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So i rethought my last discussion and appreciate the feedback, so I'm going for a more conservative build with a 4bt.
I found some good deals online that are from old bread trucks from salvage yards that they cut the front clip off and sell everything from the air cooler to the trans mission.
But I'm not sure about the fs4205 5 speed transmission. other than that the engine and what I need is all there as a package deal.
I'm basically going for a daily driver with fuel economy that is a total dog. The resarch I've done is showing me with my current set up I could get 30+ miles to the gallon.
But that is with a NV4500 trans. I'm trying to find how much difference there is between the two transmissions an if I could pull it off.
 

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The FS4205 is a good transmission but it's a 5 speed without OD. Not likely to be your best choice for fuel economy unless you have an extremely high ratio rear gear. That engine most likely has an SAE2 transmission adapter. There are now aftermarket bellhousings that allow you to bolt a Dodge Cummins NV4500 to that adapter plate. The other option would be replace that adapter and clutch/flywheel assembly with one from a Dodge Cummins. I'm assuming your engine may have the P7100 injection pump. I'd say 30+ MPG will be almost impossible unless you have a very light weight vehicle with good aerodynamics. With most installs, around 25 MPG is more average no matter which injection pump you have. Getting to 30 MPG has rarely been done but it's not impossible. A lot of factors affect MPG.
 

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I totally agree with char1355 comments. If you "need" daily driver fuel economy, buy a VW TDI passenger car and keep it in decent repair.

That said, look at the truck in my signature below. 9 inch Ford rear end with a high gear ratio swapped in. Note larger rear wheel and tire. I do not recommend this Ford 5-speed over-drive transmission due to the "PAIN-IN-THE-A$$" procedure involved in frequently replacing the internal clutch slave cylinder (transmission has to be removed).

I get in the low-mid 20 MPG. Smiles per mile is good. Effortlessly does the speed limit (or maybe a little more...) using the cruise control. 37 states so far.
 

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Russ, that internal or "infernal" slave cylinder has been a constant complaint on those small block ZF's. Same goes for the small block Ford/Mazda transmission. The best of the ZF's to use is the one that came behind the F250/350 7.3 diesel. Has better gear ratios and a external slave. Only problem is it costs a good bit more to adapt to the Cummins. The parts cost in the $1500-2000 range. The NV4500 is cheaper but doesn't have the best gears for a 4bt. There just isn't a perfect transmission at a reasonable price for the 4bt. As far as fuel mileage goes, higher numbers can be done but at a cost. I had a magazine article where some guys took a 6bt Dodge to see how good they could get it. Did everything they could to make it more aerodynamic, never drove over 55 MPH, no ac, windows always rolled up. Very uncomfortable in hot weather. They got it to a tad over 30 MPG, but it wasn't something most people would want to live with. Now if you were a young person with tons of money to play with, a company in the UK named Trident Sports Cars may have the perfect car. It is powered by the 6.6 L Duramax diesel and comes in 3 power levels. Base is 379 HP with 700 lb ft torque with options of 430/950 and 660/1050. It meets all European smog regulation, has a top speed over 190 MPH, and in normal driving gets 58 MPG. Oh, and it's drop dead beautiful. Comes in a coupe or convertible. That $162,000 price sticker, which may be higher by now, is just sort of a limiting factor to most of us.
 

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So i rethought my last discussion and appreciate the feedback, so I'm going for a more conservative build with a 4bt.
I found some good deals online that are from old bread trucks from salvage yards that they cut the front clip off and sell everything from the air cooler to the transmission.
Hey Yosemite, congratulations if you actually found a 4bt 3.9. Every time I thought I found a frame unit, it turned out to actually be an ISB-3.9 that was incorrectly listed.
 
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