MPG wise, my 95 Disco I with Automatic Trans has averaged 12.13 MPG over 18,129.4 miles. Most of my driving is below 40mph on hills in Cincinnati.
When I drive the highway for a whole tank, I usually get 15. No matter what. I even get 15 when pulling a camper.
As far as accuracy, I run the stock tire size. My speedometer is off by 4-5MPH, but using a GPS, my Odometer is off less than .1 miles over 100 miles traveled. I've tested it 4 times. That seems pretty accurate to me, especially for a mechanical odometer.