Got my Scangauge-pretty eye opening - Fuelly Forums

Click here to see important news regarding the aCar App

Go Back   Fuelly Forums > Fuel Talk > General Fuel Topics
Today's Posts Search Click Here to Login
Reply
 
Thread Tools Display Modes
 
Old 05-22-2008, 07:43 AM   #1
Registered Member
 
Join Date: May 2008
Posts: 85
Country: United States
Got my Scangauge-pretty eye opening

As I was just posting somewhere else, it's amazing how insightful this little doo-dad is. Oh man, all I can say is I'll never use cruise on anything but level ground again. Watching it on the scangauge, the difference was when it was on, going uphill I'd drop to around 9 mpg even though I was still maintaining vac and not slipping to turbo boost. Off, just letting the car lose a few mph, I was able to keep the instant miles in the high 20's/low 30's. Unreal.
__________________

__________________
bkrell is offline   Reply With Quote
Old 05-22-2008, 09:50 AM   #2
Registered Member
 
Join Date: Mar 2008
Posts: 408
Country: United States
That's very interesting. Is your car automatic or a stick? If the former, did it switch to a higher gear when you eased off the throttle? Just curious.
__________________

monroe74 is offline   Reply With Quote
Old 05-22-2008, 09:59 AM   #3
Registered Member
 
Join Date: May 2008
Posts: 211
Country: United States
Quote:
Originally Posted by bkrell View Post
Watching it on the scangauge, the difference was when it was on, going uphill I'd drop to around 9 mpg even though I was still maintaining vac and not slipping to turbo boost. Off, just letting the car lose a few mph, I was able to keep the instant miles in the high 20's/low 30's. Unreal.
I've noticed that effect too.. Efficiency plummets when you try to keep a constant speed going uphill. I've found that gradually slowing going uphill, a slight speedup when starting downhill then coast until near the bottom and an acceleration near the very bottom or on the flat between hills seem to boost overall efficiency the most.

Some kind of "adaptive cruise control" that would read engine load and efficiency as well as speed would be very useful.
__________________
94 Altima 5 spd.. Stock.. 29 mpg combined with basic hypermiling techniques ..

89 Yamaha FZR400 Crotch rocket, semi naked with only the bikini fairing, no lowers, 60 plus mpg

87 Ranger 2.3 5spd.. Does not currently run..
fumesucker is offline   Reply With Quote
Old 05-22-2008, 10:13 AM   #4
Registered Member
 
GasSavers_RoadWarrior's Avatar
 
Join Date: Feb 2008
Posts: 1,652
Beware the instantaneous MPG leading you astray, it's not as efficient to accelerate at 23mpg for 2 miles when you could accelerate at 15mpg for 1/4 mile to get up to your 28mpg cruise....

.01666g +.0625g < .0869g
__________________
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice
GasSavers_RoadWarrior is offline   Reply With Quote
Old 05-22-2008, 10:23 AM   #5
Registered Member
 
Join Date: May 2008
Posts: 85
Country: United States
Quote:
Originally Posted by monroe74 View Post
That's very interesting. Is your car automatic or a stick? If the former, did it switch to a higher gear when you eased off the throttle? Just curious.
auto. Stayed in 4th (O/D) the whole time.
__________________
bkrell is offline   Reply With Quote
Old 05-22-2008, 10:29 AM   #6
Registered Member
 
Join Date: May 2008
Posts: 85
Country: United States
Quote:
Originally Posted by RoadWarrior View Post
Beware the instantaneous MPG leading you astray, it's not as efficient to accelerate at 23mpg for 2 miles when you could accelerate at 15mpg for 1/4 mile to get up to your 28mpg cruise....

.01666g +.0625g < .0869g
Right, but I was just talking about hills and momentum. Good point, though
__________________
bkrell is offline   Reply With Quote
Old 05-22-2008, 10:40 AM   #7
Registered Member
 
Join Date: Mar 2008
Posts: 408
Country: United States
Quote:
Originally Posted by bkrell View Post
auto. Stayed in 4th (O/D) the whole time.
Thanks for answering the question.

