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russlars
09-30-2009, 03:16 AM
I am looking at a new tow vehicle which is available in either a 3.31, 3.55 or 3.73 ratio limited slip axle. Can someone explain the pros and cons of a higher vs. lower axle ratio? I would think that this would affect fuel economy, however, all three vehicles have the exact same EPA Fuel Economy Estimate.

Jerseydave
09-30-2009, 04:52 AM
Higher the number, the better it will be for towing. Depending on how much you will be towing with the truck, and how hilly your area is I would lean toward the 3.73 gears. You may suffer a little with gas mileage while solo, but well worth it IMHO. My suburban has 4.10 gears, a real towing machine but poor economy. 3.73 would be a better choice.

What vehicle are we talking about? Some of the new GMs have 6-speed transmissions vs. 4 speeds making them even better for towing.

Check again, I cannot believe the EPA estimates would be the same for all 3.

trickskier
09-30-2009, 08:48 AM
I have a 3.42 in my 09' Tahoe with a 6 speed transmission. My Suburban has a 4:10 with 4 speed transmission. Both have the 5.3 L engine. I prefer to pull with my Tahoe - Does a better job on the hills than the Suburban.

JimN
09-30-2009, 09:45 AM
I am looking at a new tow vehicle which is available in either a 3.31, 3.55 or 3.73 ratio limited slip axle. Can someone explain the pros and cons of a higher vs. lower axle ratio? I would think that this would affect fuel economy, however, all three vehicles have the exact same EPA Fuel Economy Estimate.

It's the number of teeth on the ring gear vs the pinion gear in the differential. If you ride a bicycle, think of how the crank gear relates to the gear cartridge at the rear wheel. If you use a smaller gear at the crank and the largest one at the rear wheel, it's easier to pedal uphill but you can't go very fast and as you increase the number of teeth on the crank gear but use the same cassette gear, it becomes harder to crank uphill but you can go faster. The larger cassette gear has a higher ratio to the smaller crank gear than when you use the largest crank gear.

They may have the same EPA estimates but I'd bet the transmissions are different.

EarlyriserX9
09-30-2009, 10:25 AM
It's the number of teeth on the ring gear vs the pinion gear in the differential. If you ride a bicycle, think of how the crank gear relates to the gear cartridge at the rear wheel. If you use a smaller gear at the crank and the largest one at the rear wheel, it's easier to pedal uphill but you can't go very fast and as you increase the number of teeth on the crank gear but use the same cassette gear, it becomes harder to crank uphill but you can go faster. The larger cassette gear has a higher ratio to the smaller crank gear than when you use the largest crank gear.

They may have the same EPA estimates but I'd bet the transmissions are different.

Jim, wouldn't it be easier to pedal up-hill if the gear on the crank is larger, not smaller? I thought it would be harder to pedal up-hill if the crank is smaller and the wheel gear is larger?

russlars
09-30-2009, 10:34 AM
Higher the number, the better it will be for towing. Depending on how much you will be towing with the truck, and how hilly your area is I would lean toward the 3.73 gears. You may suffer a little with gas mileage while solo, but well worth it IMHO. My suburban has 4.10 gears, a real towing machine but poor economy. 3.73 would be a better choice.

What vehicle are we talking about? Some of the new GMs have 6-speed transmissions vs. 4 speeds making them even better for towing.

Check again, I cannot believe the EPA estimates would be the same for all 3.
2009 Ford F-150 with the 5.4L engine. I have the window sticker from all 3 vehicles and they all list the EPA gas mileage at 14/18. I wonder if this is a generic estimate for all F-150's?

russlars
09-30-2009, 10:36 AM
It's the number of teeth on the ring gear vs the pinion gear in the differential. If you ride a bicycle, think of how the crank gear relates to the gear cartridge at the rear wheel. If you use a smaller gear at the crank and the largest one at the rear wheel, it's easier to pedal uphill but you can't go very fast and as you increase the number of teeth on the crank gear but use the same cassette gear, it becomes harder to crank uphill but you can go faster. The larger cassette gear has a higher ratio to the smaller crank gear than when you use the largest crank gear.

They may have the same EPA estimates but I'd bet the transmissions are different.
Thanks Jim. Good explanation that I can relate to being a bit of a mountain biker myself. I will check again, but I am pretty sure that they all have the same transmission though.

Andyg
09-30-2009, 10:40 AM
I have an 06 F150 with the 3.55 gears and it doesn't tow as well as my 03 F150 did with the 3.73 gears. I would recommend the 3.73 gears for towing in the F150. Hopefully Ford has developed a better method for changing the transmission shift points other than turning O/D off. When I tow my 197 on the interstate I am lucky to get 10 mpg. Granted the newer engines are supposed to get better gas mileage, but I would be surprised if they do when towing.

JimN
09-30-2009, 11:11 AM
Jim, wouldn't it be easier to pedal up-hill if the gear on the crank is larger, not smaller? I thought it would be harder to pedal up-hill if the crank is smaller and the wheel gear is larger?

Nope- Lots of revolutions at the pedals (small gear) and not much at the rear wheel (largest gear) is easier.

JimN
09-30-2009, 11:12 AM
Thanks Jim. Good explanation that I can relate to being a bit of a mountain biker myself. I will check again, but I am pretty sure that they all have the same transmission though.

