Gear Ratio for Dummies? 4speed vs 5speed

Hey folks - I am so ignorant on this so I am looking for a little help.

Currently I have s stock '77 280Z coupe with a 4-speed. It runs 4000RPM at 80mph.

I am looking at buying a 5-speed transmission core and rebuilding it. I have been told that would improve my highway MPG by almost 30%.

Would this be worth it? Do you think this would help at all? Are there other things I could do instead?

Thanks for any and all help.

Tommy

I am only getting 13mpg on the highway now and about 10mpg in the city.

I doubt the 30% increase claim, but if you're only getting 13/10 I'd say you have other issues. My 78 gets roughly twice those figures. You might try searching 'mpg' on this site.