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Old 05-10-2008, 10:50 AM   #1
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Hi I'd like to get some input and opinions on a Cummins N-14.
I have a 2001 Century Single axle with a 370 HP N14, 10 speed (.74 OD), 3.91 rear gear, 22.5 tire. According to my GPS the motor turns 1550 at 60 MPH and 1650 at 65 MPH. I tow a 36 foot gooseneck car trailer. The trailer weighs 12,000 14,000 pounds at the very most.
I'd like to be able to roll along at 75 mph at 1400 rpm. I used the Roadranger website and their road speed calculator says to use a 3.08 or 3.25 ratio depending on the tire (498 or 501 revolutions per mile).
I haven't reprogrammed the computer to "owner operator" spec. as I wanted to do that after I changed the gears.

1) Do any of you have any experience with gear ratios and the "pros & cons" of a longer gear verses a shorter gear and how it affected their trucks performance, economy etc.

2) Also what are your thoughts on increasing horsepower output? How does this effect the fuel consumption? There are two schools of thought. The first is If you increase HP you would increase fuel consumption. The second is- If you increase HP the engine works less and therefore consumes less fuel. Does anybody have any experience with this?

Right now the truck gets right around 8.5 9 MPG at 60 mph towing the empty trailer mentioned above (7000 lbs.).

Any input would be appreciated.

Chuck
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Old 05-10-2008, 09:19 PM   #2
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.....speed will determine your fuel mileage......my turned up 500hp N-14 will do about the same with 3.58's at 22,000# tractor and 12,000# trailer.....if you want to increase that is cheap go to 24" tires and rims.....otherwise It isn't really worth the $$......unless you are high mileage over the years......and at $4.199 a gallon this weekend it will take forever to pay for itself......mine is set for 105 mph and will do it with ease.....8.5 to 10 is the range with a lite foot at 58 to 60 above that It will drop off .....geofkaye
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Old 05-12-2008, 05:11 AM   #3
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Chuck,
With the weight and configuration I think by changing the rear end ratio is not going to present you with any performance issues. I think changing the rear end ring and pinion would be your cheapest route unlee you were already going to change wheels and tires. The fuel mileage will depend on how aggressive you drive it. uprating the hp will burn more if you keep your foot in it. I would uprate it and change the ring and pinion if it was me.
Wick
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Old 05-12-2008, 10:54 PM   #4
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Thanks for the input. Changing wheels and tires isn't really an option for me at this time. I have polished rims and good tires. I'm going to run it that way. As far as a gear ratio change. It's the cost of the ring and pinion and bearings/seals if required - I'll provide the labor.
What horsepower rating do you suggest? 430/430 or in that range? 370 is pretty light but it does have good "juice". I have to make a trip through the Rockies with it next week so that will tell the tale. I am hoping to get into Cummins and get the computer changed up this week if possible. I want to keep it in the torque range at Interstate speed. I just think if I do want to cruise at 70-75 mph sometimes, I don't want to have to twist the motor to 16-1700 rpm to do it.
I just did a routine check over on the truck today and felt a little too much play in the front bearings. This means a hub change because it's a unitized hub assembly. Priced an new hub today over $700. Ouch!!
Chuck
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Old 05-13-2008, 07:54 AM   #5
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We had some of those hubs on our 2000 models and we replaced the entire front axle assembly instead. Went to a local truck graveyard and bought the whole thing for $500.00.
Wick
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Old 05-13-2008, 09:29 PM   #6
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....hold up here for a moment!...have you ever done a ring and pinion on a road tractor before...do you have the equipment?.....ain't like a no race car at all dude!....geofkaye
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Old 05-13-2008, 10:43 PM   #7
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geofkaye
The answers in the order asked:
1) Yes I have
2) Yes we do
3) No it isn't

Chuck
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Old 06-18-2008, 11:33 PM   #8
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This gear thing has been rung out on one of the other forums, sorry I did not pick up on this thread last month. Decades back the smart owner ops geared their rigs for cruising speed with the transmissions at 1:1 and save the overdrive(s) for faster and/or easy going situations, long gradual downhills like we have out west, etc. 1:1, the shaft going straight thru the tranny, saved on gear whine, countershaft gear loss inefficiencies and bearing wear.

With 22.5 tires and your trany in 1:1 (9th) a 2.5 rear would get you 65mph at 1400rpm, but don't know if can get a 2.5:1 gear set. 2.64 is available and it would get you about 63mph at 1400rpm and 67 at 1500rpm. Drop it into 10th and you'd get 66.5mph at 1100rpm, 73 at 1200, 79 at 1300 and 85mph at 1400rpm, etc.

Why do we need our highway cruise to be in the topmost gear with all the other gears stacked up down below? Put one of those gears above for those situations still leaves you 8 gears down below, plenty for our loads and lets you cruise with tranny in 1:1. Worth considering?
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