First off the only way to clock mileage is to fill the tank, drive & clock the mileage, then refill and do the math. You cannot use stand alone units. Or instant mileage features on the dash. There were times that my Acura was showing 40+ mpg on the dash when in reality is was about half.
Back to the ultra guage, how do you know its within 1/2 mpg? Im assuming thats sarcasm when you say @ 55mph your tahoe gets 25mpg??
Ok, first things first, I am educated and have been an ASE Master tech since 1991. While I fix the machines that make microchips now and I am not a current tech anymore, I know my shit.
Of course I filled the tank to the top for my tests!!! I even used the same station and same pump. I hand calculated with a calculator and compared to the Ultra Gauge. Therefore I know it is accurate. I thought you were reading my posts about it? BTW there are correction factors you enter to match your hand calc to the U-G. So last time I used 2/3 of a tank of mixed driving and the U-G was only .5 mpg off. So YES mine is now calibrated to my truck and is accurate. Therefore if I look at the instant mileage at any time, it is actual.
Getting good steady state mileage is all bout aerodynamics, and being in the engines sweet spot (for your gearing). Add some bolt ons and you can get better. Remember my truck is not stock.
Wait till I tune it and add more cruise timing, and I will get even better mpg.