I have an 87 Sunrader 4x4with a 4.10 geared full floating 1 ton axle. Front is also 4.10 geared

I have found a brand new rear full floating axle but it's 3.9 geared.

Can I run a new rear full floater with 3.9 geared with a front geared at 4.10?
I know they are close, and I have a 4x4? Any issues or do they both need to be exactly geared?

How do I tell the axel I'm buying is a 3.9. I counted the fins on the outside it has 4 on each side, same as mine.

 

The gears must be exactly the same ratio, or expensive trouble will ensue if you attempt to use 4wd..

No reason you could'nt regear your old front to 3.90 I guess, but I'm thinkin that 4.10s' are none too low for your large truck, as is.

Whats wrong with your current axle???

 

Nothing wrong with it. I just came across a cheap brand new full floater. Thought I'd swap it out, and resell my 28 year old one! And have a few hundred bucks in my pocket.

How can I tell what gear ratio the new axle is? It's stamped 399?

 

Anyone know what the axle stamp 399 is? It's on the new axle. Would like to know the pinion gearing before I pay cash for it.

My old axle is stamped 3Y8
Or how can I tell what gearing is the axle by looking at it?