Why does real-world fuel economy differ from the trip computer after six months?

[AbigailQR]

I’ve been tracking my fuel costs for the last six months, and I’m starting to wonder if my car’s real-world fuel economy is actually getting worse, or if it’s just the changing mix of my driving. My commute hasn’t changed, but the numbers on my spreadsheet don’t seem to match up with what the trip computer says anymore.

[SophiaFD]

I've been logging miles and gallons for six months. The trip computer would spike MPG after a few long highway days, but the spreadsheet kept nudging the overall MPG downward. My commute didn't change, but the mix of short trips and idle time on some days did.

[EricGM]

I did a real check: filled to the same level at the same station, then calculated MPG from miles since last fill and gallons pumped. After standardizing the fill method, the numbers lined up within a tenth or two, not the big gaps I worried about.

[JerryWG]

It might be measurement noise. Trip computer averages speed and distance and rounds; the spreadsheet relies on my odometer and reading the pump gauge. If tires were a bit underinflated for a while or fuel trims changed, it can skew things.

[GaryG]

Maybe the root cause isn't the engine at all but the driving mix in tiny ways. More short hops than I realized, more cold starts, or higher A/C load recently could burn more gas even if the route stays the same.