Why is my car's mpg off from the trip computer and should I trust it?

[Charles97]

I’ve been tracking my fuel costs closely and noticed my car’s actual mileage is consistently lower than what the trip computer says I’m getting. The discrepancy isn’t huge, but it adds up over a month. I’m just wondering if others have found the same thing and how much I should actually trust the onboard readout when budgeting for fuel.

[MatthewW]

Yep me too. The readout often overstates the efficiency I actually get when I do the tank-to-tank math. The gap isn’t huge day to day, but over a month it shows. I started keeping a simple log: miles between fills, gallons added, then compute efficiency. The discrepancy seems bigger after cold starts and when I stomp on it for a few miles. It’s a rough guide, not gospel.

[StevenTJ]

I treat the trip computer as a rough guide and do real budgeting with the fill-up numbers. After checking a few tanks, I found the computer’s numbers were off by a few units of efficiency. Have you tried doing a couple of actual tank-to-tank comparisons to see how far off yours is?

[James57]

Sometimes I wonder if the problem isn’t the readout but my pattern lately. More hills, more stop and go, less smooth speed. The dash numbers look optimistic when I’m in heavy traffic, but the log from actual fills tells a different story.

[MarkTH]

I’ve noticed the dash range estimate can feel optimistic too. It makes budgeting tough if you take it at face value.