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Rear end swap or keep

MEMO43

MEMO43

1/2 ton status
Joined
Mar 24, 2018
Posts
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Location
Gardenrville NV
So I've been considering doing a rear end swap.
Currently have 14 bolt semi float. Wanting to go full float.
My truck is an 85 k20 with and l31/th400.
Gears are 4:11, tires 265/75r16. summit brand efi.
I'm planning on doing a cam swap (going close to stock) current one doesn't created much vacuum.
I use my truck for truck things as well as occasional daily driver up in the Sierra Nevadas with a 53 mile commute each way.
With my current set up I get close to 13-14 MPG.
I'm thinking of re gear down to 3:73, but not entirely if I want to with the current axle.
I don't carry anything heavy, my off road is forest service road.

What are the pro and cons over semi float vs full float?

Thanks in advance

Memo
 
@eodcoduto thanks for the reply
kind of what of what i was leaning towards it.

@83chevy rough "gestimate" gains in mileage would be an additional 2mpg.
I already have the front end stashed away with 3:73 gears. Scored it from my son whom is getting ready to do a Cummins swap on his k20.
He is doing a dana 60.
I guess the L29 he stuffed in it does everything he needs except the gas mileage

Swap cams an eventually re gear what I have?
 
A 31/32" tire doesn't provide much clearance under a 14BFF. Although you can shave them, it seems like more axle than the setup demands. Now if you're supercharging your big block, that's another thing. If MPG was the main goal, the 3-speed transmission would go.
 
13-14 mph is fantastic, I would kill for 10.
14bff is heavier than the semi, brakes are harder to service. The only real benefits are hauling more truck weights and bigger trailers.
Reducing weight, lowering truck, and remove bug shield running boards and any sun shades light bars roof racks etc to improve milage.

Frankly if you are getting 14mpg I'd leave it alone. Maybe tire size change and keep the air pressure on the high side.
 
Tire size change can be the equivalent of a gear change. 31 to 33 or 33 to 35 can be equiv to going from 4.10 to 3.73 or 3.73 to 3.56
 
A semi float will do all you need. You can likely pick up a sf with the gears you want for much less than the cost of regearing.Then you could sell your current pair of axles to recoup some of the cost.

Changing tire size would be even easier, and an extra set of rims for the new tires would give you the option of changing back easily if you needed the lower end for a towing job, wanted a different tread style, or simply didn't like the change. Check out some of the gear calculators (Grim Jeeper,etc) and choose a tire size that would give you the same ratio as your gear swap would give.

If you use it enough to justify the effort, an OD transmission would be nice. It would take a while to recoup the cost, although drivability may be worth it anyway. Like others though, 13-14 would suit me fine.
 
Even borrowing a pair of larger tires on suitable wheels to run on the rear for a short trial would give you an idea the drivability. You'd have to live with the rake while trying them and for goodness sakes do NOT lock the front hubs !
 
Thanks for all the input
I forsee in my future
Cam swap which I want to get done to increase vacuum, especially at my elevation going 4500' at my house an driving to wrk not only is it 53 miles each way but the elevation to 7100'.

I'll play with the tire size
although I'm pretty close to stock size with my current ones. Playing with air pressure as some have mentioned, vs pulling the trigger an doing a gear swap.

Maybe someday a 4l80e swap for the extra gear.

The joys of square body lifestyle an wanting to DD it everyday, I know here pretty soon up in the Sierra Nevadas mother nature is gona try an bury us in the snow like last year (fingers crossed) Can't beat an old heavy chevy driving through the snow.

Sure glad I pay for the membership here
you guys are awesome at advice an helping me spend money,
now if I could only win the monthly draw, I may be able to fill my tank(s) ;)
 
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