MPG Phenomena

[Bearhog]

I was not sure how to search for this one so just decided to post.

Pretty much everyone has experience the phenomena where MPG is better when the tank is filled sooner, i.e. fill when fewer miles have been traveled and fewer gallons are required. My first guess would be that as the truck travels more miles, your average fuel economy drops due to more varied driving conditions. I believed this to be the case until today. Filled up my truck this morning and immediately jumped on the interstate and proceeded to travel 23 miles in an hour. All stop and go on I85 in ATL. So out of curiosity, I filled up at 53 miles and got this result. I averaged a nearly 2 MPG improvement over my normal average. 16.1 vs. 14.3. I figured I'd have some of the absolute worst driving conditions for MPG but low and behold, better MPG.

Anyone care to hazard a guess? I'm baffled.

 

[emjohn4]

Margin of error is greater on a smaller fill-up.

Running the tank closer to empty gives you a smaller margin of error, and therefore a more accurate calculation of MPG.

 

[DarrenT]

I'm happy to accept that my MPG is a constant variable rate. I know I average around 15/16 but that depends how I drive, the surface I drive on, the tires I'm using, the aerodynamic coefficient of the vehicle etc etc, too many variables to accurately provide a figure. Or at least a constant figure that doesn't require re-calculation every twitch of the gas pedal or gust of wind.
Far too much time is spent worrying about how much gas we burn, and not enough time appreciating the vehicle itself it's a truck with a big motor, miles per gallon, or gallons per mile?
I know my dad's 2001 sport holds the same amount of gas, but out does my 96 by over 100km per tank....
I have torque pro reading back from my diagnostic port and doing calculations based on ECU and GPS measurements. That's about as good as it gets.... I'd say save yourself the headache and grab a $10 OBD2 adapter along with the $5 for torque pro app and let it do the rest. You might find there is actually no phenomena so to speak, just inaccurate estimates.

 

[Bearhog]

Great point. No argument at all on the greater margin of error on short tanks.

'94 is OBDI. Is there anyway to hook up a real time monitor such as a Scangage to a pre OBDII.

 

[DarrenT]

Great point. No argument at all on the greater margin of error on short tanks.

'94 is OBDI. Is there anyway to hook up a real time monitor such as a Scangage to a pre OBDII.

My bad, I negated to check the model year..
You can get OBD1 to 2 adapters, and attach the newer scanner module. Don't always work though, I've only ever seen the code scanners for OBD1 so I couldnt testify as to functionality though I'm guessing the ECU's run largely the same PIDs.
In all honesty, I spent $20 on my setup a few years ago, and it's proved its worth many times over, so if a shot with the obd1->2 adapter doesnt work, you've still got a highly effective obd2 diagnostic kit for all your other cars.... or friends, or family, or strangers in the street as it happens... they all come out of the woodwork when they know you have it

 

[crazzywolfie]

you can do it the way the old timers did it back in the 40s and 50s. Simply install a vacuum gauge and drive the car keeping the engine at maximum vacuum all day long.

 

[jawzs2]

you can do it the way the old timers did it back in the 40s and 50s. Simply install a vacuum gauge and drive the car keeping the engine at maximum vacuum all day long.

I remember my grandfather driving his Ford that way in the rain to keep the wipers going fast enough

 

[crazzywolfie]

lol it does work. i am thinking about getting one for my daily driver. i got one in my 81 already. they also come in handy for detecting if your engine is having issues or vacuum leaks like a blown plenum before they get super bad