Also when you refill try to use the same pump, or at least same model pump, and fill until first shutoff. GPS and mile marker technique will be difficult unless you are checking mileage on a single trip (which makes using the same pump a little more tricky), but results will be more accurate (won't be affected by non-stock tire size, etc.). I seem to average around 25mpg or so between fillups (mixed driving). Best tank average was just over 27mpg.
Mine in both of my T-5R's have been pretty close, within 2 mpg. The best I have done at 70 mph is about 29 mpg. I can get 32 mpg at 63 mph. Mine actually reads a little lower than actual. I ran the car out of gas once to see how close the remaining was and it read 5 miles to go and I started running out 2 miles later.
Thanks everyone. I took it out west from Michigan and I think I am getting about 25 mixed, this does not include the mountain driving though. I quit counting in yellowstone. Way too much uphill stop and go, she handled it well but used a lot of gas. Overall a great ride though. She blew an intercooler hose going up a steep climb in wyoming on the way home which i had to replace with 3" radiator hose from the cheyenne auto zone but it did the trick. I bought it 3 days before we left for the trip so I was very happy she made it home. Great bonding trip with the car.... And oh yeah the wife and kids too.