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Old 02-26-2009, 04:52 PM   #43
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Originally Posted by cbgenrich View Post
2009 Ford F-150, 3.73 axle ratio - Must have?

That is what I have to decide now.



It is extremely difficult to find a 2009 F-150 in the super-deluxe trim that has the 3.73 axle ratio. Makes me wonder if I'm too focused on that.

The initial Aistream to tow is 5,600 + 2,000 = 7,600 lbs

In the super-cab form factor:

1) 3.15 Non-limited slip axle ratio -> not an option per this thread
2) 3.55 Non-limited slip axle ratio -> not an option per this thread
3) 3.55 Limited slip axle ratio -> 9,700 max conv. trailer
4) 3.73 Limited slip axle ratio -> 11,300 max conv. trailer

I now get that a fifth wheel is out of the question.

In order to get 3.73 I will have to order a custom built truck. I am willing to do that, but is this really necessary?

-CB-
If it was me I would buy the truck with the 3.55. I know I said 3.73 earlier but the 3.55 should be perfect for your present trailer.

If you do decide you need lower gearing, a set of low profile tires will give you the same effect as 3.73 gears. That is, if you later buy a heavier trailer and you need more power.
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Old 02-26-2009, 04:56 PM   #44
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Lessons anyone?

Wow....didn't know we have to take lesson's, know a good Instructor?

I'll turn it off now...
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Old 02-26-2009, 04:59 PM   #45
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CB, didn't you want to have some room to buy a heavier trailer if the 27' you're interested in doesn't work for you? Does the 3.55 (option 3) give you room to do that? If you follow the 80% "rule", you might not want to tow anything heavier than the 27' with the 3.55.

Are you committed to Ford? There may be more options with other brands. There may be a Ford for you with a 3/4 ton, i.e., 3.73 may be more common with a F250. You may be able to find the truck you want via the internet and not have to restrict yourself to nearby dealers. I don't know enough about axle ratios to understand why a 3.55 doesn't work for your situation, but I do know the majority on this Forum prefers higher numbers. Is this myth or conventional "wisdom" at work here?

It's good you're willing to figure this out carefully. By now I might have just pulled the trigger, but I'm not all that patient.

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Old 02-26-2009, 05:01 PM   #46
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The gear ratio you "need" depends, in part, upon the tire diameter. 3.73:1 works well with LT265/75R16 tires typical with 4WD equipped trucks. If you're looking at 2WD, the tires that come with it may be short enough that 3.55:1 produces about the same rpms.
3.55:1 and 3.73:1 are pretty close.

Personally, I prefer not to tow more than 7,000 pounds or 25 feet with any half-ton. Yes, 7,600 and 27 feet is close and I've done at least that much. I wouldn't go through the grief of ordering just to get 3.73:1 on a 2WD, if you're getting 3.55:1 LSD without ordering.
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Old 02-26-2009, 08:33 PM   #47
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Pulled the trigger on the Airstream!!!!

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Originally Posted by CrawfordGene View Post
By now I might have just pulled the trigger, but I'm not all that patient.
Well I just pulled the trigger on the Airstream!!!!

Paid a deposit, subject to inspection by my brother-in-law on Saturday morning or whenever works from him. But I've seen many photos that give no indication of a problem (eBay Motors: 2007 AIRSTREAM 27'FB SAFARI LS TRAVEL TRAILER QUEEN BED (item 350169150007 end time Feb-26-09 15:17:45 PST)) and do not expect a problem with a 2007 Airstream that looks to be in good shape.

You're probably thinking, "He's going to order a truck and wait?". It won't be easy.

I'm thinking this axle ratio is about one of these two things:

1) The axle ratio only matters for the top of 6 speeds in the transmission? The others are all shifted as needed by the load and speed, but once you hit the top gear it just has to rev up to go faster?

2) The axle ratio only matters for the bottom of 6 speeds. It can't start lower than first, so heavier loads just need the higher ratio to get going without overburdening the engine?

Or am I all wrong about that? Because I said earlier that I don't drive over 60, so in case 1) above maybe that bolsters the case for the 3.55.

-CB-
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Old 02-26-2009, 09:14 PM   #48
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Congratulations! Looks like a good price. Is a left over one that never sold? A repo? Even with some problems, still a good price.

Axle ratio: As I understand it, it's about torque. The higher the number, the more torque you'll have at any speed, any rpm, any transmission gear. Torque gets you started and helps you accelerate to get on an interstate, go up a mountain, accelerate out of danger, pass a really slow vehicle on a busy interstate, and tow a load of gold bars. The higher the axle ratio, the less work for the engine. With the high axle ratio, comes a lot of fuel, so transmissions come with overdrive (2 overdrive gears on my beloved Tundra) to compensate.

