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Rear End Gear Ratio


Alie&Jim's Carrilite

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No Ralf, no green band on the tach...

Company is moving me as of Friday so I won't get this particular rear end, but now my curiosity is going. If we stay somewhere long enough to do a swap I'll do it.

OK but keep in mind with my revs at the top end and even a bit beyond 'most efficient ' , I still get large variation in MPG at the same revs.

At our very light loads revs may not be the most important factor, load is - then rpm.

2005 volvo 670 freedomline singled
Newmar Torrey Pine 34rsks
woot woot

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With the 13spd, we get two OD ratios. So with a 3.08 I could run 50-60 mph in 7th over(direct) which would be perfect of our lower CA speed limits, 8th under (.86) for 60-70mph out of state, and 8th over (.73) for unloaded times where speeds are higher. This way I could actually use the lower gears available with this transmission too.....I never use low or 1st ever. This should theoretically lower both trans and differential temps with less RPM's turned.

 

The only downside I can see is a taller overall reverse gear.

There's two camps out there: one camp says overdrive transmission ratios improve efficiency across the board, while the other camp says there are parasitic losses with overdrive ratios, as output shaft bearings are going to see a relatively higher load trying to twist the driveshaft even faster than before, etc.

 

In the end, a 13 is a 5-speed with a two-range set and a splitter set, with L disabled in high range and the splitter disabled in low range. They're going to make one gear combination direct drive...silly to not do so. However, the gear spacing from 7 to 8 would also impact the spacing from 3 to 4 (which in turn impacts the spacing from 4 to 5, even though 5th gains the splitter). Even if they did rework the spacings, there's a certain amount of strange math going on when 4th is an overdrive BUT the low range is un-overdriving it.

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With the 13spd, we get two OD ratios. So with a 3.08 I could run 50-60 mph in 7th over(direct) which would be perfect of our lower CA speed limits, 8th under (.86) for 60-70mph out of state, and 8th over (.73) for unloaded times where speeds are higher. This way I could actually use the lower gears available with this transmission too.....I never use low or 1st ever. This should theoretically lower both trans and differential temps with less RPM's turned.

 

The only downside I can see is a taller overall reverse gear.

 

Same here, with 13 speeds, 13th(.73) is 75 mph at 1450 rpm, 12th(.86) is 65 mph at 1450 rpm and 10th(1.00) is 55 mph at 1450 rpm. I have 3 reverse gears so I hope the taller gear will not be a problem. As scrap told me once when I started this process, he didn't like "dock thumpers" and I don't like "fast" when backing up.

2017 Kenworth T680
2015 DRV 38RSSA Elite Suites
2016 Smart Prime

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What is your top trans ratio Phil? I have 3.36's in mine with a .73 top OD netting a final of 2.45 rolling on a 42" tire and would like one more OD when traveling on the flats out of CA when speeds are 70+MPH. I used to leave in 4th unloaded and 3rd loaded, but now one gear lower since swapping out the 3.90's. What I found as the written sweet spot really doesn't apply to me only running below 40K gross combined. I would do 3.08's or 3.21's if I had it to do over again.

 

As with all 12-speed FreedomLines -- by far the majority of FreedomLines installed, as the 16-speeds and direct drive twelves aren't all that common -- twelfth gear is 0.78. When I singled I went one step taller, from the 3.55s in the tandem set to 3.36 in the single axle that replaced them, just as in your truck, but with my slightly shorter OD, my final drive ratio is 2.62. I toyed with dropping another step to 3.25 at the time, but didn't. That would have put my "typical" cruising speed of around 67 at 1449 RPM instead of 1498, but would have meant that anything under 60 or so would have dropped me down to eleventh (direct). Like you, I've found that it seems to get it's best mileage around that 1350 to 1450 RPM range.

Phil

 

2002 Teton Royal Aspen

2003 Kenworth T2000 - Cat C12 380/430 1450/1650, FreedomLine, 3.36 - TOTO . . . he's not in Kansas anymore.

ET Air Hitch

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I'm not a big fan of overdrive gears and agree with the theory of less drag in the trans and lower driveshaft speeds. That said, if you want the most out of a multi-purpose rig, I think OD is a necessity.

 

If cruise speed, road conditions, weights, and wind drag were consistent at most times, I would lean to a direct trans and the proper diff ratio.

 

If the op ever makes the swap, might want to consider installing a locker at the same time.....just like the 9", just a lot bigger!

 

Mine should be good for 109mph plus drive tire growth, but have yet to get it there......

