Yes that is too good to be true and heres why,
1) If your tire size is currently larger than actual it will make the odometer read more distance traveled per tire rotation than actual. Make the distance traveled larger than actual and you increase the calculated mileage.
2) Never go by computer readouts for things like mileage. That needs to be only on paper because the Edge programmers rely on MAF sensor values and corresponding mpg values. It then makes a running average. Using the MAF sensor inherently brings in error. Usually making the mileage value larger than reality. When you lift your foot off the gas pedal and roll or start to slow down, that portion of time is counted as 40 MPG, WOT runs are usually around 5 mpg because you are moving while using gas. When you compare many 40's averaged with a few 5's and a general majority around 15, the 40's will pull the average up very quickly. For a silly experiment you can reset the mileage calculator and do nothing but WOT acceleration then roll (no brakes) until you stop. Do this for an entire tank. The average will say you are getting good mileage, your paper calculation will not agree.
The programmer influences the speedo/odometer by changing the tire size. Your odometer calculation is done by keeping track of tire rotations. The only way the truck knows how far you went is by the tire circumference you enter. That mileage you have calculated could be adjusted by going back to the trip distance, multiplying by the squish value (.97 or whatever is accurate) then dividing by the number of gallons used.
An easy way to get your odometer is to program the truck to a tire size you know is wrong, like 20" diameter. Drive about 10 miles. You can either use a GPS, map or highway markers. Then use this formula
GPS odo / Truck odo x tire size you entered (20") = Correct tire size.
Distance increases the accuracy of this formula. My programmer uses the actual circumference in MM so that also helps but it should still work on diameter since you are only scaling the value.
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