"Partial fuel-ups and fuel-ups following a partial fuel-up will not display an MPG"

[docwisdom]

But when you do a fill up in Fuelly, it requires you to input how many gallons you added. (or it calculates it based off of total price/price per gallon)
If you incorporate the number of gallons actually put in the tank then my formula works just fine.

In your example, the car drove 250 miles on 10 gallons and drove the same distance on 5 gallons. Of course the calculation will be wrong then. On the other hand if you drove 500 miles in the 10 gallon example, the efficiency would be the same.

I could have a tank that holds 10,000 gallons and never fill it up. I still only need two numbers to do the calculation, miles and gallons. Miles Per Gallon.

Real life examples from my log book:

Another view

[Matt715]

Originally Posted by docwisdom

But when you do a fill up in Fuelly, it requires you to input how many gallons you added. (or it calculates it based off of total price/price per gallon)
If you incorporate the number of gallons actually put in the tank then my formula works just fine.

I'm with RobertV on this; I still don't see how you could get an accurate reading. If I started with a full tank of gas and then drove 100 miles and then put some random amount back in without topping the tank back off, I don't see how you could accurately figure out how many MPG you're getting.

The only way I can see figuring out accurate fuel economy with partial fills is by recording the amount you've added in partial fill ups to your next full fill and then divide by your total miles driven during that period.

[Charon]

If you don't like the way Fuelly does it, don't use Fuelly. Pretty simple.

[Jay2TheRescue]

The only way I see partial fills being reasonably accurate is if Fuelly knows the tank capacity, and every time you fill you accurately record before and after fuel gauge readings. You're getting into some pretty advanced calculations there, and it becomes more of a pain to enter fillups because you have to have two fuel gauge readings for each fill.

[docwisdom]

As long as you start with a full tank baseline at least once, and record every fill up, even partial, all your numbers should line up fine...but I guess ill concede to the MPG math gods on the forum.

[DPTyphoon]

Jay: Doc and I agree that a MPG can not be calculated for a leg that ends with a partial fuel-up.

What we're asking for is that a MPG be calculated for a full fill up after one or more partial fuel-ups. Fuelly will presently only calculate a MPG if the current fuel-up and the preceding one were both full fuel-ups.

Doc is right. As long as you don't MISS any fuel up, even a partial, it will work.

Odometer now 2500. At last fill was 2000 = 500 miles traveled.
Add 10 gals to full = 50 MPG.

Odometer now 2,500. full fill add 3 gals to full. Sum of all gallons (3+2+2+3) since last fill = 10 into 500 miles = 50 MPG.
partial fill add 2 gals
partial fill add 2 gals
partial fill add 3 gals
Odometer at last fill = 2,000 (gallons don't matter, tank is full).

It still works just fine. You have to add up all fuel added since last fill-up.

If the "this is a partial fuel up" box is checked, sum up all gallons added since the last time the box was NOT checked.
You're right, you can't calculate MPG for a partial fuel up. But you can for EVERY full fuel up. That's the difference between what doc and I desire, and the way it is now.

That every full fuel-up will have a MPG calculated.
Not just the second consecutive one the way it is now.

My Super Duty log has 53 fuel-ups, but only 13 MPG calculations. Using my suggestion, the same log would have about 22 MPG entries which would give a more detailed perception of the fuel mileage obtained.

Thanks for taking another look.

[DPTyphoon]

I need to add a couple things, but don't want to make the above TLDR.

Although MPG can be calculated between any two fill-ups, I can think of a field that shouldn't be calculated using partial fill data. "Most miles on a Tankful".

And if the "I missed a fuel-up" box is checked, then the MPG should not be calculated for the next full fuel-up, as it would be inaccurate because not all fuel added between fill-ups was recorded.