This is probably a stupid question, but. . .

We are not supposed to run our gas tanks dry.
We are not supposed to top off the tank.
So, can we really get accurate MPG calculations?

Consider this scenario:

Let's say my tank is getting low.
But I buy only, say, 7.21 gallons of gas on January 1.
(I don't fill it up.)

Then I drive a while, but the tank is NOT getting low.
Then I buy, say 3.12 gallons of gas On January 10.
(Again I don't fill it up.)

Then drive till I have 1/4 a tank left (or whatever).
Then I buy, say 6.53 gallons of gas On January 18. . . . etc..

As long as I document how many many gallons I bought - and how many miles I drove since the last gas purchase - it doesn't matter if I filled the tank or if I topped it off.

Also, I think it does not matter that we are not running our tanks dry, and not filling up with any particular amount of gas left.

I will still get accurate MPG calculation.

Right?

In other words, I guess my question is, do we need an exact repeatable physical reference for low (such as a bony-dry empty tank, or the instant only one bar is left on the fuel gague) or an exact repeatable physical reference for full, such as in topping off the tank.

Actually I think it IS necessary to run bone-dry to topped off to get accurate MPG calculations, I think I just pushed the scenario to the extreems to make the inaccuracy more obvious.

 

I suppose the most accurate way would be to first ensure your tank is bone-dry...that is no puddles of gas remains in the tank or any in the lines up to the injectors....then measure a single gallon of gas....not by the pump calculation but by what ever way gallons are accurately measured....pour that gallon of gas in and see how far you can go until it stops.

Because that is not very practical I just fill the tank till it clicks off.
I personally don't mind if it varies slightly.
This is the way MPG's have been calculated since the gravity gas pumps dissappeared.
(I guess that's what they were called)

 

Over time the per tank inacuracies will average out and the average will be acurate.

 

I run a tank nearly dry and then always fill up the same way.

It's the way I do it and I never, ever put 10$ at the time...that's just useless IMO.

The way a diesel tank is arranged, it's fairly easy to fill it up the same level everytime for max accuracy fuel economy calculations.

VW puts a vent button, that when you push it you hear air coming out.

That is called "venting" in dieseler's term.

When venting you fill up the tube that goes from the tank to the brim of the filler neck.

Mine holds 25 litres, most Golf/Jetta/NB will hold about 7 litres.

It's the way I and many TDI owners do it so it gives you a good idea of your mpg per tank to a pretty accurate level.

 

Hi All:

___Just one more reason why I fill from almost dry to max is so I don’t have to waste my time nor burn up a few ounces of fuel just to drive to the station and back or stop and start from 0 mph with a slightly colder ICE after the fill up. When I run extremely low near work, there is a BP only about 1 mile out of my way so that one isn’t so bad. When I run low at home, it is ~ 10 miles roundtrip of wasted effort just to fill the little beauty up. God knows I wouldn’t want to waste any fuel like driving around the state a few times for no ungodly reason

___I also know within .1 to .2 mpg what my actual is other then when I fill the vapor recovery HW. Then I have to wait until I burn off that tank to average with the previous for an accurate fuel economy reading. Filling up at 2 to 4 gallon intervals will never tell how you are doing on that last tank in the least. You will eventually get an accurate lmpg but what about tank over tank? They would be all over the place if you didn’t fill her up. An example is if I placed just 4 gallon in her now after 1147 miles. That is over 250 mpg which is absolutely meaningless as you can well imagine.

___Good Luck

___Wayne R. Gerdes
___Hunt Club Farms Landscaping Ltd.
___Waynegerdes@earthlink.net

 

I agree, no one will purposely run the tank dry. If you fill a bit at a time like you're describing, your lifetime mpg will certainly be ok.

What you'll miss is the ability to compare the mileageyou get out of different tanks of gas. Say 87 from Shell, vs 87 from Amoco/BP, or one with ethanol, etc.

I think it's more of an exercise in futility, but I'm certainly interested in trying it out!