Long 5th gear ratio - is it worth it?

[kraM]

Morning,

As the title says... the box I've bought to cannibalise has a way longer 5th,
the calculator says it'll drop the rpm by nearly 500rpm in top.  I'm not looking for top speed, just economy,
and a chance not to go deaf, 300 miles to Cornwall at 4000 rpm with a 2.5 jetex exhaust is tiring at best!

Opinions please.   

Cheers

Mark

[jv]

Is there a down-side? Seems like a good idea