How to screw up checking our gas mileage...

ember1205 · September 16, 2015

Fuelly tracked miles vs Actual miles on the truck are off because of tire size difference. I can calculate the actual mileage and update that in fuelly, where I can't add that in the truck (nor do I really want to )

Huh? How do you figure?

The odometer is going be accurate to whatever level regardless of tire size. If the odometer is off because of tire size change, then so is the trip odometer. You aren't using GPS-calculated miles to enter into Fuelly... You're inputting information directly from the truck. Right or wrong, it's from the truck. The reason that the tracked miles and actual miles are different is because you're using trip odometer readings (miles traveled) instead of odometer readings.

Rawyzf · September 16, 2015

Huh? How do you figure?

The odometer is going be accurate to whatever level regardless of tire size. If the odometer is off because of tire size change, then so is the trip odometer. You aren't using GPS-calculated miles to enter into Fuelly... You're inputting information directly from the truck. Right or wrong, it's from the truck. The reason that the tracked miles and actual miles are different is because you're using trip odometer readings (miles traveled) instead of odometer readings.

#1: The OD is not accurate if you change from the factory tire size. (unless you have that corrected along with the speedo (via tuner)).

#2: The OD and Trip are off the same amount regardless if you track your tank by OD or Trip.

#3: I don't track via GPS, that would be less accurate in my opinion.

#4: Fuelly miles traveled will not match OD because of tire size difference per tank and the calculation you will see below.

Not sure the best way to explain this. Lets go with this.

My truck has 10,000 miles on it and I fill the tank and reset the trip. After driving 300 miles, I am down to a quarter of a tank and want to fill up before the weekend. So, I stop at the gas station with 10,300 miles on my truck and 300 miles on the Trip. I take that 300 miles and multiply it times 1.045 because my 275/60/20 tires are 4.5% larger than the 265/65/18 tires that come on the truck. 300x1.045=313.50. Now, I know my OD says 10,300...but I really went 313.5, but the OD has no way of knowing that (because I didn't program the tire size into it).

So, in fuelly, I enter 313.50 miles travled. I then start pumping gas. Lets say the pump stops at an even 18.00 gallons. I enter 18.00 gallons into fuelly. Fuelly then calculates that my MPG on that tank was 313.50/18=16.66. I think reset my Trip and start this all over again until I fill up. You can see, based on this one fill up, fuelly and my OD are already 13.5 miles off. Now I have tracked like 30 tanks, so imagine they are off a bit over time.

Now to the point of your comment on the fuelly miles tracked vs actual OD being off, that is correct and I expect that. My OD should be around 4.5% less miles than I have tracked in fuelly (if I always have this size tire). However, I change tires like I do underwear, so I've been 2% bigger, 4.5% bigger, and now I'm gonna go back to 3% bigger with 17" tires).