Gas Mileage query Holley 318

Thanks everyone for your feedback...
I am using 98 octane fuel (equivalent to US 95 I believe), and engine/heads are from around 73 vintage (I was told by seller) Cam is just a bit spec up from stock specs. Those figures are from me driving sedately at highway speeds around 50/55 mph, being approx 2100 - 2400 rpm. I once had a 68 Dart with a 383 running a Carter 600 which I think was MUCH better on highway trips ; I plan to replace the Holley with a TQ for the record!