My mom's 07 Grand Cherokee 4.7 gets 12 with E85 and 17 with regular 87. Both are long-term averages, several consecutive fill-ups.
Current engines are not designed to take advantage of E85's high octane rating. The compression ratio is a compromise since you can run regular or E85.
High Octane Does NOT equal performance or economy, just that it helps high-performance engines perform best.