Rear-End Gear Ratios

[marek4792]

So it appears I just had a differential failure it started hollering for a few hundred miles when I would put the pedal down in the middle of nowhere (I-84 Oregon) got into Portland got a green light and as soon I loaded the clutch " bam bam bam" got another driver to get the trailer. I could move but not really fast with a very light foot and had to lock the power divider to go up inclines.

My question tho I'm running 3.42 Rockwell gears which puts me at 1500 at about 70mph with 295 75R 22.5 drive tires the next ratio available would be 3.21 (no 3.36 available from Rockwell) which unless I want go 80 at 1550rpm everywhere seems extreme for a truck that goes 48 states unless putting it in direct (8LO) on a 13 speed. Would I be better off sticking with the 3.42 or switching them out for the 3.21 and run in direct in the 65mph and below states and possibly sacrificing pulling power at the same time? I pull van/reefer at varying weights up to 80k gross.

[Nilao]

There are no 3.55 or 3.58 available? I wouldn't want to go to 3.21.

[marek4792]

(08-04-2017 )Nilao Wrote:  There are no 3.55 or 3.58 available? I wouldn't want to go to 3.21.

They do but I think it would bring my mpg down in the 70+ states and considering I got 6.9 mpg from Maryland to Nebraska 1500 miles averaging 68-70 mph but the 3.21 I feel would fall on its face as soon I hit Wyoming.

[Rawze]

Sounds to me like your trying to gear it around your bad driving habits. 6 mpg totally sucks and 70 mph beats the S$iut out of the truck.

[Billdozer2]

I have 3.42's in my truck and run 65 at 1400. Half the time I'm only loaded with 30k in the trailer and the other half, the box is empty. If my diffs go, I'll be going up around 3.60's 3.70's so that I can run 60 at 1400. Right hand lane of course. But that's me. I do the same run everyday. So for me it's about maximizing my costs per mile. I'd love to run 60 with my 3.42's but that puts my RPMs at 1300. So any ant hill I approach, engine falls on its face.

If you like to run fast, 3.21's would obviously lower your cruising RPMs, but would make your engine work that much harder when you hit those western states with that kind of weight.

I'd say, if you are happy/content with the 3.42's, stay at 3.42's. I don't think you would gain anything by going to a lower gear ratio other than a lower cruising RPM. Fuel mileage would probably stay the same at those speeds.

My $0.02.

[marek4792]

(08-04-2017 )Rawze Wrote:  Sounds to me like your trying to gear it around your bad driving habits. 6 mpg totally sucks and 70 mph beats the S$iut out of the truck.

Well I mean you aren't wrong I like to run the speed limit and sometimes schedules dictate that. I do see where you are coming from though with the wear and tear though. It probably would be cheaper for me to do just to replace it with another 3.42 which the truck came with and then the power divider before it fails when I get back home.

I mean lets say I drove 55 like you do with 3.42 it would put me around 1134 rpm and if I dropped a half gear (13 Speed) it would put me around 1335 rpm which you say is low for a Cummins especially under torque which is why I tend to drive closer to 65 in those 60-55 states just because I'm not geared for those states (blame the factory) but 3.55 would put me at a disadvantage when it comes to cruising on the 70+ states when I need to get it there yesterday.

I drove an older volvo with a CM570 with 3.58s I think and I was surprised how well the mpg was at 70 mph so its not outta the question that 3.55-58s are pretty good I'm just trying go weigh my options here considering the situation I'm in at the moment.

[Rawze]

(08-05-2017 )marek4792 Wrote:  

(08-04-2017 )Rawze Wrote:  Sounds to me like your trying to gear it around your bad driving habits. 6 mpg totally sucks and 70 mph beats the S$iut out of the truck.

Well I mean you aren't wrong I like to run the speed limit and sometimes schedules dictate that. I do see where you are coming from though with the wear and tear though. It probably would be cheaper for me to do just to replace it with another 3.42 which the truck came with and then the power divider before it fails when I get back home.

I mean lets say I drove 55 like you do with 3.42 it would put me around 1134 rpm and if I dropped a half gear (13 Speed) it would put me around 1335 rpm which you say is low for a Cummins especially under torque which is why I tend to drive closer to 65 in those 60-55 states just because I'm not geared for those states (blame the factory) but 3.55 would put me at a disadvantage when it comes to cruising on the 70+ states when I need to get it there yesterday.

I drove an older volvo with a CM570 with 3.58s I think and I was surprised how well the mpg was at 70 mph so its not outta the question that 3.55-58s are pretty good I'm just trying go weigh my options here considering the situation I'm in at the moment.

