As posted on Sprinter talk, says it best.
The Scan GaugeII won't do the maths as precisely as you can with a calculator or spreadsheet at each fill up, but it can get you close.
The real value of the Scan Gauge II is that when you are driving, you can see the current % engine LOD, as well as the current fueling rate (GPH. Based on the % Load and the fueling rate, you can decide (or simply experiment) to manually downshift your Sprinter and see
if running at a lower % LOAD but higher RPMs results in an improvement or a decline in your GPH fueling rate.
The RPMs increase, the % Load will usually decrease, as will the GPH fueling rate and the turbocharger boost. So, you are operating more efficiently
and getting better fuel mileage per mile. If you make the downshift, and the %Load only decreases < 5% and the fueling rate only changes by
a few 10th's of a liter, you can simply upshift back into the higher gear as the downshift was not getting you enough of a decrease in %load and
fuel flow GPH to make it worthwhile.
I've found that a great deal of the time if my %LOAD increases above 65% (60% actually is my break point between getting good mileage and getting great mileage @ < 60% Load)
I can get improved economy by running the engine up in the 2700-3200 RPM range where it makes it's best and most efficient power