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Old 04-27-2014, 10:03 AM   #1
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Gearing a 6.0 PSD for 35" Tires...

Hey ya'll, long time no talk but you're always there when I need you! Thank you!

I did all the BulletProof Diesel upgrades including studs last year and I've been really happy with an aggressive tune and lots of power, but my next upgrade is to 35s!

I currently have 3:55 gears on D60s and 33" tires and its sluggish off the line. With the amount of power my upgraded motor has it WILL pull the van up moderate grades in OD but the RPMs are still lower than I'd like - the van lugs a bit until it hits about 65-70mph.

So the question is what ratio and which manufacturer of gears should I use considering I also want to move up to 35s?

I think with 4:10s I'd be happy enough (I've been told it will put me back around the stock ratio, but now I have much more power than stock) but the thought of having 4:56s and having a mean off-the-line/around town beast is attractive....just don't want my revs to annoyingly high at 70-75 mph (my typical long range cruising speed) or have disgusting fuel economy.

Anyone have 4:10s with 35s and feel like its perfect? Anyone with 4:56s and can comment?

THANKS EVERYONE!
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Old 04-27-2014, 10:53 AM   #2
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Re: Gearing a 6.0 PSD for 35" Tires...

Good to know your extensive mods are working out for you

I have 35" tires on 16" rims and 4.10 gearing. Atlas 40 FICM tune and 65 rwhp PCM/TCM tune by Eric at Innovative Diesel. My rpms are ~2000 at 68 mph.

Seems about right for both off-road crawling and hwy cruising. I can't really comment on the effect of 4.56 on a van with a 6.0 similar to our rigs, but I'm not sure I would want that gearing unless I went to 37s and/or did a lot of towing.
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Old 04-27-2014, 11:49 AM   #3
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Re: Gearing a 6.0 PSD for 35" Tires...

I assume your D60 is a semi-float?

If yes, then stick with 4.10 gears. If you put 4.30 or 4.56 gears in your semi-float, you have go with thin-cut gears, and that makes the pinion gear weak. I actually broke a pinion gear tooth in my 4.30 gearset, and that's what prompted upgrading to the larger carrier and thick-cut 4.56 gears (Mine came with the full-float, which was actually a D70 housing with the D60 gears inside.

I got the gears too hot towing the big tailer up to Alta, and this may have weakened them. About a month later I was loading snowmobiles into the trailer, which required a long run and jumping them into the trailer because of the ramp angle. I did not set the parking brake, and when I pulled away I instantly heard "thunk thunk thunk" off a broken gear in the diff. Still was able to drive the shop though!

Also, I would worry that gears lower than 4.10 could put too much torque through your axle shafts. In fact, I'd say just putting on 35" tires would probably pushing the limits of a semi-float axe.


So.... with all that said.... I plan on upgrading to F450/550 axles front and rear, pending parts availability and scheduling at QuadVan. That means there may be a full-float Dana 70 with Limited Slip coming up for sale soon
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Old 04-30-2014, 11:02 PM   #4
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Re: Gearing a 6.0 PSD for 35" Tires...

Thanks for the tips guys! I think I'm going to run my 33s for a bit until I can find a FF axle. CarringB - you think 4.10s and 35s are pushing a semi-float? Arghhh. I'm at 10,000lbs already!

With this information, it just doesn't seem worth doing a gear ratio change with a semi-floater when I could find a used (and relatively economical) FF axle to install and increase the safety, durability and reliability of the rig.

Let me know if you do swap the D70, that could work out!

If I can't find a FF by the time my 33s wear out (and they're getting there), I may just wind up doing the gear change and taking my chances with the SF axle with 4.10s

Thanks!!!
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Old 05-01-2014, 09:28 AM   #5
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Re: Gearing a 6.0 PSD for 35" Tires...

FF axles are available. Most, if not all, the cutaways have them. Any good truck wrecker should have a couple in the right width and ABS setup for your van.

Mike
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Old 05-01-2014, 11:04 AM   #6
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Re: Gearing a 6.0 PSD for 35" Tires...

Quote:
Originally Posted by khalilsc
you think 4.10s and 35s are pushing a semi-float? Arghhh. I'm at 10,000lbs already!
I do think 35's are too much. Taller tires = more stress on the hubs and also more unsprung weight. Its not that they are guaranteed to fail, but it certainly raises the possibility, and the failure mode on a semi-float is not pretty.

Also if your van is 10,000 pounds, I would be concerned that you may be over its weight rating already. My Dana 70 is rated for 7800 pounds and I'm way over that sometimes, and hence the reason it will be getting a Dana 80.
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Old 05-01-2014, 05:58 PM   #7
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Re: Gearing a 6.0 PSD for 35" Tires...

Geez, now I'm paranoid my rear end is going to fall off on my 10,000 rig
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Old 05-03-2014, 09:47 AM   #8
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Re: Gearing a 6.0 PSD for 35" Tires...

Quote:
Originally Posted by khalilsc
....cut....

Anyone have 4:10s with 35s and feel like its perfect?

.....cut.......
Isn't that essentially the same as stock tires with 3.55? Sounds within typical factory range.
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Old 05-03-2014, 10:53 AM   #9
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Re: Gearing a 6.0 PSD for 35" Tires...

@Chance Yes, in terms of ratio same as stock. But the concern is that with the larger tires and lower gears there is more stress on the rear end beyond its limits, especially considering the weight of our vans.

@ CarringB Yeah nervous indeed! Its always best to be on the safe side - I'm with you there! Anyone know of actual failures of the semi floats with the weights of our beasts? Aren't 4.10s and semi float rears what come on all the Sportsmobiles?

I feel like I should just save up for the FF with new 4.10 trutrac gears and do it all in one shot. Thanks for the feedback folks.
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Old 05-05-2014, 08:04 AM   #10
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Re: Gearing a 6.0 PSD for 35" Tires...

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Originally Posted by khalilsc
@Chance Yes, in terms of ratio same as stock. But the concern is that with the larger tires and lower gears there is more stress on the rear end beyond its limits, especially considering the weight of our vans.

....cut........
In my opinion driving style and/or differences push equipment limits more than the upgrades or modifications themselves. It just happens to be that most people who upgrade equipment do so because they push limits. So it's no surprise stock equipment fails, although I'm not convinced that upgrades are necessarily that much better. Some drivers are very easy on vehicles and others are not. Maybe not because they are bad drivers, but because they ask more of them.
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