Why is my real fuel economy off by about 10% from the trip computer?

[LarryB]

I’ve been tracking my fuel consumption for months, but my actual mileage at the pump never matches the trip computer’s estimate. It’s consistently off by about 10%, which adds up. I’m just trying to figure out if this is normal for modern cars or if something might be wrong with my calculations or the car’s system.

[JonathanOW]

Yeah, that gap sounds familiar. My readout tends to swing by about 5 to 12 percent depending on the day. On highway days it’ll show higher numbers, and in city crawl it drops. After a few tanks I stopped trusting the readout and started tallying by brim-to-brim MPG myself. It’s never perfectly aligned, but the trend is what matters, and it stays around that 8–10% area sometimes.

[AnthonyUW]

I think the readout is just an estimate. It uses sensors and last trip data; it can lag or be biased by last segment. If you use a different tire size or a miscalibrated speedometer, the odometer can throw off the fuel math. Make sure tires are inflated; small changes change fuel economy a few percent. A lot of times I found the average readout drifts when you reset it mid-cycle.

[Nora.W]

Have you tried zeroing the trip every fill and staying consistent about brim level?

[MichaelL]

I kept a log for a couple of months, filling to the brim and comparing. I saw a few tanks where the pump-based math and the computer disagreed by about 7–9 percent. It seemed to bounce around with temperature and when I had a long highway leg. I’ve stopped chasing a perfect match and just watch the trend.