How many gallon fillup for accurate MPG readings?

[darcane]

As everyone else said, the larger the fill the more accurate the results.

Therefore, I recommend at least 30 gallons per fill.

Mike
(It works for me)

[gone-ot]

...with the SGII newly installed on my '09 Vibe, I noticed that after the third fillup that I didn't have to "tweek" the ±gallons value unless: (a) I had filled up at a different station (BIG change!) or (b) was using a different pump at my usual gas station (small change).

[taredog]

Thanks for the link from "full tank vs. partial fill ups..." to here. I'll bump this year old thread.

FWIW, and it ain't much, I have found over the years that filling up the tank when it is 3/4 or more full almost always gives me a better FE figure.

This seems to be true now and goes back to the days before auto shutoff pump nozzles and when you could see the gas in the tank or filler neck after carefully listening for that sound of fuel coming up the neck. It was easier to make sure you filled to the same level every time. Same thing after the auto shutoff nozzle. After it clicks off, you top it to the same level.

Unless of course if you were going to park it in the sun for a few hours afterward.

This is purely anecdotal as they say. All those pocket note books I carefully kept over the years are gone.

I have no explanation for this nor will I offer one. Just sayin'. I would see up to a 10% improvement when only burning a few gallons out of the tank.

IMHO, burning it down to 1/4 or less is more accurate.

[jamminjimmy]

thanks for the input...

I was on a different line of thinking, I guess. I'm not looking for very, very tight accuracy(+- 1 mpg if fine with me considering ALL of the many variables that affect car MPG readings). Also, my crx only has a 10 gallon tank- so its really not that accurate to begin with.

It averages in the 37-39 mpg range and I knew I didn't want to wait weeks between fills(about a month if you average three tanks for decent mpg feedback) to find out if one modification was worth it or not... At that time, I had many modifications planned.....

I was looking for a consensus(that I could prove/disprove on a quicker method for somewhat accurate results(quicker testing, but also fruitful testing)- I ended up buying a mpgguino and called it good....

thanks for your input!

[Joenavy85]

Originally Posted by gasstingy

I rarely ever buy fuel until I'm down to 1/4 tank unless on a road trip, then I buy fuel when I have the need to use their rest room.

Rule of Thumb for Road Trips: "Fill your car's tank when you empty yours"

[neilzero]

I think the problems with frequent small fill ups is exaggerated. If you average over 50 gallons it should not matter if this was 4 fill-ups or 10 fill-ups. Neil

[roosterk0031]

It all averages out, if I used less than a tank a week but more than say 5 gallons, I'd pick a day/station/pump before or after work and refill weekly.

[darcane]

Originally Posted by neilzero

I think the problems with frequent small fill ups is exaggerated. If you average over 50 gallons it should not matter if this was 4 fill-ups or 10 fill-ups. Neil

It's not averaged, frequent small fill-ups that is a problem, it's the individual small fill-ups.

You aren't going to get to exactly the same level every time you fill, it just isn't going to happen. The volume of the fuel in your tank is going to vary a little with temperature, the pump doesn't shut off at the same point every time (espeically with different brands), a slight incline one way or the other will affect the fuel level at the fill hose, etc. Factor in all of these and the amount of fuel in your tank will vary.

You know how much you pumped in, but that isn't how much you used since you last filled. How much you used is going to vary based on the conditions when you last filled AND the conditions from your current fill, which doubles your error. In other words, you may have been at one extreme last time and the other extreme this time.

With a big fill, you may not notice this, with a small fill it will definately affect your mpg calculation for that tank. Not a big deal if you average 10 small fills, but if you use that one small fill to determine how effective your recent mod is, then it has to be a BIG change or it will get lost in the measurement error.

[Joenavy85]

Originally Posted by darcane

You know how much you pumped in, but that isn't how much you used since you last filled. How much you used is going to vary based on the conditions when you last filled AND the conditions from your current fill, which doubles your error.

Yep, this is why we always have to adjust the calibration on our SGII's and UG's.