It seem as if all for nothing

[shonus]

So today my dad and I had to drive a total of 90 miles in my car and I felt it was the perfect opportunity to measure my aerodynamic mod improvements. Most of the trip I was driving 60 or 55 mph with a few areas 45-35 for only a few miles total, cruise control used the entire time. On the way up I had the ac on for all highway for +55mph areas and windows down through the city while on the way back the ac off and windows only cracked an inch. I re did my front undertray so it levels off with the chin spoiler, have a belly pan with only the exhaust, catalytic converter, tranny, and oil pan exposed, rear undertray sloped at 10 degrees with a little duct for the hot air from the exhaust to follow out by the muffler and pieces of coroplast to guide air from the wheels that comes inside out the rear bumper. My car is lowered for those of you whom have never seen it, the front of the chin spoiler is 5" and the sides are 4" from the ground and the skirts are around 6" from the ground. I have rear tire deflectors that I made as well. Additionally I sealed all the big gaps around my headlights and hood with weatherstrip and foam. I partially have my front grill blocked and covered the foglight holes. After doing all of this I only netted 37.55 mpg. I know I should be thankful for just that but it makes me mad all the work and money I have put into my undertray and belly pan and sealing up things and striving for aerodynamic improvements. Yet the last time I made the trip it was the same, 37.55 and I didn't have undertrays, tire deflectors, no sealing, belly panels, fog light holes were exposed, grill wasn't blocked at all, and my dad had the window open about 4 inches, nor I didn't have cruise control. Additionally that mileage was with other city driving and a few other highway trips on that tank as well.
Does anyone have any suggestions because I just saw on here another saturn owner with the same model but a 99 and got 38 mpg with no mods and bone stock and in much colder temperatures. I thought I should have gotten at least 40 mpg if not more all the time I have put into improving my aerodynamics.

[Jay2TheRescue]

Florida has been phasing in E10, I think the entire state is E10 now. Could it be possible that this trip was E10, and the previous trip was E0? Also, changes in headwinds/tailwinds can make a difference too. I would not make a hasty decision based on one run.

[shonus]

As far as fuel I'm not really sure, the last trip was about 2 months ago. Thank you for pointing out about headwind jay because there might have been one on the way there because something felt weird like my car either had a lot of down force or something hanging down acting like an air brake but I checked and everything was still all buttoned up.

[theholycow]

One trip does not a trend make. Jay pointed out a couple variables but there's always more. It takes a lot of data to determine the effect of a chance. It's a long-term thing.

That said, aerodynamic modifications can be tricky. Sometimes you try aero ideas that usually work and they don't work. Given a wind tunnel, smoke wands, and an expert, you could figure out what went wrong...there's always things mere mortals can't understand or can't predict.

[shonus]

I will try to get up some pictures today and perhaps that will help with some assessment. I understand what you are saying holy and I always appreciate your input, this was my first time that I actually had both undertrays on for an all highway speed run and was hoping to get accurate data because I already know my combined FE to be 35-36 mpg which is about 8-9 mpg above what I'm supposed to be getting. It discourages me when I see bone stock saturns on here getting about the same mileage and some even better. Could my tires being 4 psi lower than max made a huge difference?

[Jay2TheRescue]

Exactly. An extended air dam did not work on my pickup truck, I eventually ripped it off. Some things work, some don't. Each vehicle is different.

[Jay2TheRescue]

Tire pressure can make a difference, and when it comes time for tire replacement, a good set of LRR tires would help too.

[theholycow]

It's difficult to compare to other people. Here are some major variables that usually differ, other than configuration and condition of the car itself:
- Route
- Driving
- Traffic
- Road conditions
- Fuel quality

Route: One person's route may go up and down hills, another up and down mountains, and another on flat land.

Driving: No matter how much we discuss driving style and techniques, there will always be major differences. We are aware of some, and maybe unaware of others. Does one driver open the throttle faster than the other? Does one define "long before the red light" as 300 feet and the other 2000 feet? Does one make rougher steering corrections driving straight than the other?

Traffic: For one person, highway driving might mean drafting a tractor trailer at a steady 60mph with 10 miles between on-ramps. Another might have to jockey for position every mile when traffic gets on and off. Still another may have constant changes in speed, or be in stopped traffic on the highway.

Road conditions: Is one on smooth, low rolling resistance concrete while another is on rough-aggregate bitumen and still another is on a highway full of potholes and expansion joints?

Fuel quality: Is one person getting 4% ethanol and other 15%? Does one get a lot of absorbed water in stale fuel and another clean fresh fuel?

The more I learn about this stuff, the more vague it all gets. That applies to fuel economy as described in this post, and aerodynamics as described in my previous post which works the same way except us mortals will never have the same level of understanding that we have of fuel economy in general. Where is the flow laminar and where is the flow planar? When you smoothed your undercarriage did the reduced turbulence and drag there help, or did it have a larger negative effect on flow somewhere else? That's why it's trial and error.

I guess my point is don't feel disappointed in yourself. You can't change what your last tank was. Just keep looking forward at trying again, maybe making minor adjustments.

[benfrogg]

Shonus-
Is it my understanding that you used cruise control on your most recent trip with aero mods? And that the previous trip without aero you did NOT use cruise? That could easily be your loss of FE point. Cruise (under most circumstances) uses MORE fuel than hypermiling/not using cruise.
Do you have an instantaneous way of measuring FE?
B

[shonus]

Thank you again to everyone for replying to this thread and may I apologize for replying so late, I have been busy with soccer tournaments and working underneath my car.
I have done much since I first posted this and have since covered the tranny and oil pan with only a small cut out for heat to escape. The cat is still exposed and the rest of the exhaust and I think I will keep them that way or cover them with an aluminum sheet. My only concern being stuck in a traffic jam.

-Holy thank you again for pointing out all the different factors that I should consider when trying to compare my car with others, I forgot about comparing the roads and fuel quality.

-Ben yes everywhere I go now I use my cruise control because I can't maintain a steady speed to save my life and I thought it was more efficient because it will not fluctuate like your foot can on the throttle?
When you refer to hypermiling are you referring to using say a pulse and glide?
I don't do that anymore because I'm either in areas where it is too congested and I will piss people off or I annoy my passengers and have to listen to them complaining the entire time.
I already have people flying around me to a red light because they don't like that I'm in neutral coasting.