Filling the tank with repeatabilty
 

People usually say to get an accurate reading, you gotta use the same pump at the same station all the time. I think it is because there is a huge difference in pumping speeds between stations.

So heres what I have been doing for years to get repeatable results over pumps from anywhere. And it works really well.

First of all, if the car is sitting considerably nose up or down, itll affect the fill. But I only know of a couple of stations like that, and it only affects me 1 mpg one way or the other. Next tank averages out. I tend to avoid those stations unless the price is better than the others.

Then what I do is always pump the same speed. Which is about 0.1 gallon per second, I just imagine a clock ticking in my head as I watch the gallons readout. Then when it kicks off, I pump about half that speed till it kicks off again, and it usually does in about half a gallon. Then I call it good.

Actually, this started back with one of my old cars, when I had a problem pumping the gas too fast or itd trip the spillover valve and stop the pump. Then I just got used to pumping slower and once I started really logging mpg it came in handy.

Does anyone else do this? Or does anyone else do anything better?

my car seems to suffer from this - sometimes the gauge is stuck way over full, and others at about7/8 full. It seems the load in the car stuffs up the fill as well. It makes for some great mpg numbers - good and bad depending on the previous fill. I always just stop at the first shut off - but i'll experiment with your slow fill method next tank or two and see if it helps - i'll be back in a month or 2 - only ever fill up every 3 or so weeks!!!

Sometimes my Neon tank needs to "burp". So I've taken to bumping the car pretty hard after the nozzle shuts off. Almost always good for another 1.5 gallons...

Wow, I didn't know about this. I guess I've been lucky that I prefer to not only use the same station but the same pump. Does it matter if I pump the gas manually or use the clip on the handle? I usually leave the pump going automatically and wash my windows or check my tires in the meantime.

My car can't even take full speed on the nozzle.. it just clicks off right away.. so i do the following..

The nozzle always goes in a certain way.. otherwise it clicks off early on my car.. seems the filler neck is just slow draining..

I always use the same station.. when I can the same pump.. but all there pumps are the same type.. I set the gas nozzle "cruise control" to the first little notch on the nozzle... let it fill, then when it clicks off I just trickle it in (half of the first notch on the nozzle) till it clicks off again.. the i quit.. It never over fills it but I found that if i didn't trickle it in at the end id get off fills..like .5 gallons difference sometimes.. at 50 mpg that can be a big difference.

I don't think we can 'learn' much from taking mpg readings from one tank of fuel... sure, we might get bragging rights that justify something to somebody (may only ourselves) but we simply can't do accurate enough filling in one tankful to defiatively learn weather or not that latest driving technique or do-dad made 1mpg difference or not... the sample size is too small, the equipment not accurate enough, and the environment is not controlled enough... that's the bad news.

The good news is this, so long as all the fuel from the pump goes into the tank, (not spilled on the ground) and the pump is measuring accurately (controlled calibrations performed (monthly?) by the State Board of Weights and Measures) averaging takes care of the uncertainty of 'how much fuel did I just burn?'. Through averaging you will always get an answer to the question "what's my mpg?" your real fuel usage, which is what you care about anyway.

Keep in mind that the error is proportional to how much you put in the tank. In other words, an error of +/- 0.5 gallons will be 5% if you're putting in 10 gallons, as compared to 10% if you only need 5 gallons to fill it.

Temperature seems to make a notable difference, too. I try to fill at the same time (preferably when it's cool outside) so that doesn't make so much difference.

Make sure you measure (roughly) the amount of nozzle in the filler neck. Some nozzles are more curved than others and some are straight, lastly don't jam the nozzle all the way in the filler, this really distorts the differences between nozzles. ---nozzle rolls off the fingers nice once you've typed it many times.

I use the same pump as often as possible, but it's not always possible. I just fill her till it clicks, and then I go for a second click just it case it clicked early. Usually the second click comes in almost instantaneously. But anyway, in the long run all those variations even up.

In Massachusetts they took those clips way from us. We are not to be trusted! Some people figured out that they could stick their gas cap in the handle to keep the nozzle open, so gas stations put up signs saying "do not stick any object in nozzle for continuous flow".