Swapping things around.. inside

[anesthes]

I have two trucks. One is an '85 with a mint frame, good clean diffs, good springs, etc. 4x4 with a manual trans and a crappy engine.

Other is an '87 with a good (recently redone) cab, good engine and automatic trans, broken leafs, and rotted diffs. 4x4, TBI (starts in the mornin!)

I'm planning on swapping cabs, putting the 87 cam + front on the '85 frame, and swapping engines + tranny. So i'm transplanting the cab, wiring, engine, and 700R4 w/ transfer case to the '85 frame.

The only problem I noticed. The nice clean diffs on the '85 have 2.73 gears. The rusty/rotted diffs in the '87 have 3.08s. Will going to a 2.73 make a huge difference, make the truck horrible, and unable to continue plowing?  Or will it be fine?

Any advice appreciated.  I *could* sandblast + paint the diffs out of the '87, but they are pretty rough, and might turn into swiss cheese.

-- Joe

[Captkaos]

The gear ratios aren't going to make a huge difference if one is 2.73 and the other is 3.08.

[Blazin]

My 84 K 10 has 2:73s, with an SM456. It has a 305 in it now but when I got it it had a 283. With an auto I would imagine it might be doggy. I was horrified at first, then I realized the good millage I got from it. The I threw a plow on it for a spare truck. It plowed a hundred times better than my 91 2500. I have yet to have a better plowing truck. I figure the gears are high so the tires don't spin easy. In Low it will push like no other pickup I have ever plowed with. A bunch of years ago we got a 45" storm in about 24 hrs. It pushed strait blade on most of my driveways. It was light and fluffy but at 45" deep its still a crapload of snow to move. I did plow with the storm but there is always areas that don't get plowed when keeping up with it. Plus at 40 driveways most of them had 12" or more on them at any given pass while the storm continued.

[anesthes]

I ran some numbers:

4spd w/ 2.83 @ 1500 RPM = ( 7.7) 14.1,   32.2,    50.6

700R4 w/ 3.08 @ 1500 RPM =        14.6,   27.5,    44.9,        65

700R4 w/ 2.73 @ 1500 RPM =        16.5,   31.1,    50.69,      72

The top is how my '85 is setup right now, the middle is how my '87 is setup. I plan to put the engine + trans into the '85
frame and use that, which is geared 2.73. So it looks like using the 700R4 i'll end up with the third entry. It looks like
first gear is gonna bog a little bit maybe, 2nd will actually have a little more bite than the manual, and 3rd gear
is the same. Overdrive looks as if on 'highway' driving figure between 1400-1500rpms at about 70mph.

My concern is 1st gear. Is the truck gonna take off from a dead stop (especially towing or plowing) like it's hauling 100,000 lbs??
Or is it such a minor difference it won't matter?

-- Joe

[anesthes]

hrmmmm

[VileZambonie]

I think you are a bit off on what you are trying to accomplish. The final output ratio to you may look good on paper but take a few things into consideration. #1 being where your throttle has to be open to acheive the necessary propultion to move that vehicle in any range. Things you should consider - your engine peak efficiency, your tire size and what you are doing with the vehicle.

A truck with a 350/700R4 with a 3.08:1 with 35" tires on paper may look good but in reality your foot is going to be in the throttle sucking down the fuel to keep it going 75 MPH on the highway.

The same set up utilizing a 3.55:1 final drive will actually consume less fuel at the same speed. Something many people don't take into consideration and obviously early GM engineers didn't either.