Never had a scangauge but here is an excerpt from the ultragauge manual.
Calibrate MPG/Fuel
MENU Vehicle Setup .. Calibration.. Calibrate MPG/Fuel
This calibration is used to fine-tune UltraGauge to accurately measure fuel usage. This calibration is critical, especially for vehicles
which use a MAP sensor, diesels and alternative fuels.
For vehicles that use a MAP sensor*, see the menu section on Adaptive Volumetric Efficiency before proceeding.
Prior to this calibration, consider performing the distance calibration. See the section: Calibrate Distance
The gauges that depend on fuel usage will not be accurate until this calibration is complete. To perform calibration, please follow
these steps.
1. Fill up the fuel tank. Once filled, press and hold the UP key to cause UltraGauge to recognize the fill-up
Zero the Average MPG. MENU Gauges Zero Ave MPG, G/H.
2.
At the next fuel fill-up, note the number of gallons used (pumped). Select MENU Vehicle Setup.. Calibration..
3.
Calibrate MPG/Fuel and enter the fuel used. Press MENU when complete to set and save the calibration. Use the same
Gas station and pump for best results. Alternatively, record and add the actually gallons used over several fill-ups to
improve accuracy.
4. The Calibration factor will be displayed at the bottom of the screen.
Congratulations, you have successfully calibrated UltraGauge to your vehicle.
Make note of the calibration factor. If you should ever need to clear your configuration, the calibration factor can be used directly
to set the calibration. Simply jump to step #5 and increase or decrease the reported gallons until the calibration factor matches.
* To determine if your vehicle has a MAF sensor, access the menu; MENU UltraGauge Setup.. Version. This will display
MPG sensor: MAP, MAF or None
Manual may be downloaded here:
UltraGauge Support