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Keep in mind that reducing gearing is not always the answer either. I can personally vouch for this.
I once mated a CRX HF transmission to a slightly modded 1.5 liter engine... I LOST even baseline efficiency, to the extent of going from 40MPG down to 21 MPG.
I started researching how this could happen... to find that:
the CRX HF gets its' max torque at something like 1800-2000 RPM... meaning that doing 70 in 5th gear puts it in it's max efficiency area (area of peak torque)
Putting that same transmission against an engine that makes considerably less torque in that area will not gain net efficiency... you will lose it.
Please keep that in mind when choosing gearing.
This is the reason that most Honda transmissions place cruising speed at close to 3k RPM... it's where max torque usually is in those engines. It's also key for efficiency.