Wrong MPG Calculations
 

Ok, I noticed some of my mpg's didn't sound correct. It seems that fuelly calculates everything from one fuel up when it should calculate current miles traveled then divided from previous gallons added. Does that make sense, it took me a while to figure this out.

I suspect these errors are because the partial fillups when it's impossible to tell your mpg from the distance/volume, because they don't necessarily belong to the same distance. On the long run it's possible, but I'm not sure if fuelly can do such a thing if you don't have complete fillups.

Even if you did fill up, it is still only using info from your current input information when it should be using info from your last and current fill ups. Seems like the only way this would be accurate is your first fill would have to be of an empty tank and all future fill ups would have to be complete and not partials.

richardharo, Fuelly is designed to calculate your fuel consumption and mileage based upon fill ups, not partials. It sounds as though you are expecting Fuelly to calculate your mileage based upon partial fill ups which is mathmatically impossible. Fill your tank and reset your trip meter or log your odometer reading. Then drive until your next fuel stop, fill the tank again and record your trip meter or odometer reading. The number of miles driven between fill ups divided by the quantity of fuel added is your mileage for that tank. Reset your trip meter and continue to drive, refuel fully, log your miles driven, and repeat. This will yield your fuel economy in a meaningfull manner. Partial fill ups don't work because there is no way of telling how much fuel was consumed between partial fill ups since the tank didn't start out full and end full thus giving a fixed quantity of fuel consumed.

Partial fill-ups can work, but only when accounted for as part of a total fill-up. For example, you only have 1/4 tank left. You pull up to the pump and decide to fill it up. However, the pump is really running slow so you only put in 2 gallons. At this time you would not reset your trip odometer. The next time you try to fill it up it works better and you get it full. Write down your trip mileage and reset. Now for gallons filled you have to add the partial fill gallons and the total fill gallons. Your MPG for that "tank" is the mileage before you reset the trip meter divided by the total gallons. Obviously this can yield some funny figures, like 800 mile tanks and 40 gallon "fills", but as long as you don't reset the trip odo until your full fill-up and divide by total gallons used since last reset your MPG will be accurate for that "fill-up".

AFAIK, it seems it for a specific date counts /( - ) to measure l/km (or gpm), turn that around for mpg.

richardho....It may also mean that you expect that fuelly would give you the gas mileage based on the actual fuel you used. This is a common question on here.

In fact, the mileage is based on the replacement fuel. It took xx gallons of fuel, to replace the fuel that you used driving yy miles since your last fillup.

just joined this site today, as my new-to-me diesel is poorly and poor fuel consumption is part of the problem.

I got my first car (same as new but with smaller tank and petrol)just a couple of weeks before this site was set up and not finding a calculator online, used a spreadsheet for years. It seems to work in the same way and I got over the partial fill up thing by trying to keep the tank full.

I can only afford to partially fill my tank regularly, if I couldn't afford to replace what I'd used entirely I stopped calculating and restarted calculations when I could - filling the tank from 3/4 to full costs as much as filling it from empty to a quarter full :)

If I start on a full tank when my car is well enough to drive, use part of this and refill fully, then I believe entering mileage as a full tank should work? if I can't fill up fully then I tick the 'I missed a fuel up' box

https://www.fuelly.com/driver/ken555/impreza?fu=5383501

can anyone explain fill up number 5 ?

67.1 miles on 14.19 gallons? I don't see an error here.