Theoretical transmission upgrade P-30 454

Go with the Gear Vendor overdrive.
I had one in my 86 P-30 and really enjoyed using it. 6 speeds ahead!
Leave the rear gear as it is.
You'll up your fuel mileage about 1 mpg. If you go up any serious grades, you'll certainly like it. The gear Vendor has two driving modes, you can use it like a six speed or just an overdrive.
I went from 8 MPG to 9 1/2 mpg with the Gear Vendor and some tune up effort(Carb work).

Thanks for the informative responses. This is more of an intellectual exercise. I bought it 1 year ago this month and on the trip home I got about 7.8 mpg. Mainly 2 lane 50-55 mph.
I'll be doing a full tune-up(plugs,wires,air filter etc) and other things that will bump it up to the 8 club...I hope

User,

I suggest that you very seriously consider your options.

If this is a plan to get better fuel rate, you may just not get there. Even if you get an extra 1.0 mpg out of it, the payback will be forever. The problem is that the chassis was sold for that service with a best guess final drive ratio. That will put the engine near its torque peak at cruise. If you back down from there, you will be running higher cylinder pressures (more ring wear) for the same speed.

If, on the other hand, this is a hobby move, I suggest you find a recent wreck of a C or G3500 with 8.1 liter. Buy the whole thing and then pull out the driveline (Do Not Cut anything) with all its associated controls sensors and of course the ECU and body controller (may be needed for other things. and then scrap the steel.

Good Luck and keep dreaming, sometimes thing work out.

Matt

That unit should have a turbo 400 transmission; the overdrive replacement is the 4l80e, but it requies a computer for it to work. Best option would be a gear vendors bolt on OD unit for your transmission but they run about $2500. With the 454, od probably wont save enough to justify the cost.

What transmission do you have? Three speed Hydra-matic 4L60 or 4L80 four speed?
If you have the three speed just upgrade to the four speed.

Yes, a new drive shaft and you'd probably want one of the none computerized trannys. Maybe a 700r4.
You might be better off just installing and overdrive unit. That way you the engineering is all done for you. My FIL went this route and it worked great, fully automatic just like it was an AOD trans.