gear change

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Sep 15, 2005 | 08:44 AM

superduty lee

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From: southern california

gear change

I have an 02 crew cab short bed 4x4 with 3.73 rear end , 5" lift with 35" tires and pull a loaded toy hauler that weighs about 8500 lbs . I was going to change to 4.30 rear end but the shop i want to do it says i should go to 4.56 , does anyone have any input on this change. I have also thought about removing my lift and going back to stock tires ,but i do like the way the truck looks .

Sep 15, 2005 | 09:41 AM

ken04

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From: Vancouver Wash USA

Quote:

Originally Posted by superduty lee

makes sense, your tires are nearly 20% larger in diameter than the stock size so a 20% gear tooth increase puts you at 4.47-1 to maintain your 3.73 ratio. I may even go a tad lower (numerically higher) to give yourself a 4.10 ratio. If you like the look of 35's, and who doesn't, the gear swap is the only realistic way to restore power and save wear and tear on your running gear. Towing 8500 lbs of trailer with 3.0-1 gears is going to beat your drive line parts to death. I know first hand after I saw my brother replace 2 trannies, a carrier bearing set and pinion bearings in his 4.10 geared F350 with 38"s.

Sep 15, 2005 | 09:49 AM

2000silverbullet

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From: Gilbert

I guess we'll see how long my 3.73's last me with my 37's then. ha ha. Going on 3 years now with 'em and I can't even remember what it's like to have the truck the way it used to be. I figure as long as I still get my BSEG's, I'll be ok. Unlike the most of you that might get your BSEG's from the speed of the V10, mine comes from the grunt of driving over things.

As for the initial question, I'd go with 4.10's (4.30's at most) so you can still get some decent gas mileage on the highway when empty. My .02.

Sep 15, 2005 | 01:38 PM

krewat

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From: Long Island USA

Let's put it this way:

The Superduty ('04 and earlier) in stock trim is good for towing up to 12,500 lbs with STOCK-sized tires and 4.30 gears. With 3.73 gears, that drops to around 10,300 or so, depending on the empty weight of your rig. In reality, 3.73's are good for about 8000 or so, MAYBE in real-world driving.

So, you have 8500 lbs, 3.73 gears and tires that are 4" taller than stock (mine are just shy of 31"). The taller tires take away 13% of the torque reaching the ground compared to the stock 31" tires.

Going from 3.73 to 4.30 is a 15% INCREASE in torque to the ground, so 4.30's will bring you back close to what your truck was putting down stock w/3.73's.

4.56 is 6% better than 4.30's.

So, what I'm trying to say is, a 3.73 geared truck with stock tires pulling 8500 lbs is "OK". With 4.30's, and stock tires, it's much better (according to many here).

If, with 35" tires, you want the performance other people get with 4.30's and stock tires, you need 4.88's.

4.56's are a good compromise, being right inbetween the performance of 3.73 and 4.30 gears, using your 35" tires.

Does that make sense?

art k.

Sep 15, 2005 | 10:04 PM

BFR250SD

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From: Phenix City, AL

I'm running about the same setup. Get the 4.56's. That will take into account the additional rotating mass of the tires as well as the size. Pulling the trailer, you'll be glad you did.

Sep 16, 2005 | 09:14 AM

ken04

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From: Vancouver Wash USA

Quote:

Originally Posted by krewat

So, what I'm trying to say is, a 3.73 geared truck with stock tires pulling 8500 lbs is "OK". With 4.30's, and stock tires, it's much better (according to many here).art k.

Agreed, at somewhere around this weight, I tow OK, at around 10K it's more of a struggle. I don't mind rpm's when I'm towing but it's not so good for gas mileage. 4.10's or 4.30's would put my rpm's in the peak hp/torque range for towing that I do not have with the 3.73's. But with up to 8K it's not that big of a deal plus the lighter weight overall of my 5200 lb rig helps. I even dropped a tire size to put my effective gear ratio around 3.95-ish which makes a significant difference with a heavy load. The 4.30's would be so much better.