I set, and saved, all vehicle info, engine size, fuel type, tank size, I even added 1 to the engine speed because I put slightly bigger tires on the car than OEM.
If your odometer is recording a different mileage than the SG, then your miles to empty etc. will never be right. Setup the SG so it shows the same speed as your speedometer. It won't be accurate but will be more useful.
When filling up, I like to split the difference between pump gallons and SG gallons. That way, a single fillup won't spoil the next 2 tanks' readings. I also keep a running average of pump gallons vs uncalibrated SG gallons (back-calculated from my offset). It does settle down to the about the same numbers in both cases.