Gas Mileage query Holley 318

Hi all... Just did a trip in my 68 Dart coupe, very mild 318, .030 over rebuild with around 13000 miles, headers, Fresh complete overhauled Holley 650 DP, Auto w 3.23 rear - both near new rebuilds.
Sitting on 55/60 mph, highway/country driving mostly, and getting approx. 13-14 miles per gallon (Exactly 17L / 100KLM in our language!)...
Interested to know if this is normal or high, thanks...