ultra gauge set up help

so I been having my ultra gauge for a week or 2 now and I can't seem to get the calibrations correct. mainly the distance traveled is never correct. I have 265/75/16 tires which, according to tire rack, says I should travel about 3.8% further than my odometer says so wouldn't my calibration factor for my UG be 1.038? its always a different factor for some reason and the distance traveled isn't what I get when I multiply my odometer by 1.038, which effects my distance and mpg gauges which is why I got the thing...and opinions on this?

 

the issue is that the stock odometer and speedometer are not exactly correct. I discovered that with my gps. In fact the speedometer is closer to right with my 32" tires than it was with the stock 31's.

 

im having the same problem. also, i could never figure out what the exact size of a 2004 Prerunner gas tank size is!

is it 18.5 or 16??? my UG never can tell me the exact amount that is left in my tank.

 

prerunner tank size is 18.5...i just went fill up and the UG told me i had used 10.7gallons and the pump only put in 9.7 gallons when it cut off..so i entered that into the calibration. as far as the distance, i drove an 80mile trip, straight section of the intersate, an the calibration factor was pretty close but i just tried to clear it and re-calibrate it and i think i made it worse. it now tells me my distance to empty is 370 miles wen i usually only average 265! i guess ill have to go recalibrate the distance soon when i can get my hands on a friends gps sincei have just been multiplyingmy odometer reading by 1.038 and entering that as the distance traveled

 

I know kind of an old thread...

But you guys think that gps is accurate enough to do the distance calibration on the UG?