I don't wish to rain on anyone's parade, but I must ask: are your odometers accurate? Remember, TDR is about accurate, meaningful information. With the way tires and sizes are swapped around, there's a fair chance your truck is not registering elapsed distance correctly, which will skew your fuel economy calculations. But there's an easy way to calculate the true MPG; it requires going for a 200+ mile drive.
Note your odo reading at a particular mile marker, and note it 100 mile markers later. Then subtract the first reading from the second reading, divide by 100, and divide your calculated mileage by this quotient. This will be your true MPG:
Code:
Formula:
MPG_true = MPG_calc / ((Odo_miles_end - Odo_miles_begin) / 100)
or (more simply)
MPG_true = MPG_calc x 100 / (Odo_miles_end - Odo_miles_begin)
So, suppose you calculated 18 MPG, but your odometer indicates 110 miles passed when you really travelled only 100 miles. Your true MPG would be
16. 4 = 100 x 18 / 110, because you really travelled 10% fewer miles than you thought (fewer real miles per gallon).
Or, suppose you calculated 18 MPG, but your odometer indicates you only travelled 90 miles when you really travelled 100. Your true MPG would be
20 = 100 x 18 / 90. because you really travelled 11% more miles than you thought (more real miles per gallon).
Highway mile markers are rarely accurate by themselves (I've seen them up to 10% off), but are pretty accurate over longer distances. I've found that 100 miles in mile markers is pretty consistent around the country. I used this method to set my TruSpeed unit; my speedometer and odometer are accurate within 1%.
If you want to make the math easier, you can track how many real miles passed in 100 odometer miles. Getting the actual miles travelled this way requires a little more thought; but the final math is much easier:
Code:
Formula:
MPG_true = MPG_calc x (Milemarker_last - Milemarker_first) / 100
or
MPG_true = MPG_calc x Milemarkers_actual / 100
Now you simply multiply your calculated MPG by the actual mile markers passed, then shift the decimal point of the result two places to the left.
Technically, you can use
any ratio of actual miles to odometer miles, but you want to use enough miles to average out the inconsistencies in highway mile marker placement. I use 100 because it's easier to compute usin' me noggin. You can even use this method to verify the accuracy of your GPS unit, or use GPS to double-check. You can even test your kids'/spouses' math skills: keep them busy (and quiet) doing something useful and meaningful. And you can see how long it takes them to realize that the actual-to-indicated ratio doesn't change much from tank to tank.
Attention: TDR Forum Junkies 
:-laf
I've got an auto, so that accounts for some (but not all) of the difference. 
I just cruised for hours then. Montana didn't have a speed limit and with a Passport 8500 I was covered in the states that did care. The mileage wasn't bad then... 15 MPG at 105... but alas... many moons ago. 
