Miles/gallon: Newer vs Older Cars

I have a bone stock 50,000 mile '69 roadrunner. I've never measured it but guessing it gets 14-15 MPG highway with my foot out of it. Drove it a total of 1,500 miles the last 3 years.

Also have a Pro Street A990 Clone '65 that gets 4-5 MPG and drove it a total of 150 miles last year. Don't care how bad of mileage they get, statistically, they produce far less emissions then my daily car which gets 40,000 miles a year.