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  #1  
Old 04-02-2010, 10:56 PM
340king 340king is offline
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Help Gear Selection

I have a Johnny Cash mobile that I need to determine the best gearing for. It is an '84 D350 crew cab dually. I grafted a '91 6BT non-intercooled engine mated to a '97 NV4500 trans into the unit. It has 4.10 gears and I would like a more highway friendly gear. The truck does see towing duty, most of it generally light, but I do plan to get an enclosed car trailer in the future.

The only mods to it are a 16 cm² turbo housing and it is straight piped. Otherwise it is just as I received it.

I am torn between the 3.73 and 3.54 gear sets for the Dana 70 rear axle. I see benefits both ways. I am leaning toward the 3.54. My only concern is that when pulling the enclosed trailer that it will want more gear. Currently I use all of my pedal to make rpm and not really using it for pulling. I have been told it runs like a tractor. You set the rpm and it just runs there.

In a perfect world, I would really like to install the ARB Air Locker kit, but the budget just doesn't have it in it right now.

Any suggestions are welcome.
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Old 04-03-2010, 10:11 AM
peg leg peg leg is offline
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Default That engine

Was rated at 170HP and around 360 torque, if memory serves me. If you intend to pull that enclosed trailer, or any heavy weight, you had better consider staying with the 4.10 ratio.
You can bump the ratings of the old twelve valves pretty nicely, with injectors, timing plate and an intercooler.
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Old 04-03-2010, 09:05 PM
340king 340king is offline
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Default Enclosed Trailer

Thanks for the reply. I was thinking about the 3.73 as middle ground to reduce the rpm at cruise and still be geared a little deeper than the 3.54. That way I would stay in the better rpm range for the smaller turbo housing.

I have pulled some enclosed trailers before. The last was a tall 8.5 by 20. We filled it and the dually with stuff and headed out on a 700 mile trek. Pulled OK at 62 to 65 mph. Checked it out at the scales when I got home and we were right at 20,000 lbs gross. It got over 13 mpg on the flat and dropped down to around 11 when we started pulling the river hills. We would slow to around 45 mph on the big hills. Otherwise it was pretty good pulling.

So I was thinking if I can gear a little taller, it would be OK with a normal load. Pulling that same trailer down to Oklahoma empty we got 15 mpg at 72 mph. Everybody that has used that trailer in the past has said it pulls like a parachute. I thought it was just fine. I race a 2,200 lb car with modest supplies, so I won't be grossing anything like that last big trip.
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Old 04-03-2010, 09:58 PM
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ehostler ehostler is offline
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Default

What size tire? It changes the final ratio.
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Old 04-04-2010, 01:21 AM
340king 340king is offline
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Default Rear Tires

The rear tires are 235/85R16. They roll out to 98.25" with their current amount of wear. The 4.10 gear makes it scream just to get up to highway speed. It tops out about 82ish right now.
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Old 04-04-2010, 09:38 AM
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ehostler ehostler is offline
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Maybe keep the 4.10 and go with a GV OD unit.
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Old 04-04-2010, 11:59 AM
peg leg peg leg is offline
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Default Tires!!

Man, that 85 aspect ratio tire is changing your final drive ratio quite a bit. Those trucks most likely came with a 65 aspect. So, you are really running close to a 3.55 already, maybe 3.73.
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Old 04-04-2010, 04:07 PM
John Kunkel John Kunkel is offline
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The NV4500 has a .75 fifth gear so that 4.10 becomes 3.08 in top gear which means you're only turning the engine about 2000 rpm at 60 mph.

I used to run my old '92 hauler with 3.54 gears and the same trans at 15,000 pounds in the Sierra Nevada mountains...it didn't set any speed records but it did the job. Had to pull it out of fifth on steep grades.

I consider 3.54 to be the ideal gear for the Cummins.
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Old 04-04-2010, 10:02 PM
340king 340king is offline
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Selection

Again, thanks for all the replies.

I think I will venture for the 3.73 ratio. That was originally my idea, but had many suggest that I purchase the 3.55 set instead. I have an '03 Hemi Ram that I use for highway trips now, so empty economy isn't as important as if this were my only truck. I hope to get my '79 short box step side on the road this summer. It has something like 2.75 rear end gears with a 3 spd OD trans. It turns around 1,400 at 60. Don't know as I haven't had a speedo or tach in it. It just chugs in 4th at about 55 (I was following another car).
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