So, I've been pondering how to calculate real-world fuel economy that accounts for partial fillups. Let me know if you guys think it makes sense (I think it does mathematically).

For any given tank D-o is the Delta of the odometer reading at the fillup from the last fillup and D-v is the amount you put in the tank. Under normal circumstances, your milage is simply D-o / D-v. That works great as long as you fill up every time so you know truly that it took you that much fuel to go that far. So, I've begun marking the odometer reading and if the tank was full or not at each fuel add. readings look like this:

mysql> select fa_date, fa_vol, v_odo, full from fuel_add order by fa_date desc limit 10;
+---------------------+--------+-------+------+
| fa_date | fa_vol | v_odo | full |
+---------------------+--------+-------+------+
| 2007-10-27 12:53:00 | 16.677 | 21184 | Y |
| 2007-10-18 13:28:00 | 18 | 20862 | Y |
| 2007-10-12 08:46:00 | 10.005 | 20656 | N |
| 2007-09-30 15:00:00 | 19.173 | 20271 | Y |
| 2007-09-12 08:08:00 | 18.406 | 19869 | Y |
| 2007-09-01 13:22:00 | 19.273 | 19454 | Y |
| 2007-08-25 10:04:00 | 20 | 19065 | Y |
| 2007-08-15 20:31:00 | 19.556 | 18650 | Y |
| 2007-08-04 10:03:00 | 16 | 18248 | Y |
| 2007-07-28 14:01:00 | 14 | 17892 | Y |
+---------------------+--------+-------+------+
10 rows in set (0.01 sec)

- on october 12, there was a partial fillup. The way I figure, if you add the volume added that day to the NEXT time that you get a reading and use the odometer difference over two fuel-adds, it should give an accurate average over the total volume of fuel. That way, even though your datapoints might get a little farther apart, the overall fuel use curve should be accurate as long as you figure your datapoints when you do indeed get a full tank (in order to have a reference point for the next time).

What do you guys think? Too much math and computer stuff for a Tuesday?

P.S. ultimately, I'll be putting a web-based front-end on it. I'm also keeping track of fuel type, brand, which vehicle, etc, for some neat graphs. I'll probably give away the source when I'm done.

_________________

2005 Silver CRD Limited
245/75R16 GoodYear Duratracs
Fumoto drain | ProVent CCV Filter
Stanadyne FM100 filter | Cummins fuel pump
GDE Eco | SEGR | BoulderBars | FrankenLift | Frankenskids

I always fill the tank up (unless there's a problem with the pump)

take miles/gallons - plot on a trend chart

you can see patterns drifting upward or downward, it can tell you in advance if somethings going wrong - and a step function change you need to figure out why.

(you can also do an SPC chart but people think you're weird)

_________________
2005 CRD
stuff
Skeptic quod gratis asseritur, gratis negatur

I appreciate the idea. my visa has been limiting me to $50/swipe, which was not a problem with the $2.11 B20 I enjoyed when I purchased my jeep in 2005. however, the $3.39 B-nothing forced on me recently has been causing persistent partial fills.

my EVIC has been acting funny too, so i have no idea what the real, or fake, MPG is anymore.

Dgeist, your method, albeit that it looks rather complicated in your presentation - should be correct. I have used this method a few times in the past, except I would just use the trip odometer reading as Reset from the previous to the last FULL fill up and simply adding the amount of gallons (or liters). Then take total trip odometer reading divided by gallons for mpg. The key is not to reset the odometer until after a FULL fill up.
It boils down to total miles driven (from the trip odometer)(or last odometer reading less earlier reading at full fill up) divided by total gallons burned.

_________________
It's got a CRD, sweet! (KJ Newbie since 2002!)
- KJ 14 mpg replaced with WK 28 mpg
- OEM hitch and OEM tow harness added
- Reese Pilot brake controller

ya thats just what I do, keep the receipt in the cup holder and add those gallons into my fillup and total trip odometer recording.

_________________
http://www.Colorado4Wheel.com
"Its not about what you can DO with your Jeep, its about where you can GO with your Jeep."
Knowledgeable - But Caustic

Yep, that's exactly what the formula does. The big advantage for me is that all I need to keep is the reciept and note the jeep's odo reading at the top. All the rest can be extrapolated. I'm just trying to make it easy to keep track of and let the darned computer do what it's for (crunching numbers). Here's my first stab at a quick and dirty output: http://www.polter.net/~dan/automotive/. Eventualy, there will be select boxes for date range, vehicle, graphing, etc.

Oddly, I'm not getting as good economy as I thought I was... Darned EVIC.

_________________

2005 Silver CRD Limited
245/75R16 GoodYear Duratracs
Fumoto drain | ProVent CCV Filter
Stanadyne FM100 filter | Cummins fuel pump
GDE Eco | SEGR | BoulderBars | FrankenLift | Frankenskids