Why is my real-world mpg lower than the trip computer shows?

[Andrew14]

I’ve been tracking my fuel consumption for the past six months, and I’m trying to figure out why my real-world mileage is consistently about 15% lower than what the onboard computer displays. It’s frustrating because I budget based on that reading, but my actual fill-up costs are always higher. Has anyone else found this kind of discrepancy with their car’s trip computer?

[MiaH]

Yeah, I’ve run into this. The trip computer will say one thing and the real math says another. Mine showed around 28 mpg, but when I filled up and calculated, it was closer to 24–25 mpg. I started tracking actual gallons and miles for a few months and the gap stayed in the 12–15 percent range.

[HarperC]

In my case it got worse after I swapped to slightly bigger tires. The odometer and the computer live on wheel rotation, so bigger tires throw the estimates off and the gap grows.

[Brandon89]

I tried keeping the tires inflated to the spec, doing careful top-offs, and even resetting the trip computer once. It mostly stayed the same over six months.

[Scarlett61]

Could the real problem be how the car calibrates fuel use, not the fuel itself?