ScanGauge: Confused-is my mpg actually calculated and my speedo gauge is off?

Ok, i'm confused after reading several posts.

I have ScanGauge II and i want to know:

1) Is my MPG actually calculated correctly or is it some standard # the ECU is reporting? Meaning, we all know if we haul stuff, our MPG goes down, so does the ScanGauge actually report our correct MPG.

Example: If i don't haul, i get about 320miles on a full tank. If i haul, i get about 240miles per tank.

2) I noticed my speedometer is off? Or is it my ScanGauge that's off? When i'm traveling about 75mph, it says on my SG, that i'm traveling 79mph. The faster i travel, the more range there is between my unit and actual speed.

Example: 60mph (speedo) = 63mph (Scangague).. 70mph (speedo) = 75mph (scangauge). 80mph (speedo) = 89mph (scangauge).

I don't know which to believe.

 

Not sure about the scan gauge. But the way I've checked to see if my speedometer is off is to use a gps and compare. Mines off by about 1-2 mph at 45 in town and 3-4 off when going 80 on the freeway.

 

i have the hypertech speedometer/tranny module to be put on... this is suppose to solve the speed/tire/gear problem am i correct? Does anybody know about this product?

 

you can change your speed % on your sg to match a gps (which is pretty acurate) my sg is set at 8% on the speed setting an it matches perfect with gps and my local radar signs. for the mpg it only gets info from fill up to fill up then calculates an estimate for your inst. econ number.....my question is my odometer and the scangauge read two diff numbers....does the sg pick up that since i have a 8% speed diff. that the distance traveled is longer?