New build on FL120

[kulas44]

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.

[Bob86ZZ4]

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.

[bushpilot]

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 !

[kulas44]

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.

[bushpilot]

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.

[kulas44]

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.

[bushpilot]

Originally Posted by kulas44

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.

[bushpilot]

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.

[Bob86ZZ4]

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.

[Bob86ZZ4]

I assume there would have to be some way to correct the speedometer after a gear change too.

[bushpilot]

Originally Posted by Bob86ZZ4

I assume there would have to be some way to correct the speedometer after a gear change too.

depends on where the speedo pick up is....

if its on the transmission (and not the rear diff) do you have to change the speedo (gear) at the transmission to speedo cable ?

I just learned (from scangauge) that there may be separate speed sensors or pickups on each of the front wheels (perhaps for ABS ?)

Linear Logic (makers of ScanGauge) tells me based on some CAPTURED (PID) Data that i reported to them, that i have additional items that i can monitor...including:

• Front Axle Speed(MPH)
• Relative Speed; Front Axle, Left Wheel (MPH)
• Relative Speed; Front Axle, Right Wheel (MPH)
• Relative Speed; Rear Axle #1, Left Wheel(MPH)
• Relative Speed; Rear Axle #1, Right Wheel(MPH)

Mind you the process for adding these additional data points is a bit of a PIA i.e.:

Percent Clutch Slip(%)
TXD: 00
RXF: 11F002020000
RXD: 3008
MTH: 000200050000
NAM: User Defined

so i haven't done it yet to see if i can even view the data or not.

[orangepeel91]

Why in the world would you get a re gear quote from a dealership? There are junkyards for trucks too. How much it costs depends entirely on what axle you have, what ratio you want and how common it is. Re gearing itself is easy. pull the axle shafts on each side (access by removing bolts on hub.) then attach chain and hoist to axle center section. Remove bolts that attach differential assembly to axle housing. Remove Diff. Replace with another assembly of your ratio choice. this is all bolt on. You are pulling out and exchanging entire assemblys so no need to worry about setting up proper gear lash and mesh pattern. Junkyard diff for my bus is $1k exchange. Your price WILL vary.

[kulas44]

Have'nt been here for awhile. Still working on the truck, removed all the exhaust and air wing stuff. Truck has a sleeper. Would like to remove it (cut) and go straight to "box". On that subject, I had planned to build the box but I can buy a nice 26 foot frp box way cheaper. What are the trade offs and problems going that route?

[hot rod]

On one of the other recent posts on the forum the guy was going to sell off his frp box in favor of buying a brand new box sized to his specs. Look back through the recent posts.

[hot rod]

I wouldn't spend a bunch of time and money on speedometer work. Just use a gps with a speedometer readout on the screen. Way more accurate than a mechanical speedo anyway, plus it tells you the speed limit and goes red when you go over. That's what I do in my truck.

I was going to suggest a gps speedometer like these:
Search results
but they are still a little pricier than I thought. They have models for cars, motorcycles and boats, no reason you couldn't use one in a big truck.