Overdrive Swap Economics

[JCampbell]

Starting to get serious about putting my dually back together. I've been focusing a lot on my options regarding the transmission. I've been seriously contemplating replacing the TH400 with a 4L80e to gain a little highway MPG, but now I'm wondering if the headache and expense will be worth the hassle.

It's not a daily driver. I'd be surprised if it'll see 7,000 miles/year, and 90% of that will be city driving. The only steady highway mileage it will see is the occasional 2,000 mile trip from SoCal to Washington and back. I don't do much heavy towing...maybe 10,000# (max) 2-4 times a year. It's really more of a heavy cruiser than anything else.

The truck was getting 11MPG before the rebuild with a Quadrajet. The new engine will be a TBI unit, so I assume the mileage will stay about the same. Anyone done this conversion before? What are your thoughts?

There's a link to the build thread in my sig.