Broken MPG Calculator?

[Kungfubhuda]

Like many of you, I've kept my Trip A as a running total of the vehicles total miles. Prior to my most recent fill, my average across all tanks was under 15 mpg. 33 miles into this tank, which is at 18.9 mpg (finally), I watched my cumulative mpg increase to 16.2. That seemed crazy to me, especially since on any given tank, once I'm past the halfway point, it's difficult to change the overall number much. Yet my cumulative seems to jump around a lot. Thinking back, at every tank, the cumulative has been really close to whatever I was getting on the current tank, even when that number change drastically. Since watching my cumulative mpg spike today, I'm pretty sure the total isn't being calculated right.

I'm too tired to consider all of the math, but only 33 miles into the tank, I believe that my total fuel usage should not have changed much. If my math is right:
3713 miles (total miles) / 16.2mpg = 229.2 gallons
3680 (total miles prior to recent fill)/15 = 245 gallons

Obviously I haven't used -16 gallons of fuel on this tank. Am I missing something? It seems like there's a bad algorithm in the mix.

Please weigh in with your own numbers and math.

 

[Jako]

My MPG calculators work the same. The A trip was not reset and acted the same as you describe. With close to 6,000 miles on the odometer and seeing numbers change that are not mathematically possible I gave up "faith" on the system. Based on the average of 26 fill ups and 5700 miles I find the display to be .64 mpg higher on the display versus hand calculation.

If the miles are taken into consideration on each fill up the difference is .98 mpg.
Explanation: 100 miles 20mpg, 200 miles 16 mpg, 300 miles 14 mpg.
Average [(20+16+14)/3] 16.67
Average with miles [(100*20+200*16+300*14)/600] taken into consideration 15.67.
1 mpg difference taking mileage into consideration in this example.

Anyhow based on the 26 fillups I find the display usually a little higher from .5 to 1 mpg and on 7 of 26 occaisions lower. The discrepancy was highest on the first 3 fill ups.

 

[rlc2020]

Pretty sure it only calculates the last 2k miles give or take. I'm just speculating, but mine will jump up or down every tank by amounts that aren't possible for only a few hundred miles. like a 1mpg increase on a 4k mile total with just a 1.5mpg increase in the last tank. I am about to do a 3000 mile round trip to Michigan next week and plan on keeping track of my mileage by gps and the trip computer and see how accurate the truck is.

 

[Kungfubhuda]

I stopped calculating at the pump because my numbers per tank have been within .1 or .2 mpg, which for me is close enough. It's odd that the trip numbers fall off significantly up over a greater distance. For the numbers to be that far off the computer can't be re-using its own data correctly. I suspect that when you reset one trip meter, it's effecting the data on the other, and it shouldn't.

This seems like another bug that should have been caught during testing. I'm curious how far off other people's numbers are, and if anyone can point out the exact point where the math is being done incorrectly.

 

[Jus Cruisin]

That's why their nickname is "lie-o-meter". They aren't accurate. With the F150's you can actually adjust them to make then a little more accurate.

 

[rlc2020]

I can say with certainty that the truck lies to you big time. I just did about 1600 miles checking each tank. I averaged 24mpg after the first 630 miles. Hand calculated was actually 22.8mpg, off from the trucks computer, but a decent average considering I had the cruise set at 72mph most of the way and that average includes getting off the highway for 3 stops and a lot of hills in eastern PA.
https://www.dropbox.com/s/cc2r21p2vsrhfsl/20190417_153210_HDR.jpg?dl=0
After we got back ( 3 days of local driving before heading back dropped the average a bit) the trip meter said 21.4, which happened to be the average from the last tank. But, the calculated trip average was 22.0mpg. The long term trip average reverted to the last 500 mile average giver or take after it rolled over about 900miles.

To sum it up, The average is only accurate for about 900miles before it starts to drop the old numbers and base its number off recent mileage.

That said, I'm thrilled to have gotten almost 23mpg average at 72mph.

 

[RPeezy]

Pretty sure it only calculates the last 2k miles give or take. I'm just speculating, but mine will jump up or down every tank by amounts that aren't possible for only a few hundred miles. like a 1mpg increase on a 4k mile total with just a 1.5mpg increase in the last tank. I am about to do a 3000 mile round trip to Michigan next week and plan on keeping track of my mileage by gps and the trip computer and see how accurate the truck is.

I'm pretty sure you are right about that. I don't yet have my Ram, but every other Chrysler product I have experience with operated this way.

 

[Kungfubhuda]

My individual tanks are close enough that the gauge is still useful for on the fly accuracy, give or take 1 mpg. The problem is that beyond 1 tank the number goes to hell. 5+mpg off from reality, and changes rapidly, more than on an individual tank. My issue, is that it isn't a matter of the tool having difficulty measuring that closely, It's a math error.

Rather than calculate the trip as something like:
Total Trip A Reading + Instant Reading = New Total Trip A Reading

It's being calculated at something weird like:
Current Tank Reading + Instant Reading = New Total Trip A Reading

 

[DM-SC]

I guess mine is a rare one. I've checked it multiple times and it only reads .3-ish off (better) than my hand calculated mpg.

Sent from my SM-G973U using Tapatalk

 

[Billy James]

I guess mine is a rare one. I've checked it multiple times and it only reads .3-ish off (better) than my hand calculated mpg.

Sent from my SM-G973U using Tapatalk

I was hand calculating for a while; until I determined that the difference was very small.....its not worth the effort. I'm satisfied tracking my mileage by what is displayed. Its not like tracking my mileage is going to make a difference; I'm still going to drive it every day and fill it up when its empty.