Road is making a good point. Reading instantaneous mpg can be fundamentally misleading. That's because the SG (or similar instrument) is taking into account how much distance the car is covering, but it's not taking into account work that's being added to the vehicle, in the form of potential and kinetic energy. That energy will pay you back later. And it's also not taking into account the duration of the energy use (which is basically what Road was saying, I think).

That's why it's important to make comparisons based on an entire trip, and not just a series of instantaneous readings.

And these issues are separate from other accuracy issues that come up with something like the SG, associated with the fact that it's not directly monitoring the injectors.

It's still a great tool, but it helps to understand the limitations.
monroe74 is offline   Reply With Quote
Old 05-22-2008, 10:51 AM   #8
Registered Member
 
Join Date: May 2008
Posts: 85
Country: United States
Right, but I'm not talking about prolonged situations. I'm talking about dropping a few mph on the upslope instead of maintaining constant speed. Doesn't that make sense or am I wrong?
__________________
bkrell is offline   Reply With Quote
Old 05-22-2008, 11:13 AM   #9
Registered Member
 
Join Date: Mar 2008
Posts: 408
Country: United States
Quote:
Originally Posted by bkrell View Post
Doesn't that make sense or am I wrong?
Good question. I don't know. What I'm saying is that the math is deceptively complicated.

Here's an example of what I'm talking about. Consider this comparison:

A) Travel up the hill at a steady 25 mph
B) Travel up the hill at a steady 30 mph

Contrast that this with comparison:

C) Travel up the hill at about 24-26 mph, but with the speed slowly dropping
D) Travel up the hill at a steady 30 mph

When an instrument like the SG looks at C, it's going to give you a deceptively optimistic result. Why? Because it's calculating mpg based on fuel use and distance, but it's not taking into account the fact that you're losing speed. The momentum you're losing was paid for previously, by gas you burned a little earlier. And now you're relying on that momentum, instead of fuel, to propel the car. That's why the car is slowing down.

The reverse happens when you're accelerating. The SG will give you a deceptively pessimistic result. It's going to tell you that you're burning a lot of fuel relative to the distance you're traveling, but it's failing to take into account that some of the fuel you're burning is being used to create momentum in the vehicle. It's like putting money in the bank. In a few moments, you'll be relying on that momentum.

When you add speed, you're adding kinetic energy, and when you climb a hill, you're adding potential energy. Either way, you get this energy back later. But the SG is not taking this into account, if you're just looking at an instantaneous reading. That's why you have look at a broader interval of analysis, like a trip.
monroe74 is offline   Reply With Quote
Old 05-22-2008, 11:27 AM   #10
Registered Member
 
Join Date: May 2008
Posts: 85
Country: United States
Gotcha. I already have a resettable trip gauge on the car's computer. This is what I've always relied on. I understand what you're saying, but my question is this...as long as I am staying in OD gear, wouldn't it still make better sense to use momentum even if you get deceleration that you have to make up somewhat on the downslope? I mean, aren't you loading the engine more by keeping steady speed on the upslope?

I barely passed physics. That's why I am an archaeologist.
__________________

__________________
bkrell is offline   Reply With Quote
Reply


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Saturn TDI for S-Series cfg83 Diesels 14 10-02-2007 02:24 PM
Anyone have experience making their own solar panels? MetroMPG Electric and Solar powered 17 08-29-2007 06:43 PM
Hovercraft 88HF General Fuel Topics 2 06-21-2007 11:41 PM
Belated Intro GasSavers_brick Introduce Yourself - New member Welcome 4 12-22-2006 07:48 AM
Atlanta Auto Show GasSavers_DaX General Discussion (Off-Topic) 26 05-07-2006 06:02 PM

» Fuelly Android Apps
No Threads to Display.
Powered by vBadvanced CMPS v3.2.3


All times are GMT -8. The time now is 07:28 PM.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2024, vBulletin Solutions, Inc.