MPG discrepancy

[GeneralZod]

The sunroof only weighs 100lbs. if you are driving your sunroof-equipped Sonic on a planet with considerably more gravity than Earth.

I suggest leaving the sunroof calculation @ or around 30lbs. And that's probably high.

Also, it is not as if it is tacked onto the top of the car. You must also discount the loss of weight involved in losing a large piece of the roof, including the outer skin, any reinforcement materials inside, and all of upholstery on the inside.

If you want to do real calculations, take the parts off of a real car and weigh them. Anything else is crap science, and therefore just crap.

So sheet metal is now heavier than glass? Relax, your snarky reply is full of crap. So a pano-roof in you mind should be lighter. The addition of a sunroof requires additional support. Poke the roof, the sheet metal bends easily. So you just can't tack it on there. It may not be 100 lbs but I am going by what I read, and it seems to make sense.

 

[Preloader]

Maybe OP can weigh both his Sonics, without the spare in the LTZ. Then we could apply the weight adjustment for the different wheel weights.

The additional LTZ weight would then be a result of fogs, pleather and sunroof.

Theen someone who is doing the fog install could weigh the stock fogs and we could subtract that.

Theeeeeen we would have the weight of the pleather and sunroof.

Theeeeeeeeen, if some one weighs their fabric seats I could subtract that from the weight of my LTZ seats and know the weight of the pleather.

And theeeeeeeen we could find the weight of the sunroof and accompanied reinforcements.

 

[GeneralZod]

Preloader, thanks for the laugh!

 

[RichieRocket]

How about the data plate on the driver door jam? You can get the empty weight by

Gross Weight - Max Cargo Load

If we could find two people with the same trim except with & without sunroof...that may give an estimate? Not sure...

EDIT: I just checked my 'tire data sticker', it only has max passenger & cargo load = 847 lbs. No Gross Weight. If two cars are equal except sunroof, maybe the max load would show the difference?

 

[RichieRocket]

As was said by Firstorbit and others. Tire width and wheel weight are at the heart of the loss. A few months ago I posted if I installed 195 LRR tires and light weight rims 3-4 mpg increase would be easy on my LTZ.

There is also the wheel diameter. 17" five spoke wheels smack the air around harder than a 15" 5-spoke or solid wheel. Most Bonneville speed cars use soild wheel caps for lower drag...

City mpg - tire width (rolling resistance), tire/wheel weight (rotating inertia)

Hwy mpg - tire width (rolling resistance & aero drag), 17" spokes (aero drag)

 

[GeneralZod]

Drag is another factor, nice Richie. About the drag that makes it difficult to quantify, just throwing on pizza pans over the rims is not the whole story. Even the tread can have an impact. Most times the frontal is looked at. However, the data shows air exiting a surface is just as important. It is a major area of drag for hatches.

I also did a quick google about sunroof weight. Seems 65 plus pounds is what I saw. Some were higher. Much might depend on the engineering and the safety laws for the MY. It also depends on the maker. So 100 isn't that outlandish. To add to the aero, not using a sunroof can have a negative impact.

 

[sx sonic]

Its been a long time since I handled a sunroof frame and glass but 50-60lbs does sound about right. Again its been awhile, my memory is hazy, and I didn't get to handle the same size sheet metal of a standard roof for comparitive purposes.

I do have a solution! Next SOF car meet will be at a weigh station and we can compare our cars weight.

 

[ungawd]

I have a sunroof on mine, and I guarantee I still have the lightest Sonic on this forum.

Also, even though the added weight is a bummer, I love my sunroof. It makes the cab feel roomier, and it helps the HVAC run on vent without the need to turn the fan on.

I keep my moonroof on vent and rarely ever use the fan. The amount of fuel I save doing that is comical. The following imagine, the DIC is approximately .2mpg pessimistic. It displays my mileage from work to home yesterday when the temperature was between 42 and 50 degrees and he engine was already at operating temperature when I reset it. The ultragauge is showing my tank average.

Here's an interesting project for you Preloader. Carbon Fiber sunroof weight loss plan -- for less than $350 - BMW M3 Forum.com (E30 M3 | E36 M3 | E46 M3 | E92 M3 | F80/X)

That guy weighed the assembly from his M3 out at 33lbs. The glass alone was 15lbs.

Substituting an Al alloy for whatever steel there is would retain strength and reduce weight. Changing the glass to a polycarbonate would save weight as well while retaining strength.

 

[Preloader]

Wow! Awesome link.

I have already looked into a Lexan swap........ Not cheap!

I may consider the CF idea, when time permits. Thx.

 

[GeneralZod]

I keep my moonroof on vent and rarely ever use the fan. The amount of fuel I save doing that is comical. The following imagine, the DIC is approximately .2mpg pessimistic. It displays my mileage from work to home yesterday when the temperature was between 42 and 50 degrees and he engine was already at operating temperature when I reset it. The ultragauge is showing my tank average.

Here's an interesting project for you Preloader. Carbon Fiber sunroof weight loss plan -- for less than $350 - BMW M3 Forum.com (E30 M3 | E36 M3 | E46 M3 | E92 M3 | F80/X)

That guy weighed the assembly from his M3 out at 33lbs. The glass alone was 15lbs.

Substituting an Al alloy for whatever steel there is would retain strength and reduce weight. Changing the glass to a polycarbonate would save weight as well while retaining strength.

@ those temps you are in the winter sweet spot.

I saw that mod when I googled. I would be hesitant of anything but glass. Hazing, swirls, and cracking are common issues. You could black it out.

 

[1980PontiacWagon]

...So leaving your sun/moon roof on vent rather than closed will give you better mileage?

 

[ungawd]

...So leaving your sun/moon roof on vent rather than closed will give you better mileage?

With the fan off, yes. The moonroof open to vent draws air through the ducts at the temperature specified on the temperature selector. It doesn't pull the air through as quickly as the fan pushes it, but it's plenty enough to keep the temperature inside the car very warm even when it's 20 degrees outside. Below that it can't keep up, but still good enough for the fuel savings for me. At 10 degrees and below I close the moonroof and put the fan on full for a while then turn it off. That's a rare occasion. Up to at least 85 degrees this is effective for cooling as well. It hasn't been above 85 since I bought the car so I'll find that limit in a few months.

 

[1980PontiacWagon]

Thanks, and good to know...moon roof stays open now.

 

[Stew]

With the fan off, yes. The moonroof open to vent draws air through the ducts at the temperature specified on the temperature selector. It doesn't pull the air through as quickly as the fan pushes it, but it's plenty enough to keep the temperature inside the car very warm even when it's 20 degrees outside. Below that it can't keep up, but still good enough for the fuel savings for me. At 10 degrees and below I close the moonroof and put the fan on full for a while then turn it off. That's a rare occasion. Up to at least 85 degrees this is effective for cooling as well. It hasn't been above 85 since I bought the car so I'll find that limit in a few months.

Are you supposing the mpg increase is because of lower electrical load or some kind of aerodynamic thing going on due to the sunroof tipped up?

 

[ungawd]

Are you supposing the mpg increase is because of lower electrical load or some kind of aerodynamic thing going on due to the sunroof tipped up?

Lessened engine load whether it be electrical or mechanical. While in drive with my foot on the brake: fan on full puts about 40% load on the engine, fan off puts about 26%. That was just one check I did, nothing scientific just to observe the difference.

 

[ungawd]

I pulled a Dubya and misremembered. 26% and 34%. I confirmed it this morning in the McDonalds drive-thru. So ~8% more engine load in that scenario.

 

[TRO-512]

I heard some of those sunroofs were made in Chicago.

(hahahahahahaha)