mileage expectations

I have a 302 in a 2600lb little british car... had never checked mileage before but did the other day & was surprised it was only 15 mpg; was expecting low to mid 20's.

What's the collective mileage experience here?

For comparison purposes:
* Driving was 50/50 mix of town & highway.
* Motor is relatively new (~2500 miles on it), Aluminum GT40 heads, B cam (i know it's not the best but it's what was in the motor when I got it & was too good a deal to pass up), Performer 289 intake, Holley 650 double pumper (w/as supplied jets, squirters, etc.), 3.54 rear gear (Nissan diff), 0.68 overdrive gear, 225/50/15 tires (23.86" diameter).
* 93 octane Texas gas (10% ethanol)
* My plugs look good & A/F is good at least at WOT (recent dyno run) - no idea what it is when crusing.