolen.