[PaleMelanesian]

Right. The normal way to do a calibration is the slow way - run through a tank and then adjust with the real amount when you fill up.

There are two quicker approaches.
1. adjust the engine size. bigger engine = SG thinks you're using more fuel = lower mpg reported. I like this option because it only takes 4 clicks to change the setting (2.0L, 2.1, 2.2, 2.3). It's also easy to reproduce if the unit ever resets or if you use it on another vehicle and then bring it back.

2. up until you've used 1 gallon on the tank, you can adjust the offset in the Fillup menu. Just click until it's +20%, and then DON'T hit done. Just leave it, or it will start a new tank from zero. This takes a LOT more clicking (20% in 0.1% increments = 200 clicks) and will have to be repeated if you reset or use the gauge on another car.

Whatever approach you use, I suggest recording the settings in case you need to reproduce them later. It will default back to the wrong settings if it resets.
- engine size
- fuel type
- offset
- tank size if you use the miles/gallons/distance to empty feature.