Scan Gauge not to accurate with trucks?

[91CavGT]

I have an original Scan Gauge 1 connected to my Chevy Colorado. I've been using it for quit sometime and have used it on several cars in the past. On all the cars, the gas mileage estimates have been pretty accurate, but on my truck so far in the past it seems to be a bit off.

I've changed the engine size and confirmed that it is correct, but in the past before I started doing the FE improvement driving techniques, it would be off by anywhere from 2-6 mpg (it always showed better gas mileage).


Is this because of the increased aero drag caused by the truck vs the cars?


One more question, my Scan Gauge does not have a Hybrid setting on it, so when I do the fuel injector switch mod to the truck, will Scan Gauge not show current FE?