Airstream recommends not towing over 65 and they tow so smoothly you may find yourself exceeding your personal speed limit.

Gene
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Old 02-26-2009, 10:07 PM   #49
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Gene,

My personal speed limit isn't for towing, it's always, but I thought it might relate to this.

If higher engine RPM at any given speed implies higher torque at that speed, then isn't that what transmissions do? So between the low and the high it adapts by changing gears. My thinking was that maybe this axle ratio thing is all about the lowest or the highest gear. I dunno.

-CB-
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Old 02-26-2009, 10:10 PM   #50
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Personally I like the Limited Slip or Locking Rear-end.... whether you are towing or not. I had a standard open rear-end on an older truck and found myself just sitting there spinning one wheel on wet grass. Once when I pulled over on the side of the road then trying to get back on the road and another just turning around... I'm sure there were countless other times but those were the most frustrastrating because there really was no reason other than just wet grass. Since that vehicle I have always had at least a limited slip.
Thats just my 2cents. Goodluck with whatever you decide.
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Old 02-26-2009, 10:47 PM   #51
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Limited Slip? 3L:73?

Hi, a few notes here.

Limited Slip is just that; The clutches in the differential hold the axles together in a limited way. How? If you jack up one rear wheel and turn this one with a torque wrench, you will find [in some variation] that it will only take about 75 - 250 lbs to turn. What does this mean? It means that if you are on a sandy, wet, or for some other reason slippery road surface, that when one rear wheel starts to slip, between 75 - 250lbs or less, the other rear wheel will help you get moving. If you have one rear wheel in a deep mud hole and the other one on dry pavement, then you spin your wheel trying to get out, [exerting more than the clutches can hold] you just burned up your clutches. Limited slip helps you to not lose total traction of your vehicle in slightly slippery conditions. Repeat: it is not your failsafe for when you get dug in and stuck. Is it good to have? Yes. Positraction [GM] is similar, but instead of constant spring pressure on the clutches it uses a cam that is operated by the spider gears. There are more positive type differentials made, if you're hard core off road, or you could use a spool.

Generally speaking Ford traditionally used 3L:73 gears in all vehicles with the optional tow package. Now they seem to have two levels of tow packages in some of their trucks. Just on that I would opt for the 3L:73 set up.
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Old 02-26-2009, 10:48 PM   #52
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CB,
I think you're on the right track. My old suburban spends most of its time being the family truxter. The higher range (3.42) gear w/700R4 tranny (converted to 4L60) lets me cruise comfortably when not towing, which is most of the time. When towing I just leave it in drive instead of overdrive. The 20+ year old technology doesn't like to tow in OD anyway. If your truck will have a 6 speed w/ 2 ODs, I think you will be able to find a gear that works no matter which axle ratio you have.

Mike

Had an old guy tell me one time that a transmission was simply a torque multiplier.
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Old 02-26-2009, 11:15 PM   #53
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I agree with Gene about torque. The ratio of the rear end and the transmission combine to help produce torque, but the highest torque is generally not produced at the highest rpms. Every engine has a torque band where the the most power is produced.

Think about it. 1st gear and a numerically high axle ratio doesn't require much to get the load moving because most tow vehicles produce good low end torque. As you move through the gears, the engine speed can stay fairly low and still get the job done nicely. And that helps overall mileage.

The low end torque also lets 5th and 6th gear overdrives work while towing. My Tundra usually is in 5th at about 2100 rpms to cruise at 60-65 while towing. It only has the power to stay in 6th (towing) on a real flat, smooth highway. Then the rpms drop to around 1700. It will pull really hard at 3000 (in the mountains) and keeps up a reasonable pace in the steep places. This is only about 50% of the rated red line so I'm thinking the torque band on this engine is in the 1700-3000 rpm range. Put this engine power with a 4.30 rear end and a 6 speed and it will do most anything you ask.
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Old 02-27-2009, 09:16 AM   #54
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CB, yes the transmission is used to distribute torque at different speeds, but unless you have a split axle, the axle ratio does not change. Thus, a high number makes all the transmission gears lower, more torquey.

You won't find a split axle on a "light truck", but it gives you two gears in the differential and a lever to shift the axle too. Thus when you are shifting you go back and forth between low first, high first, low second, high second, etc. I don't think a split axle has been offered in many, many decades on any light vehicle, but it turned a 3 speed transmission into a 6 speed. Auburn had them in the '30's on some of their cars.

I agree with seeleylaker about the way the Tundra operates. I can go up mountain passes at 65 and still have plenty more power at 4,000 rpm, but the price in gas isn't worth the thrill of passing Porsches. Well, occasionally it's worth it and it's easy to do.

