Confused About the Reported Average MPG for the 2024 Toyota Sienna on Fuelly

[Zafira01]

Hi everyone,

I was checking out the 2024 Toyota Sienna stats on Fuelly and noticed something that doesn’t seem to add up. The page shows:

4 vehicles registered

47 fuel-ups logged

20,519 miles of driving

Combined Avg MPG of 34.49 with a 1.02 MPG margin of error

Here’s where I’m confused: the four vehicles listed individually show 35.5, 35.0, 34.7, and 35.5 mpg. Logically, I would expect the overall average to be close to (or slightly above) 35 mpg. But the combined average reported (34.49) is actually lower than the lowest individual average.

Am I misunderstanding how Fuelly calculates its “combined average”? Is it based on total fuel-ups rather than the average of individual vehicles, or is there possibly an error in how the numbers are displayed?

I’d really appreciate if someone could explain the math behind this, or share if they’ve noticed similar discrepancies on other vehicles.

Thanks in advance for any insights!

[Jay2TheRescue]

AFAIK it takes total miles traveled by the vehicles by total fuel used. It doesn't simply average the totals of the vehicles.

[rosannana]

Fuelly calculates the combined average using all fuel-up records together, not by averaging each vehicle’s MPG. So if one car had more low-efficiency trips or more fuel-ups logged, it can pull the total average down. You can check the “fuel-ups” weighting on each profile for confirmation.

[chifetter]

Originally Posted by rosannana

Fuelly calculates the combined average using all fuel-up records together, not by averaging each vehicle’s MPG. So if one car had more low-efficiency trips or more fuel-ups logged, it can pull the total average down. You can check the “fuel-ups” weighting on each profile for Snow Rider 3D confirmation.

The overall average is weighted by total fuel consumption, not the simple mean of each car’s MPG. So vehicles with more miles or fuel-ups influence the combined result more heavily. That’s why the total average can appear lower (or higher) than any single vehicle’s figure, depending on how the data’s distributed.