1974 CB Rear Axle Question

[Ecavalino]

Alright, how can i figure out which axle ratios i have on my rig? Also any suggestions to best ratio for road/highway as its really just a pavement queen …

 

[thegreatjustino]

https://classicbroncos.com/forums/ams/whats-my-gear-ratio.286/

 

[Apogee]

The link above will help you determine your gear ratio, but as for the best ratio for your application, that will depend on several variables including engine/trans, tire size and intended use.

 

[ol gray mare]

Not to ignore the obvious, if there's a metal tag on one of the axles, it will be embossed on the tag.

 

[DirtDonk]

The Warranty Plate in the glovebox will have an axle code for what it came with originally from the factory.
If you're lucky the little metal data tag gray mare mentioned will still be under one of the third member bolts. Maybe one on the front cover too, giving you the same usable data.
If there's any question of the gears having been changed in the last 49 years or so, then a physical check is needed.

What is the known history of this Bronco? Do you suspect that the gear oils have never been changed? It's not out of line for multiple PO's to have ignored that bit of servicing.
If so, then this is your chance to pull the front diff cover, drain all the old oil, count the ring gear teeth while you're in there, and then renew the gasket and re-fill with new lube. Especially a good idea if this turns out to have a clutch type limited-slip diff.
I forget, but it seems to me that by '74 maybe front limited-slips were no longer available from the factory. But it might have been later, and I also seem to remember that some years got skipped for the fronts.

Another way to check with an external test is to turn the wheels and count the turns of the driveshaft.
You can either mark a tire and mark the shaft and drive the vehicle forward while someone counts the turns of the shaft (or you set up a camera to record it) while you rotate the wheel/tire one full revolution.
With both tires on the ground this should be a direct ratio comparison.
If you lift one wheel off the ground to do the test by hand, an open diff will give you a skewed reading by half. But that's factorable as well. If your shaft turns 1.75 turns to one turn of the wheel, you have a 3.50 ratio. If it turns approx. two turns, you have a 4.11 ratio. Both should still have been available in '74.

If the one-wheel method is hard to do because the wheel is hard to turn, as if the brakes were on (you did release the parking brake, right?!) then you probably have a limited-slip diff in the rear and will need to lift both tires off the ground.
Doing this should get you a full direct reading however. Showing 3.5 or 4.1 turns of the driveshaft to one turn of the wheel.
So that's not a bad thing.

Good luck.

Paul

 

[Ecavalino]

Thanks guys … Looked at the door tag and matched to the Marti Report …. Says Rear Axle is 3.50 “Conventional Rear Axle” … I’m 99.9% it’s original ….

 

[bulletpruf]

Thanks guys … Looked at the door tag and matched to the Marti Report …. Says Rear Axle is 3.50 “Conventional Rear Axle” … I’m 99.9% it’s original ….

You also asked about the best all-around gear, and that really depends on a lot of things, but a 3.50 is a pretty good start, especially for something that doesn't send any time off-road.

To give you a more detailed response, you'll need to provide more info - what engine and is it stock or has it had a performance build, what transmission, what size tires, and what sort of driving do you do? Long interstate trips or just in/around town?

 

[Ecavalino]

Original V8 302 which was rebuilt … have 4 speed manual and run 33” tires … all local weekend cruiser , cars & coffee and stuff … just feel it’s needs an extra gear or longer legs for highway cruising


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