I wish there was a definite, clear answer for you about the best truck, but each brand and model has different characteristics and different reliability. Then there's how they look and other emotional factors like "my Uncle Joe had a Dodge in 1953 and it was awful". I didn't have an Uncle Joe, but my father pledged in 1943 not to buy another Plymouth because of something silly that he didn't understand about a warranty dispute. He was wrong, but he never did buy another Plymouth, or any other Chrysler product, and I haven't either; when I met my wife she had a '85 Dodge and it was a bad car, so I guess my father was right. We buy for different factors—for us it's reliability as a prime factor. If we had a bigger trailer, we'd have to look at other brands. Too bad that trailer didn't come with the tow vehicle.

Gene
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Old 02-27-2009, 10:29 PM   #55
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I may have nearly made a horrible mistake.

I was mulling over the whole axle ratio thing, and fired up a spreadsheet to see, well, ratios of ratios. Sounds complicated but 3.55 vs 3.73 is like 1 to 1.05. Not that different. Hmm.

I decided to make sense of the weight ratings while I was in there. It's always seemed like the various numbers that are kicked around are part of some arithmetic that's just a little too intertwined for my mind to *completely* sort out. And I like to be complete in my considerations you may have noticed.

I won't bore you with the spreadsheet, but that's how I got it through my thick head that 1760 lb of payload won't cut it. Plenty of towing capacity with the 3.73, quite possibly adequate with the 3.55 (11300 & 9800 respectively). I was too focused on that. But 1760 of payload works out like this:

760 lb of tongue weight (10% of 7600, the GVWR of the Airstream)
200 lb of gasoline
200 lb of me
125 lb of daughter #1
125 lb of daughter #2
050 lb of dog
-------
1460 lb before pickup cap, lawn chairs, dirty underwear, etc.

I don't really see how the 3 of us could get out the driveway without exceeding the payload rating. Normally it's just me and the dog, but clearly I wouldn't need the long box, because all I couldn't put anything in there denser than foam rubber.

And I see what you all mean - ain't no kind of fifth wheel going to work in this plan.

Now I turn to F-250 vs F-350, 5.4L gas vs 6.4L diesel, 3.73 vs 4.1, blah blah. After punching in the specs for a good many of these combinations I lean towards less is more. The minimum scenario here is F-250 5.4L gas 3.73 axle ratio.

F-350 gives more payload, but it's not clear I need it.

4.1 gives more towing but it's not clear I need it.

Diesel gives less payload but more towing, but it's not clear I need it.

This is where you all come in, if you're still with me. You can help me understand what I need. Because it's largely driven by how much stuff I will put in the truck and the trailer.

Probably been written about in other threads and I'll go look, but...

Full-timer with generator & fuel, fluids and batteries and food for dry camping at times, and inflatable boat and small outboard, a tendency to keep tools and computers with, and the occasional change of clothes...

How much cargo weight in the trailer? How much in the truck?
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Old 02-28-2009, 07:58 AM   #56
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I may have nearly made a horrible mistake.

[deleted]
Now I turn to F-250 vs F-350, 5.4L gas vs 6.4L diesel, 3.73 vs 4.1, blah blah. After punching in the specs for a good many of these combinations I lean towards less is more. The minimum scenario here is F-250 5.4L gas 3.73 axle ratio.

F-350 gives more payload, but it's not clear I need it.

4.1 gives more towing but it's not clear I need it.

Diesel gives less payload but more towing, but it's not clear I need it.

This is where you all come in, if you're still with me. You can help me understand what I need. Because it's largely driven by how much stuff I will put in the truck and the trailer.

Probably been written about in other threads and I'll go look, but...

Full-timer with generator & fuel, fluids and batteries and food for dry camping at times, and inflatable boat and small outboard, a tendency to keep tools and computers with, and the occasional change of clothes...

How much cargo weight in the trailer? How much in the truck?
I have a F-250 Diesel. 8100 or so lbs empty 10,000 fully loaded. The only difference I have been told between the 250 and the 350 is a leaf spring in the rear. Same axles, same frame. 06 Classic 31. 7100 lb empty 10,000 lbs loaded. It usually runs 9,000 or so lbs.

After nearly three years of full timing My 250 diesel and my 31 classic have served me well.
I have a hard topper. Approx 250 lbs. lots of stuff in the back and lots of stuff in the trailer. The truck on the scales comes in at 9800 lbs full fuel. I usually travel alone. I had a friend travel with me last year on my trek east. truck was maxed out at 10,000 lbs. trailer was 9,200.
I have a 3.73 limited slip rear axle that has been fixed once. The clutch pack "burned up". 4 wheel drive(or should i say three) got me out of the mud. Crew cab will reduce you pay load and the long bed will reduce your payload. I have an extended cab, long bed.
Did any of this help?
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