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The Dash says fuel economy dropped
Free: the Dash says when a car’s fuel economy falls well below its usual. Check the tire pressure first; Got it hides the card until the economy recovers and drops again.
When the economy drops
When a car’s last 3 tanks together average 8% or more below its usual, and each of them is below it, the Dash says so under your fuel economy. It says what to check: the tire pressure first, then extra weight, short trips, winter-blend fuel or a dragging brake.
The usual is the same weeks in earlier years once the log reaches back a year and has a few tanks from them (so winter is compared with winter), else the 10 tanks before.
See each tank opens Stats → Fuel economy; Got it hides it until the economy recovers and drops again. With Fuel economy drops on (Settings → Alerts) a reminder comes that evening (the next evening when it first shows after 6 PM).
Not for plug-in hybrids or electric cars, nor a car without a tank in the last two months.