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Old 01-08-2015, 06:24 PM   #1
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Default New build on FL120

Just signed up, but ive been watching you guyz for awhile now. Have a few ?s. I think I need a 24 foot box for the layout I'm planning. At least 22 foot, I would like to keep it as short as possible. Will have the master bed in the overhead above the cab. Planning to keep the sleeper and install seating, add passenger door (s). 4 bunks in the rear, water and waste tanks underneath in rear. House batts and 7.5 diesel kohler gen as far aft as possible. The rest will be just normal stuff. House type appliances. I'm trying to keep as much weight of the front as I can. Is this neccessary ? Also would like to keep rear overhang as short as practical. What are your reccomendations for axle placement. I have the M11 originally rated at 370 hp, with a 500 hp ecm bump and hx55 turbo. It has 4.10 rear gears and 9 speed Eaton trans. What rear gear ratio would you like to have in this rig. Thanks guys.
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Old 01-11-2015, 12:55 PM   #2
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I don't think I'd like that gear ratio. My truck is single rear axle, 26' living quarters. 10 speed Eaton autoshift. I don't know if the 10 gear is higher than the 9 on your trans. I know at 70 mph. I'm turning about 1425 rpm's. I'm 29k lbs. with full water and fuel. I'd even like a lower numerical rear axle so I could drop the rpm's at 70 to like 1200-1300? It's not worth it for me to have the gears changed out to do that. But, if the rear axle were to need work I would certainly look into that. I bet with 4.10's your going to be screaming along at 70 mph.

I wouldn't go any shorter than 26' living quarters. My wife and I have gone on a few 3 week trips. Nothing longer. Ours is just great at this size, one slide out in the living room. I think smaller would not be as comfortable. Plus you lose storage space underneath. My storage bins are all pretty full.
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Old 01-11-2015, 02:30 PM   #3
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Im with Bob !

At 72mph I'm turning about 1650 rpm and it puts my MPG in at a consistent 9.9
when I've had to travel in the 1200/1300 rpm range my mpg was slightly over 13 (towing our wrangler too).

I cant remember what my door tag stated as to the rear gearing on our coach but i know its lower than a 4.10 (its a 3.xx something) and id prefer to have an even lower gear set - we don't run the coach around town - we run it on the highway for great distances 100s of miles at a time.

Our coach is programed to start out in 3rd gear - and if i could i'd re program it (myself) to start in 5th or 6th.

If i need to move slow or leverage low rpm torque ill shift it manually, til i get to 3rd or 4th gear, then let the computer take over and provide some skip shifting !
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Old 01-12-2015, 09:25 AM   #4
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My trans has a .73 final drive ratio. With 4.10 gears and 42 inch tires this gives me about 77 mph at 1850 rpm. The lower gears are mostly worthless for this application. My calculations show that with a 3.50 rear ratio top speed is 90 mph and at 1250 rpm its 61 mph. 3.00 ratio gets me 105 on the top and 71 mph at 1250 rpm. !00 mph is a little to quick for my liking but 1250 rpm at cruise would be nice. Most of my driving is on the flat but even for pulling the long western hills direct (8th gear 1 to 1 ratio) should work fine. More thoughts on rear gear ratios would be much appreciated. I need to decide befor I start looking for a frame section at my local truck wrecker yards, i plan to buy the section with axle attatched. I will seriously consider the 26 foot box. It would be easier to build it that way than stretch it later. Thanks guyz.
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Old 01-12-2015, 10:06 AM   #5
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i was gonna say i think mine are 3.73's but i didnt want to provide misleading information & your rpm's at the given speeds sound familiar to me & my experiences.

id be lying if i said i hadn't (on more than one occasion) buried my 85mph speedo - but its not something i TRY to do....although the frequency of touching 80/85 seems to increase w/ time, experience and comfort.

at 1100 rpm my MBE4000 (aka DD13) is almost bogging (when in 11 or 12 gear). Im not sure that the move to a 3.50 would be worth it to me, especially since the interstates that i run on are all 70 if not 75 mph.....but the 3.00 would seem perfect (assuming i kept it at 70 all the time)....i wonder what it would be like w/ the 3.00 in backroading to/from highway.
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Old 01-12-2015, 01:01 PM   #6
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I generally like to run the posted speed, but its nice to be able to shift up on the easy flats and long down hills and just loaf the engine along and still maintain that speed.. And, theres less gear drag in the 1 to 1 hole so 8th would be the normal highway gear with overdrive reserved for the easy stuff. Theoretically anyway.
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Old 01-12-2015, 01:58 PM   #7
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Quote:
Originally Posted by kulas44 View Post
I generally like to run the posted speed, but its nice to be able to shift up on the easy flats and long down hills and just loaf the engine along and still maintain that speed.. And, theres less gear drag in the 1 to 1 hole so 8th would be the normal highway gear with overdrive reserved for the easy stuff. Theoretically anyway.
i agree, for the most part i set my cruise for all its worth (72 mph).
In virginia (i81) thats about all anyone will run for fear of getting a ticket from the VSP.

Running i10 from texas to florida the posted limit is 75 in many places and up north on i40 between amarillo & abq the posted limit is 80.
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Old 01-19-2015, 07:59 PM   #8
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correcting my earlier posts (my referenced RPM's were too high).

I was out in the coach this weekend for a local car race and recorded more accurate numbers.

1300 at 65 mph & 1450 at 72 mph
I would certainly prefer to be in the 1200 range at 75 mph.

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Old 01-20-2015, 03:19 PM   #9
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That's about what I'm at too, Don. Anybody know what it would take/cost to change out the gears in the rear pumpkin? Both Don and I have single rear axles. I'm going to check with my local Kenworth dealer when I'm passing by there next.
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Old 01-20-2015, 03:19 PM   #10
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I assume there would have to be some way to correct the speedometer after a gear change too.
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