89 p30 diesel highway capable transmission?
 

Hi there. First post.
I have a 1989 Chevy P30 Diesel Canada Post step van.
It stopped working in October and my mechanic thinks it's the high-pressure injector pump.

So if I'm getting that fixed anyway, I thought it may be a good time to swap out the tranny, or do something else to get it to idle at highway speeds.

I dont even know how to change my oil, so I put it out here:
is there a gear kit, overdrive add-option, different transmission, or am I looking to replace the rear differential to get good highway economy from this thing?

Many thanks;

- Andrew

which diesel 3.9 or 6.2? either way you should have the th400. the only OD trans that can hold up imo is the 4l80e but you have to get a standalone computer (from 500-$1k)to shift it, a 91 6.2 diesel (if you have the 6.2) or a "black box" which will convert the auto trans to manual (for a fer hundred). the 700r4 might be able to hold up to the 3.9 and 6.2 if its built. but its going to be the low end Tq that kills the trans. i would call a trans shop and ask their opinion. when i called for my 5.9 cummins swap i had a guy straight up tell me he wouldnt touch the 700r4 if it was going behind a 5.9 because it wouldnt hold up. if the 700r4 can be built to hold up to your engine it might be the cheapest, but im not sure about the T.V. cable

I have the 6.2 GM Diesel. Oh, the things I'd do for a Cummins powertrain swap that was affordable.

700r4 should be a cheap swap if you can find a donor