My EVIC has been very accurate so far. I jot down the #'s and clear the Trip B odometer at every fill up.
So far, the hand calc'ed has been within 0.1% of the EVIC.
Also jot down the #'s and clear Trip A odometer whenever I hitch or unhitch.
So far the average unhitched mileage has been 18.6 mpgUS, mostly rural driving, shopping, and so on. On the freeway at 62 mph, less steep hills and no stoplights, the EVIC says over 20 mpgUS.
Towing mileage is over 11 mg so far, but need to put more miles on it to get a good average.