1999 Peterbilt 385 C12 430/1650 13spd

2006 Dodge 3500 DRW 4x4

2010 Hitchhiker Champagne 36 LKRSB

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I'm not a big fan of overdrive gears and agree with the theory of less drag in the trans and lower driveshaft speeds. That said, if you want the most out of a multi-purpose rig, I think OD is a necessity.

 

If you want the most out of a multi-purpose rig, overall ratio range within the transmission is probably what matters more. If the gearbox is 12:1 from low gear to high, you can gear it for 109mph top speed and 65mph cruise whether it's OD or not, but the lowest gear is still going to be relatively tall. Put in a gearbox that's got a 16:1 range and now the 109/65 combo gives you an easier start under heavy load.

 

I know you're talking about traditional 13-speeds here, but interestingly the Volvo I-shift comes in direct-drive and overdrive versions. The only thing that's different is how they've "plumbed" the splitter box: as an overdrive or as an underdrive. Everything else is the same, so the percentage difference per gear is the same and the overall range is the same. Find two trucks where one has the direct box and the other has the overdrive box, such that the rear axle ratios are "matched" to give them the same RPM at 60mph, and the only difference will be the driveshaft speed (plus the bearing wear, etc.)

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If you want the most out of a multi-purpose rig, overall ratio range within the transmission is probably what matters more. If the gearbox is 12:1 from low gear to high, you can gear it for 109mph top speed and 65mph cruise whether it's OD or not, but the lowest gear is still going to be relatively tall. Put in a gearbox that's got a 16:1 range and now the 109/65 combo gives you an easier start under heavy load.

 

I know you're talking about traditional 13-speeds here, but interestingly the Volvo I-shift comes in direct-drive and overdrive versions. The only thing that's different is how they've "plumbed" the splitter box: as an overdrive or as an underdrive. Everything else is the same, so the percentage difference per gear is the same and the overall range is the same. Find two trucks where one has the direct box and the other has the overdrive box, such that the rear axle ratios are "matched" to give them the same RPM at 60mph, and the only difference will be the driveshaft speed (plus the bearing wear, etc.)

My trans is the 16.9:1 allowing lots of room to play with.

 

For the OP and others with a direct box, my feeling is that you wouldn't want to be in a position where you would need to operate in an underdrive (lower than direct) gear for 1000's of miles right? Or is that essentially the same as running in OD in terms of bearing wear and heat? I do know since my taller gear swap my loaded gear box temps are much lower than my unloaded temps were on my initial trip home after purchase, but I suppose that could be Amsoil lubes doing a better job too......

1999 Peterbilt 385 C12 430/1650 13spd

2006 Dodge 3500 DRW 4x4

2010 Hitchhiker Champagne 36 LKRSB

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Making the rear a locker is part of the swap to a lower gear.

Cool! In Rockwell I just had apart last week, the Detroit locker was just a drop in that replaces the spider gears just like my Eaton. Simple as splitting the differential ad swapping the stuff. You mentioned re-lashing in you original post? Not something I would recommend......I would measure backlash and duplicate the original reading on reassembly. That gear set has been running and wearing in to that lash measurement it's whole life so setting it tight again might only gain you noise and poor gear contact at the expense of a tiny bit of drivetrain lash. With the Detroit's 1/8 turn play, you would never notice .020" extra lash.

1999 Peterbilt 385 C12 430/1650 13spd

2006 Dodge 3500 DRW 4x4

2010 Hitchhiker Champagne 36 LKRSB

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Cool! In Rockwell I just had apart last week, the Detroit locker was just a drop in that replaces the spider gears just like my Eaton. Simple as splitting the differential ad swapping the stuff. You mentioned re-lashing in you original post? Not something I would recommend......I would measure backlash and duplicate the original reading on reassembly. That gear set has been running and wearing in to that lash measurement it's whole life so setting it tight again might only gain you noise and poor gear contact at the expense of a tiny bit of drivetrain lash. With the Detroit's 1/8 turn play, you would never notice .020" extra lash.

Ok, didn't take 400k in miles into consideration.... I was thinking "new" setup, not swapping used for some reason. Looking thru some of the ads, there are lockers available. For 2 air lines and 4 U-bolts, rolling in an axle is even easier.

Alie & Jim + 8 paws

2017 DRV Memphis 

BART- 1998 Volvo 610

Lil'ole 6cyl Cummins

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Jim;

 

Have you thought about changing tire size? I went to 24.5 low profile that put me in my "sweet" spot.

 

Curt

2001 Freightliner Century, 500hp Series 60, Gen 2 autoshift, 3.42 singled rear locker.

2004 Keystone Sprinter 299RLS (TT)

2 & 4 Wheelers!

2013 Polaris Ranger 800 midsize LE

Our motto "4 wheels move the body, 2 wheels move the soul!"

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