I know my comment sounded harsh, guess I should have elaborated a bit more. It was late and all I could see was the "dry/reefer" + "70-mph" + "6 mpg" + "busted rear" and right away it makes me think back at when I first got my own truck and got poor fuel mileage. I had a thousand reasons to "go fast", only got 6.8 mpg on my best days, went 70+ out west, and was exactly where you are at ..> All while wondering why I had hardly any money after bustin my arsse' so hard at the end of the month. This knowing I was hauling the crappiest, most competitive type of freight in America, dry/reefer specifically, didn't help much, as it does not pay for any of those extra costs of "keeping up with traffic". I thought I had it all figured out, I was finally driving an "un-governed" truck instead of that company P.O.S. that was held back to 65 mph.

Back in those days, I wished I had 3.42's or 3.36's because I also thought that was the answer, but I had 3.55's. -- FF a year later and I am damn glad I didn't part with them after I figured out that I just had to knuckle down and manage my time better so that I could keep the speeds down. I managed to get down to about 63-ish and was happy with that for a while, and the 3.55's were perfect for that. Of course, I started getting addicted to the extra money and higher fuel mileage, so I kept slowing down to the now ridiculous 55 mph that I go now. Sorry, but I am just too addicted to keeping extra money in my pocket, not working on my truck nearly as much, not being beat to death by the roads and people getting in my way any more. I have even driven loads at 53 mph on occasion when time and other factors permit. I do however speed up a few mph when in heavy traffic or in construction zones as a courtesy to others, but when out west,.. 55-57 mph it is most of the time (except when rolling down a hill) as there is no traffic. I am just a stubborn bas#tard that way I guess.

=======

Re-gearing a truck is not an easy decision and is expensive. The rear diff is fairly cheap to replace with a different gear ratio, but it can get pricey when you include the front. As a small business owner, I would look at it like this...

Spending money to replace the worn out component is inevitable, but what steps could I take to turn that money into an investment that provides a return/profit for me?. Many repairs can be turned into investments that can provide a long term profit, and replacing the rears is clearly one of them if done right. I could just outright replace the one failed component, but it is likely that the other is worn down just as badly inside too. Replacing both is not out of the question when looking at it like that, so not it is a matter of re-gearing it while doing so. If I re-geared it, I could get a return on my investment, but only if I take other steps to ensure it. Those steps would be to take a hard look at my driving style, speeds, etc. and ask myself,... "Can I discipline myself to make that extra money to re-gear and replace both boxes by slowing down a bit pay off?". That would be the biggest question to answer towards this, and hence -- The harsh comment and words above.

My recommendation is that you take a hard look at how you drive and what place you want to be in a year with your driving style. If you don't see yourself changing by very much (I.E> say, driving 63 in a year vs 70 for example), then anything past "replace what is broken" will be completely wasted money. My point was simply to get you to think about it past just a simple "re-gear it and make my faster speeds work better" because you will never really get a return in that. The few tenths mpg would take years to make up in cost difference and that is not a realistic investment return.

On the other hand,.. the repair and costs you are facing presents now,.. the opportunity to make a change. A change in the direction towards slowing down, getting a return on the money spent long term, and becoming a safer (simply by slowing down a bit), better driver, but that takes effort and discipline that not everyone is willing to make.

Take a hard look at where you want to be in a year with it and go from there is what I say. 3.42's if you want to keep what you got, 3.55's or 3.58's if you want to put it in the zone where you can make a profit on that money spent while slowing down to 63~ish match that new gear ratio.

[Hammerhead]

Hell, I get 7 USmpg with my big-rad non-aero truck with my wind grabbing parachute of an equipment trailer empty w/ a jeep & booster stacked up weighing 70k. That's at 55-60 with 4.10 rears w/11R22.5 not low rolling resistance drive tires pushing those great big definitely not lrr 385 wide steering tires.
If you pull a box trailer, especially if you've got the side skirting and stuff you should be kicking my @$$ all over the place in the fuel mileage game.
The "old myth" that countless truckers spew about "I get better mileage at 70 than I do at 60" is such BS I don't even laugh at it any more. Physics doesn't change, that's why they call them "Laws of Physics". A physics fact, as speed increases linearly, aerodynamic drag increases equally, however as aerodynamic drag increases linearly, the power required to overcome the aerodynamic drag SQUARES!!!

[marek4792]

So just an update so I ended up ordering another 3.42 rear end back at home and going to bring it over to Oregon. I'm really hoping this is the source of the vibration I've been feeling and mysterious noise I've been hearing. Fingers crossed!