Car was with mechanic - let idle for a while - how to handle?
February 12, 2012 1:46 PM 
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You could just skip your next fuel-up and pick up again next time. Or maybe just add a note about the type of driving that happened to help you remember why things might be a little off there.
Ok - I think I'll just do the "I missed a fuel-up" when I fill up next, which should then ignore my odometer from the last fuel up to the next, right?
Dude, no reason to leave out a bad tank MPG due to maintenance.
I have had a couple of sessions such as these, I just made a note about it, and left it as is. Tracking your fuel mileage isn't like sorting good from bad fill ups.
Just because someone gets a bad mpg one tank (in my case, the van) does that mean I should not report it, so it doesn't throw my numbers? No. All those are considered part of your real life usage. So don't throw it out, just add it in, it will be fine. The average will swallow the difference and months, years from now, the impact of that will be nothing on your average.
My van has over 160 fill ups. Nearly 45 thousand miles. The fiance is driving it for in-town use right now. The MPG has gone from an average of over 21 to 11 with her driving. I still count them, even though they are actually dragging my average down. You only have one that will affect it only slightly. Don't worry about it! Winter for me also brings my averages down, but I still count them.
You can can check "missed a fuel-up" and Fuelly will skip an individual tank calculation there. And you can use Fuelly however you want but I sort of feel the same way DTMAce does. Stuff happens over the life of your car so why not record it so you can go back and see what happened when?
While I agree, this is not ordinary use. I would never let my car idle for 1 - 1.5 hours. Since I only have 1 or 2 fuel ups it's going to WRECK my average. Sure, it'll come back over the life of the car, but given I drive maybe 70 miles a week, that would be quite some time before I get a real handle on what the overall average is.
I consider idling for that long and driving hard to get the catalytics warmed up for emissions testing to be an extraneous event that shouldn't be considered in my MPG calculation.
Idling for 1.5hrs miiiiight have burned a half gallon of gas at the most. Most cars burn less than .3 gal/hr at idle. So even in a 10gal. fillup, at 22mpg, that would be a loss of less than 1mpg. Hardly going to wreck anything. How bad is the calc average for this tank that your mechanic ruined anyways? And how much harder did he drive it than you might if you had a wild hair or a favorable onramp? I hope you enjoy your beautiful old BMWs more than your mech...
I've never discounted a tank for any reason, just left notes for this kind of thing.
FWIW I dropped the car off with about a full tank and picked it up with just under 3/4 and only a couple miles on the odometer. It was definitely more than 1 gallon, or maybe it was longer than 1.5 hours. Either way - it doesn't represent my driving at all so I skipped it.
My inspection mechanic is a friend - considering the odo didn't move more than a few miles I know he didn't drive the car much at all. The emissions station is like a mile or two away so there's no way he did any thing "bad". The car just idled a tooooon.
Thanks all!
My mileage log includes time on a rolling-road when I was getting it remapped, and also time going around Rockingham race circuit. It's just part of the life of the car, so I still log it.
You can log how you see fit. I wanted to log this car without the endless idling, so I did :)
I wouldn't consider my other BMW (685 RWHP 525) dyno tune on mileage - the thing can empty the tank in like 10 minutes on the dyno lol
What you can do if you really want to ignore that data without losing the data from a full tank, is just fill up immediately before and after (as soon as you pick up the car) and ignore that fill-up.
That way you just throw out the data you think is bad. You don't have to wait until the tank is empty to fill up if you aren't going to use the data anyways. Throwing out a whole tank won't affect your mileage much, but it will affect your cost of ownership data.
What was the average for this tank that you threw out anyways? Just out of curiosity.
^^ Not sure what the average was. I had dropped the car off with an OBC (on-board computer) MPG average (pretty close to what Fuelly indicates) of 22.8 - 23.8 MPG. Got the car back with an 11.2 MPG rating. I can find the odometer info I think in fuelly somehow and manually calc out the average.
Looks like it went from 202442 to 202667 and took 11.72 gallons to fill, so it would avg 19.1 MPG for that partial tank.
I would still add the fill-up otherwise it's kind of like cheating your way through the best MPG. You shouldn't skip any fill ups, I almost did that once. I get 10-20% worst mileage in the winter and I tend to idle my car during cold mornings when I have to go to work.
I idle my car a bit before I leave for work too (I am in PA), but I count that. I just don't count the time the car is being driven in a manner other than usual. I think it makes sense. Like a manufacturer would only record MPG on a test track, not on the way to the test track.
My only point to add to this, is when you throw out that tank, you also throw out the amount you spent, so it affects more than your mpg. As I said before, the impact will be non-noticeable after you have had a few. I have had to get 3 tanks in a row at 12 mpg to actually cause the average to go down from 20.9 to 20.8. Crazy.
Its your fuelly, but still, something to think about.