Manual Calculation of Fuel Economy

So I'm going to start manually calculating my MPGs.
I just started today..
@31,052 miles I put in 13.686 gallons...
I'll mark this down with every fill up.

MY QUESTION is...

How do i know what was already in the tank to complete the calculation?
I know my tank is 14.5 gallons.. does that mean I had (14.5 - 13.686) gallons of gas in my tank? What about fuel lines? what about reserve fuel? How do I compensate for these things?

Doesn't matter what you "start out" with, it's how much you use form now on. And w/o very accurate fill amounts each time, the results can vary a bunch until you get several tanks and average them.
Search out the "ventecomy mod". Makes accurate fills to the top of the filler neck easier/quicker.

Each fill simply divide the miles traveled by the gallons used.
Ex: 725 miles/14 gallons=51.79 mpg.
Keep a log book in the car. Makes it easier to remember to calculate each tank.

Unfortunately you will never know exactly how much fuel you used on each tank. When you calculate the MPG for each tank, bare this mind. The accuracy will vary tank to tank.

Keep a running total of how far you've traveled and the amount of fuel spent. If you use these totals to find the MPG, the accuracy will increase the more fill-ups you have logged.

Essentially every calculation will be slightly off because you never know exactly how much fuel is used per tank. But if you keep a running total, the only uncertainty lies with the last fill-up you logged. (i.e. sure, you put in 13.686 gallons, but how can you be sure you used 13.686 gallons on the last tank?) The higher the running totals, the lower the % of error because the amount you're off is so small compared to the hundred of gallons spent in total.

I hope that makes sense.

As stated, that fuel was there [in the lines, pump, etc] when you pulled up at the gas station, therefore you never really displace it...unless you are flat out of fuel and the engine won't start due to it.

However, you may find yourself going a little too far as to carrying it out 3-decimal places. Fuel density also changes with temperature and thus the volume of the tank. Fill up on a cold morning or when the tank is hot after sitting in the sun all day will alter results. Consistency with the nozzle kick-off will be another large variable [in terms of 1/10ths of a gallon of fuel] and if you lock the nozzle full open, partial open or just barely crack it. On top of all that, you have odometer discrepancies, which could be off 2-5% off the showroom floor and also tire wear and/or differences in tire brands for the same given size.

Within a single tank calculation, unless you have weigh tanks or a expensive fuel flow meter, you'll be accurate to 1/10th of a gallon at best at any given fuel station. As mentioned, your long term average will be more useful and be less sensitive to those tolerances.

jettawreck---pretty sure there is no "ventecomy" to be performed on an '13. don't want to go confusing the new guy/gal.

mittgore---welcome to tdiclub

waltzconmigo said:

jettawreck---pretty sure there is no "ventecomy" to be performed on an '13. don't want to go confusing the new guy/gal.

mittgore---welcome to tdiclub

Click to expand...

You are so right.
Somehow, I always figure everyone has the same platform as mine. Missed it badly.
Thanks for catching that fact.


Goodysgatacuda:
Diesel fuel doesn't expand much at all. A completely full tank in cool weather and then exposed to HOT sun for a day won't change much. Nothing like gasoline expansion.

Fill variance with diesel fuel foaming can make a lot of difference per tank and may frustrate getting accurate results until there is a baseline of several tanks to average. Fueling fully to the same level w/o foaming will get you much more accurate "averages" much sooner. It's easy after a while.

I'm not about to get OCD about the exact amount of fuel I can click off into each tank. The average is what counts, and the MFI gauge can't be trusted except that it's usually optimistic. I fuel until cutoff, and keep the receipt kicked out by the pump*. At some point, I then transfer the data to Fuelly, which handles dividing gallons into miles, and let the average speak for itself. By its very nature, averaging will remove the inconsistencies in the amount of fuel tank-to-tank, so no worries.

*I keep the pump receipts in an envelope in the glove box. These have the additional benefit of serving as a paper trail should there ever be a problem with bad fuel.

Rico567 said:

I'm not about to get OCD about the exact amount of fuel I can click off into each tank. The average is what counts...

Click to expand...

Just make sure when determining the average that you take the total miles driven and divide by the total gallons used. If you just average the individual MPG calcs from each tank, you will be off (unless you use weighted averages).

EX.

20 mpg and 40 mpg = average of 30 mpg right?

in reality...

100 miles/5 gallons = 20 mpg
100 miles/2.5 gallons = 40 mpg

200 miles/7.5 gallons = 26.7 mpg (NOT 30)

And for weighted averages...

(2*20 mpg+40 mpg)/(2+1) = 26.7 mpg

(essentially you use twice as much fuel at 20 mpg, so you must account for this. It has "double" the impact on your average MPG than the 40 does)