I want to switch out the factory 7.6 Eaton 17121 single-speed rear in a 1972 F-750 for a two-speed. The problem is: I have very little experience with medium or heavy duty trucks, parts or interchange and modification... I don't know what will fit.

It looks like 17200 carriers are plentiful...and they *look* very similar...but will it fit? Is there some kind of axle/carrier interchange reference out there?

And I notice that parts sellers tend to list a single ratio when selling a two speed rear end. Like 6.14 for instance. Shall I assume that this is because they're always specifying the higher of the two ratios, yes? So... can I assume that 6.14 two speed will be 4.10 in its lower ratio setting?

 

Not sure on interchange etc, but all the two speed numbers for that size truck that I have seen here would be 6.14/8.xx something. I've seen these: 6.17/8.58, 5.83/8.12, 6.33/8.81, & 7.17/9.97.

All decodes for two speeds have shown both ratios.

4.10 would be mighty high gears for the tall 9.00 x 20s or similar/bigger tires on that truck.

 

I agree with your conclusion... 4.10 would be way too tall with a 40" tall tire. I've settled on what appears to be a common two speed combination: 4.88/6.65

It appears that the Eaton 17221 diff is quite common...but I don't know whether it'll fit.

Here's what I know about my truck:

REAR AXLE ASSY SINGLE AXLE
C8TA
4A297J
EATON 17121
7.60 RATIO

It has 2" axles with 39-spline ends

Fortunately, the two-speed Eaton 17221 comes with 39 spline side gears and a variety of other appealing ratio combinations.

Does anyone know *definitively* if this will work with the existing housing and axles? I know people out there customize their old trucks... but finding cross-references or interchange guides is difficult...and the community appears much smaller than the hotrod crowd. Having said that, I love this truck enough that I'm determined to get this done. And I know somebody's out there that knows what works. And when I do get it done, I'll post pictures.

Please share your diff-swap wisdom!

 

I talked with a Dana engineer..and here's what I learned: a two speed 17221 differential will fit in the original 1972 Eaton housing - no problem. And since the existing axles are 39-spline...and some variations of the 17221 also come with 39-spline side gears, it'll fit right in. The only modification that needs to be performed to make this work: the long side axle needs to have splines that extend 11 inches down from the end of the axle...which 7 inches longer than a single speed (which uses 4 inch long splines). So there you have it... take out single speed diff and the long side axle, extend splines to 11 inches... install the 17221 two speed.. wire up the shift motor. Good to go.

 

Is your tran's a 6 speed?
Glad I have a 10 speed with 4.56 gears., but wish it did have the 4.10's or 3.90's but scared I would kill the pulling power then.

 

The trans in the truck is a 5652 Spicer 5-speed with 1:1 5th gear. I supposed this made sense when the truck had 10,000 lbs. of pumper truck equipment installed. Now that the truck weighs 7,800 lbs. (in total) the gearing is insanely short. The motor seems real comfortable at about 2,500 RPMs... but at 55'ish MPH on the freeway, it's spun up to about 3,200 RPM and sounds like it's screaming. My last task for this modification: find the 11'inch splined axle shaft. So all I need to determine now: what's the part number of the axle shaft I need.

 

Could you add taller tires and wheels? On my Ford F-800 it has the 7.8 Ford motor, Road Ranger Trans and 4.56. Truck came with small wheels and tires. Now had 24.5 Rubber on it. I'm turning 2000 RPMs at 55 MPH and motor tops out around 75 at 2850 or so.
Check out this link to a road Speed Calculator Big Rig Road Speed Calculator: Semi Truck Road Speed Calculator