For your application John, I'd say 4.10 are the highest I'd go. You're not making this an exclusive 1/4 mile truck so any higher makes no sense. In most instances, the people who install 4.56 are Drag Racing/Street Racing where it's quick point A to point B. But to each his own.
I'm planning on 4.10 ratio to compensate for the oversized tires I have now and the bigger ones I'll have later.
Steve
DOC Pres

 

But from what I've heard, 3.92 to 4.10 isn't a huge difference and I wouldn't notice much of a difference considering the labor intensive project that it is.

Dunno. Just an idea I'm considering.

 

But from what I've heard, 3.92 to 4.10 isn't a huge difference and I wouldn't notice much of a difference considering the labor intensive project that it is.

Dunno. Just an idea I'm considering.

A 4.10 rear end was the defacto street standard back in the day. It provides fast enough stop light to stop light action while still being interstate driveable. There's a difference between it and the 3.92 but, like you said, not a big one. But not a bad one either. My 3.92 is fine I think for the D, so a 4.10 would be all I'd ever even consider moving up to (very doubtful it'd ever happen unless I need a complete replacement for some reason down the road.)

To move up past the 4.10 would limit over-the-road travel by upping the RPMs and engine wear higher than you'd want for extended travel periods. I think you'd be suprised how often that would come into play, even with around the town type driving. Any longer stretches could become an issue.

JMHO.

IndyDurango

 

John, I put together an RPM calculator based on the normal 31X10.5R15 tires on my D and here's a comparison between 3.92's and 4.56 in Drive and OD.

<table border=2><tr align=center><td> </td><td>60 mph</td><td>65 mph</td><td>70 mph</td></tr><tr align=center><td align=left>3.92 Drive</td><td>2575</td><td>2789</td><td>3004</td></tr><tr align=center><td align=left>4.56 Drive</td><td>2995</td><td>3245</td><td>3500</td></tr><tr align=center><td align=left>3.92 OD</td><td>1776</td><td>1924</td><td>2073</td></tr><tr align=center><td align=left>4.56 OD</td><td>2067</td><td>2239</td><td>2411</td></tr></table>
It's pretty simple math once you know the tire diameter and transmittion gear ratio. Seeing as they'd be constant in any single application, RPM becomes proportional to the final gear ratio.

4.10 is about 5% more than a 3.92 and the 4.56 is about 10% more than the 4.10, so overall, 4.56/3.92 is about a 16% increase in RPM and could change your position on the power curve pretty significantly.

Joe

 

Great insight Indy. Thanks!

That's great info Joe!! So, since I suck at math, here's my dimensions:



Diameter: 30.45" (difference is.46" from stock 17's)

 

Not enough to bother with John. I used 30.7" for my calcs.
Using 30.45" Diameter, RPMS go from 1925 to 1941 in OD at 70 w/3.92.

Joe

 

I have 4.56 in the front and rear but with 35" tires

There isn't much difference in highway rpms. It was a bit sluggish with the 3.92s but now it's normal. I can be doing about 60-65 at just over 2000 rpm. I'd assume that, if I went with smaller tires, the rpms would go up, but then I couldn't run over stuff so that would be boring. I also have the Truespeed module to correct the speedo? didn't help much since it wasn't off by much.

 

It's hard to compare because you're virtually running stock tire diameter.
If you want the gears just to say it cut about 3 tenths off your 1/4 mile time, then get them but, from what you've said, this will be a show truck most of the time so I'd suggest using the money somewhere else. You'll have plenty of power and in broad parameters so I can't imagine you'll need any type of gears for any purpose.
I need them for better drivability and that's it. The 4.11 will bring me back to stock 3.92 with my tires going from 3.55 to 4.11.
Steve
DOC Pres

 

Here's a follow up question - does anyone make a 4.56 for the front differential? I know Yukon makes a 4.56 for the rear?

 

gears

Give the folks at Randy's Ring and Pinion a call. They're the best in gear availability and prics.