[Big Dave]

Both matter.

For diesels engine size is not as serious at a given engine RPM as it has no throttle. For throttled gas engines it matters considerably.

Gear matters big time as engine frictional HP varies with the cube of the engine speed (for a given displacement and road speed).

My pickup has an exotic drive train that gives me a net 1.94:1 gear ratio. I get 30% better MPG than stock and that's with my aero package currently compromised. I cruise at 1325 RPM @ 70 MPH.. The same truck stock cruises at 2000 RPM @ 70 MPH.