I think I can safely say that the answer to your question is - it depends. Vague answer, yes. But really the answer depends on what inline 6 your talking about versus what v8. And the vehicle application for each engine. Of course gearing and performance modifications factor in too.

As a real example it is very likely that the mildly built 292 in my 1966 half ton pickup will get worse gas mileage than a stock 283. Probably even worse mileage than a mildly built 283 in the same vehicle. But, for another example, a well tuned stock 250 will probably do better in fuel ecomony than a 350 in most standard passenger vehicle applications.

There's no single "yes" or "no" answer to the question that will cover all cases.

-ftyler