I haven't taken a road trip yet, but I am at 22 to 23 mpg in town which is a mix of surface streets and freeway. That was using 87 octain. I am going to try 89 octaine next fill up.

 

2008 Tahoe EPA 19 highway gas, EPA 14 highway E85 (85% ethanol). Thats 5 MPG less with an 85% mix, quite a bit more fuel needed to be "calibrated" for the ethanol, then you should need to add at least a bit more fuel for a 25-35% mix. Now, if you are going to use more fuel than straight gas, then haven't you eaten up any savings from not going to premium in the first place?
Now if you have a modified engine, with more boost, and tuning capability, then ethanol offers alot of potential for more HP due to the octane, but this thread evolved into a guy wanting to save money on gas.