[Big Dave]

What you have done is a fairly common mod done by pickup guys – particularly when they don’t try to plow snow.

The NV4500 is not as rugged as the ZF6-650 but is much easier to find used. Your conversion sounds about like all the others I have heard of. The only quibble I had was I would have put fresh synchronizers in it when I had the NV4500 apart. Synchronizers are not that expensive and they are a wear item. But you can probably get away with what was in it.

I fully understand why you ditched the slushbox, over and above fuel economy. I had an 83 GMC with a 700R4 and destroyed five of them. Spectacularly. Parts and ATF scattered down the road. The 4L60E is nothing more than a tarted-up 700R4. You have freed yourself of much trouble.

Another thing. NV4500s have a reputation of unscrewing the nut holding the output shaft in place. I am told that reputable tranny shops have a sure-fire fix.

Your transmission probably is getting a bit hot. IIRC, the NV4500 does not have a cooler fitting. But you spend a lot of time in top gear which is a true overdrive. That gear mesh has some friction, so there is some heat build-up. My ZF6-650 has a cooler line. Stay with the Amsoil synthetic and watch the color. You should be OK but don’t expect the astronomical oil life a cooled transmission would get.

Looks like just the tranny swap gained you 3 MPG or so. That is exactly what I’ve heard from everybody who ever did such a swap . It is almost a +3 MPG no-brainer for a guy who has an unreliable 700R4.

I think 20 MPG is very doable. The stick opens new vistas of hypermiling. You will become a master of coasting. You can closely match engine RPM and road speed to keep the engine in a high load-low RPM regime which is where the best fuel economy is.

As for aero improvements, you’re in exactly the right place. There are plenty of pickup truck aero projects in here and you can find one that suits your needs. I found a flat tonneau got me a 1.5 MPG improvement over and open bed, but a sloped aerolid will give you about a three MPG improvement over an open bed. This site is chock-a-block with belly pans, grille blocks and air dams.

As for tires, tire manufacturers are pretty close with rolling resistance figures, but two things stand out. Quiet tires generally get better MPG. Also the OEMs have to submit tire rolling resistance to the EPA so the EPA can come up with a MPG estimate (such as it is). Since MPG is a selling point, the OEM has good reason to fit the lowest rolling resistance tires he can get. Also, if you are not gonna mess with gearing (an expensive proposition for a 4x4) stay with the OEM-sized rubber. The factory boys do have a clue about those.