C30 dually rear diff gearing options help please

[evan love]

Hello,

I just bought a 1979 C30 single cab dually with a very recently rebuilt 454 (100 miles ago) and I assume th400. It is also the camper special 10,000 GVW Silverado with 70,000 original miles.

I plan to tow a car trailer with a '73 K10 on it, so I want to retain that level of towing, but also would like to get better gas mileage and higher highway speed.

So, the question is, what is the absolute least expensive way to get better gas mileage and lower rpms to raise highway speed given that everything is in good condition and I don't really want to work on this one forever, I'd rather drive it.

It does have 4.56 gears in the rear diff, so perhaps that is the place to start.

Gear Vendors looks super cool, and easy, but $3,500 would take a while to make back up in improved gas mileage, wouldn't it... ...anybody done the math on such things?

I figure maybe 5,000 miles per year, which doesn't justify spending too much on mpg saving hardware, does it?

thanks, I appreciate input on this, and I don't actually know what mileage it gets now