Won't MPG data be skewed if not filling from 0 to 100% every time?
 

Surely Fuelly can only figure out the MPG if you fuel the car from 0% to 100% every time, as it's based on the amount of fuel going into the car times by the odometer reading?

The issue is, no car is at 0% every time you fill up.

If it's at 10% (which is probably when most people go to fill up), would the MPG data not will be skewed, therefore meaning it's probably preferable to just rely on a car's LCD MPG data?

Just a thought. I may be wrong.

You are wrong. As long as you fill to the same point every time, in my case first click off of the pump, and note the mileage either by trip miles or odometer reading, since last top up, then you can calculate the mpg. If I add 2 gallons to the tank to fill it, and have done 100 miles since the last time I filled it, I get 50 mpg. It doesn't matter if I have a 7 gallon tank with 5 gallons remaining before top up, or a 10 gallon tank with 8 remaining.
I have an on-board reading, plus a ScanGauge E, and all 3 figures are noted every top up. They are all slightly different to each other, but the calculated figure is the most accurate figure. The more you record, the more accurate it gets.

As the poster above says, so long as you top up every fill up, regardless whether you've done 5 miles or 555 miles, you'll still get an idea of how much fuel you just used. Partials where you don't fill up to the top are less accurate, you have to wait until the next time you fill up to get a figure, but the fuel you put in and the miles still contribute.

As Draigflag says, as long as you tally the amount of fuel added and the miles driven you can calculate the average mpg for that period. Last year I drove 8910 miles and put in 161.3 gallons of fuel. This calculates out at 55.24 mpg. My average Fuelly figure of 54.4 mpg (as of Dec 2018) is since I acquired the car in July 2016.

A potential issue.

I created an account on Fuelly a few days ago (early December).

I have yet to refuel my car since creating the account.

I have, however, added the previous known odometer readings as Notes:

• 01/11/2012 - 0 miles
• 18/12/2012 - 949 miles (service)
• 25/10/2013 - 8337 miles (service)
• 19/10/2015 - 27,689 miles (MOT)
• 30/12/2015 - 28,662 miles (service)
• 11/01/2016 - 28,662 miles (MOT)
• 29/07/2016 - 38,014 miles (service)
• 21/01/2017 - 46,465 miles (service)
• 13/05/2017 - 46,492 miles (MOT)
• 02/11/2017 - 48,988 miles
• 10/05/2018 - 52,099 miles (MOT)
• 29/07/2018 - 54,355 miles
• 26/10/2018 - 57,155 miles

When I go to New Fuel-Up Record, it shows "Previous Odometer" as 57,155 but that was not recorded when I last refuelled.

If Fuelly starts to base my MPG on 57,155, won't it be inaccurate from the start?

Just start your first fill up with the current odometer reading. It will be greater than 57,155. It will start from that odometer reading for your future mpg readings.

Wait you've done nearly 60,000 miles and haven't recorded a single fuel up?... I think you missed the point of fuelly.

Ah, Fuelly bases MPG solely on refuels and not Notes, then?

Excellent :)

There's no reason to only start using the app when I first buy a new car.

I only found Fuelly recently myself. Luckily I had dates and fuel ups recorded in Excel so I added them retrospectively.