mpg wrong if mph wrong?

This topic has been discussed ad naseum in a great many threads. The speedometer, odometer, and MPG display have different errors.

Speedometer displays are intentionally set to read high, for legal and industry standards reasons. The internal ECU speed reading, accessible with third party engine monitors (ScanGauge, Torq, and others) is much more accurate.

Odometers cannot read similarly high without drawing class action lawsuits about warranty fraud. Two of my previous car brands settled by giving the lawyers $millions in fees, and customers like me a 2% warranty extension, though none of my covered cars actually had any significant error.

MPG displays are unregulated. Your Gen2 model is typically quite accurate, my Gen3 is uniformly optimistic, some other models are known to be pessimistic. Third party engine monitors start with similar problems, but have the advantage of allowing user calibration.

 

+1 to fuzzy.

The car knows its speed based on wheel speed rotation. But tire sizes and even wear on the tire change the reading so it is never 100% accurate.

Then for various laws around the world, the odometer always reads low and the speedometer always reads high so the car fudges the difference. If you got pulled over for doing 66mph in a 65mph when your display showed 65mph, you would be upset. So when it shows 65mph, you are really going 1 or 2mph less. There is a greater error the faster you go, but not by much. Likewise if your warranty expires at 100,000 miles but you really only went 98,500 miles, you would also be angry. So when it shows 100K miles, you have really gone more, maybe 101K miles.

Just like the fuel gauge is purposefully inaccurate. People drive until it blinks and says it is almost completely out of fuel, then get angry when they run out of fuel 60 miles later. Imagine if they ran out of gas a couple miles after it started blinking, there would be an outcry.

The car knows, but it lies to you because the majority of drivers completely misinterpret symbols and warnings.

 

This topic almost passes for a "sticky".

Great explanation though! I learned something today. I didn't realize the speedo & odometer weren't reading from the same hymsheet so to speak. Easy to see why - now.