Engine size and car size benefit is not huge at 77 to 85 mph averages which may include 100 mph or more speeds to keep the average up.
1981 31.9 US MPG for a Colt Mirage (front drive Dodge Colt ) 2498 miles at 83.6 mph average per Modern Motor 1981, 98 mph fastest hour.
1980 13.6 US MPG for a 1980 2 liter Alfa Romeo Alfetta, 85.4 mph average per Wheels 1980. 118 mph at fastest
1977 10.6 US MPG for a 1977 302 V8 Falcon, 77 mph average per Wheels 1977. 118 mph at fastest
For the Alfetta, a 28% reduction in fuel use via a smaller car with the same performance....
the much more aerodynamic 4 x 2 speed 8 gear 1.4 liter Colt had to be trashed hard, and other additional the fuel saving from a front drive power pack. The over driven fifth was 28% higher, and saved only 9% on fuel consumption at 68 mph.
I found that big Falcon with the 207 hp 4bbl Cleveland 4.9 fuel economy information in this Wheels July 1977 article.
https://www.wheelsmag.com.au/feature...ydney-to-perth
And Wheels November 1980, another car, a 120 hp twin Dellorto twin cam Alfa Romeo Alfetta 2.0 doing the same trip.
http://www.drive.com.au/motor-news/m...508-37yu9.html
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