Good days/bad days
 

I've been keeping a daily log of my commutes (40 miles each way) to and from work for almost 2 months now. I don't have a Scangauge, so for now I'm just using my in-cab display. It only gives trip mpg--not instantaneous mpg. At some point I know I should just give in and buy a Scangauge, but let's leave that aside for now. My question is about good days/bad days.

Some days I just seem to have an exceptionally good day or an exceptionally bad day for no apparent reason. By this I mean about 10% better or worse mpg than a "normal day". I keep track of the weather (temp, press, humidity, wind) and there doesn't seem to be any correlation there. I know sometimes I just seem to be able to catch/time most of the red ligths well (or sometimes when it seems like they all catch me), and there's always traffic variations that allow me to do more/less hypermiling.. It just seems like these variations I see are more than that.

What's somewhat strange is that it seems to show up early. By that I mean, sometimes I can just tell within the first mile or two, "This is going to be a really good (or bad) day!".

I guess I just wanted to know whether others have had similar experiences and/or figured out what's going on. I'm not really concerned about anything--just curious.

Most of the fuel my cobalt burns is during the first 5 minutes, I spend the next 1.5 hours catching up. How you drive the first 5 miles has more of an effect than how you drive the next 30, because FE is weighted moreso by bad FE (even short durations) than good FE. AKA it takes a lot of "good" driving to make up for a little bad driving.

Around here I can usually hypermile a lot during warmup so it has less effect (I doubt my car really warmsup on my daily trip)

I'm not sure it's quite as extreme as what you say but, yes, I do think that the "warm-up period" has a big effect on FE. I'm working on ways to help that. I'm already trying out a partial grill block. It didn't seem to make much change in the warm-up time. I'm looking to do a full grill block. I'll also start using my block heater soon (it's already installed and I just ordered the cord for it this morning).

Sometimes after a few things ruin a trip (heavy throttle, bad light timing), I get discouraged and don't try as hard, even though the better response would be to try harder to recoup lost energy.

But your problem sounds more tangible, so perhaps the above tidbit is irrelevant.

Diesel_Dave -

You can use the fuel log to calculate how much you have saved. Your "Total saved:" is $829.22, so you could argue that the ScanGauge is "free".

In terms of MPG, the weather has a good correlation for me, but I can't win when it comes to traffic. I used to have control over my schedule so that I could avoid the worst commute time, but now I have to drive into the teeth of bad traffic and life stuff gets in the way. That has cost me about 3 MPG, so I am no longer going for summer 90-day MPG records.

There is also the possibility that I need a tune-up.

CarloSW2

Hmm, I'll see about justifying it that way. I'm sure I'll give in and but one eventually.

As far as weather and traffic go, I know they make a difference, but what I'm talking about is 2 days which seemed (at least to me) to have similar traffic, similar weather, similar driving style and 1 day has much better FE than the other.

What I'm trying to figure out is 1) Is the FE difference real, or just my computer 2) If it's real, what's happing on the good days to make them good (so I can do it again).

What made me think of this is that I set a new daily commute record today. 43.0 mpg. Previous record was 41.0. My average for this setup has been about 37-38. If it's real I want to do it again! BTW, I know it reads around 10-15% high (on average).

This happens to me all the time. I drive the same route 5 days a week, and some days are just plain better or worse then others. I can't explain it as the conditions are the same, traffic is identical, etc but it happens.

From what I can gather from my 2008 Nissan sentra, the trip calculator uses the exact same data as the scangague. The trip calculator on the car seems fairly inacurrate and speratic shortly into the trip but once it settles in, the tip on the SG and the car are identical. I know the SG is accurate because I have verified that tank after tank. I suspect your variation comes from other factors. Those "other factors" are whare the SG really shines. Sometimes you I don't know that the wind changed or I am giving too much gas on a hill untill I see my SG display a sad number and I use corrective acction.

Having said that, even with the SG, I find some days it's a stuggle to get the MPG's up and sometimes it's easy. There are so manny variables. I find I tend to set my MPG reccords near the end of the tank. "Getting lucky" on the lights to prevent full stops it HUGE and you don't realize how big a deal it is untill you get a SG and see your mpg's drop to single digits as you get up to speed.

To sumerize, get a SG! You will be glad you did. The sooner you get it, the sooner it will pay for it's self. I was able to pick up 2 mpg imediately when I got mine, and much more since then. I only drive 150 miles a week but I estomate it will have paid for it's self in 12 months. Not bad!

graydon, I agree with you totally on the "other factors". I've been able to learn a little bit more about some of these things more recently.

Wind seems to make a bigger difference than I had previously thought. I keep track of the weather in my log (from a local weather station on Welcome to Weather Underground : Weather Underground). Interestingly, 5 of my top 7 commutes have been when there's no wind at all. This even though only 16% of the days have no wind. Also, previously I'd only paid attention to component of the wind that in my direction of travel (head wind or tail wind). Based on my data, I now see that the cross wind component actually correlates more highly with FE.

Also, like you said, sometimes you just catch the lights wrong. What I've begin to notice, as well is that it can make a big difference WHICH lights you catch badly. I think bad things that happen early (before things warm up) have a more negative impact than they would if they were near the end.

Also, I've begin to notice that my focus can make a big difference. Sometimes if I'm not paying attention and really focusing, I can do quite a few things to hurt my FE without realy noticing--like not really looking ahead for the lights and trying to catch them right.

Just like a race (cycling or any other), sometime everything works out just right and you really blow yourself away but don't get cocky and slack just because "you rock at this" or it will start to slip back to your flawed old ways!