The following are some overall indications of looking at the parallel coordinates plot at https://mbostock.github.io/protovis/ex/cars.html
From 1980 to 82, pretty much all cars represented had a 4-cylinder engine, with a handful of 6-cylinders and 1 8-cylinder.
Very low weight and very high weight seems to be directly proportional to horsepower and displacement, but medium weight seems to have a varied horsepower and displacement range.
A slow car, even with a small engine may not yield better mileage.
However, a large cylinder will naturally have a large displacement, will be relatively fast (but not necessarily) and yield low gas mileage.
Not sure if the weight refers to just the engine weight or also include chassis. The plot seems to indicate a similar behavior as engine displacement.