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I wrecked my '75 High Boy in the mid 80's, then put it mostly back together. At the time I was planning on adding an overdrives, and had a set of 4.56 gears so I swapped them in, and added a Power Lok to the rear. Life got in the way and the pickup has sat until now. Over time the front axle ends rusted badly, and as I wanted disks anyway I acquired a complete '76 front axle. I am now not going with the overdrive, and plan to use tires slightly taller, than stock height. So my dilemma now is what to do with the gearing, swap in the 4.56 gears for the front, or change the rear ones back to 4.11?
I'm running 33's on my high boy with stock gears and it's still like driving a tractor. so I put a Gearvendor in and that makes it acceptable but I'd prefer it was even higher gearing.
What type of driving do you mostly do? Stop and go or steady cruising? I got slightly better mileage when i went from 3.55s to 4.10s because i do more stop/go and traffic driving. The engine doesn't work as hard to get the truck moving, so it uses less fuel. If you're just cruising along at a steady speed you'll want the lower ratio to keep RPM's down.
What axle and is there a carrier break? Swapping the rear to 4.10's wouldn't be that bad if they fit the carrier.
The other, probably much cheaper, option is find a good used 4.10 rear axle you can swap in and sell your 4.56/powr-lok rear.
Personally, I'd keep the 4.56's/Powr-Lok because I'd probably run some 35" tires which would effectively be the same as stock tires and a 4.10 ratio.
IMO, the minimum tire to warrant a 4.56 gear is at least a 35" if you plan to drive it on the highways at all, 37's or bigger would be better for that much gear. When I swapped from 30's to 35's on mine with the stock 3.50 gears, it was a noticeable difference in acceleration, but still driveable. If I were using it mostly at higher speeds I might have just left it that way. I drive mostly country roads and 55 MPH max, so I opted for 4.11 gears when I bought the limited slip carriers, but have yet to install them. I deliberately traded a little bit of acceleration for trying to keep the RPMs manageable.
Serving suggestion for gear/ tire combos: (IMO whoever made this chart likes high RPMs a little TOO much, but it gets you in the ballpark.)
I've got 4:10 limited slip in the rear of my 75 highboy. I will be running 35s. My driving is mostly short 5-10 miles with a few stops. I have a 79 front axle that I need to regear from 3.54 to 4:09
I have run 2.75's in my '79 F150 4WD with 31's years back and liked it. I think a 3.00 would be just about perfect. I would imagine that a 3.55 would be great for a 33 for street cruising.
I will likely go with 33" tires and my driving will mostly be stop and go short trips. I did some checking and 4.10 and lower use one carrier, 4.56 and up use a different one for the Dana 60.
For my original Goodyear Traction tires, with 16" split rims in my 7700 GVW package , which I believe were 31". The calculated RPM is very close to what I was actually seeing at 55 MPH:
31", 4.10, 2451 RPM
31", 4.56, 2720
33", 4.56, 2555
34", 4.56, 2480
35", 4.56, 2408
At this point I am inclined to replace the Power Lok and 4.56's in the rear, unless 4.10's can be used on the Power Lok with a spacer.
I could not locate any for the D60 with a quick search, but the D44 has "thick" gears to accommodate a lower ratio using a higher ratio carrier. Might be worth some more in depth searching before dropping the cash on a new carrier.
I have changed two 79 Broncos from 3.50 gears to 3.00 and love it. The second one I still need to do the front axle so no 4wd yet. Both 460 swapped with 33" tires. One C6 other Np435.
My current Powr-Lok with 4.56 gearing has new clutches, bearings, etc. and has not been used yet. I am leaning towards going back to 4.10, or 3.73 gearing. Can I take the guts out of my current carrier and add them to this one: https://justdifferentials.com/dana-6...ase-empty.html
You want 4.10 "thin" gears to work on that carrier, IDK if anyone makes a reverse rotation thin set, i only saw standard rotation when I goggled it.
I just googled rebuild kits and the only differences they list are 30/35 spline, nothing about carrier breaks so I'd assume everything will swap over, but you should call or write their tech line and ask. You should check out FB market/Ebay and see about picking up a used carrier if you're gonna swap all the newer stuff over, it'll be a lot cheaper and you can stuff the used guts back in your carrier and sell it. You might even find a whole axle with one for a couple hundred bucks.
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