Might have different shift points, though.

dummy
09-30-2009, 11:14 AM
This is more general info for others - I understand it doesn't really answer your question:

I've towed with that F-150 before. Trailer & Jeep combo weighed about 5,500lbs and it was up Hwy 395 into the Sierras. I think that's pretty similar to the weight of a boat/trailer. Truck had the 5.4L and 3.73s. Pulling power wasn't wanting at any time, but the Ford's ABS system would kick on under hard stops and effectively cut braking power. Rolled through one or two intersections in those small high desert towns when pedestrians stepped off the curb in front of me 'cause of the ABS.

Also towed my 230VRS and that same Jeep combo with my wife's Hemi Durango. It has 3.55s and better brakes than the F-150 (both in terms of the ABS function and its upgraded EBC Greenstuff pads and dimpled/slotted rotors). That SUV never wants for power and pulls well in all rpm ranges and will suck out your eyeballs on a hard stop.

My Cummins diesel has 34-inch tires (285/75-17s) and 3.73s and I'd never want 4.10s or deeper in it. I've towed light loads with diesels and deep gears (4.10, 4.56) and you just blow through the gears. The diesel wants to lug and tug against the gear and let the turbo spool and hang in the boost. I've found as long as the load is about 10-12K or less, 3.73 is just about perfect for most diesels out there. You really need to be hauling some heavy stuff to require 4.10s on a modern diesel.

As for which gear to run, it really depends on a number of factors like the engine, tire diameter, intended use, etc. If you're just buying this to haul your boat and a small toyhauler or other relatively heavy load most of the time, then go with the 3.73s. It'll help the tranny live a longer life because the added gear multiplication will help get the vehicle off the line easier. Plus, with the overdrive tranny you'll still be able to huff down the highway without buzzing the engine. I think those trucks come with 31-32-inch diameter tires, which will also help kill off some freeway rpm. On the other hand, if you're only gonna use it once in a while to tow your boat (which in the grand scheme of things isn't really that heavy) and will be using it as a commuter/daily driver every day, then I think the 3.55s make more sense. You'll be able to lope along with the tach at 2K or so on the highway and the 5.4L engine makes enough torque to get you off the line and up the on-ramp without becoming a road hazzard. Either way, I wouldn't consider the 3.31s.

russlars
09-30-2009, 03:34 PM
This is more general info for others - I understand it doesn't really answer your question:

I've towed with that F-150 before. Trailer & Jeep combo weighed about 5,500lbs and it was up Hwy 395 into the Sierras. I think that's pretty similar to the weight of a boat/trailer. Truck had the 5.4L and 3.73s. Pulling power wasn't wanting at any time, but the Ford's ABS system would kick on under hard stops and effectively cut braking power. Rolled through one or two intersections in those small high desert towns when pedestrians stepped off the curb in front of me 'cause of the ABS.

Also towed my 230VRS and that same Jeep combo with my wife's Hemi Durango. It has 3.55s and better brakes than the F-150 (both in terms of the ABS function and its upgraded EBC Greenstuff pads and dimpled/slotted rotors). That SUV never wants for power and pulls well in all rpm ranges and will suck out your eyeballs on a hard stop.

My Cummins diesel has 34-inch tires (285/75-17s) and 3.73s and I'd never want 4.10s or deeper in it. I've towed light loads with diesels and deep gears (4.10, 4.56) and you just blow through the gears. The diesel wants to lug and tug against the gear and let the turbo spool and hang in the boost. I've found as long as the load is about 10-12K or less, 3.73 is just about perfect for most diesels out there. You really need to be hauling some heavy stuff to require 4.10s on a modern diesel.

As for which gear to run, it really depends on a number of factors like the engine, tire diameter, intended use, etc. If you're just buying this to haul your boat and a small toyhauler or other relatively heavy load most of the time, then go with the 3.73s. It'll help the tranny live a longer life because the added gear multiplication will help get the vehicle off the line easier. Plus, with the overdrive tranny you'll still be able to huff down the highway without buzzing the engine. I think those trucks come with 31-32-inch diameter tires, which will also help kill off some freeway rpm. On the other hand, if you're only gonna use it once in a while to tow your boat (which in the grand scheme of things isn't really that heavy) and will be using it as a commuter/daily driver every day, then I think the 3.55s make more sense. You'll be able to lope along with the tach at 2K or so on the highway and the 5.4L engine makes enough torque to get you off the line and up the on-ramp without becoming a road hazzard. Either way, I wouldn't consider the 3.31s.
Thanks dummy. That does help. I appreciate your assesment. You know, I think your a lot brighter than your screen name would indicate.:)

EarlyriserX9
09-30-2009, 04:11 PM
Sorry Jim, you're right ;)





I tow my X9 on a tandem axle MC trailer with an 08' F150. It is a crew cab 4x4 with the 4.6and 3.73 gearing. I have no complaints, never in need of more power. However, I live in South Carolina, which is somewhat flat. I have also towed much heavier trailers and boats without a problem.

russlars
09-30-2009, 04:19 PM
Sorry Jim, you're right ;) .
He usually is!:worthy:

TMCNo1
09-30-2009, 05:03 PM
Towing with a 4.3 liter, 160 hp '88 Chevrolet Astro Van with 3:73 locking differential and factory towing package, including engine oil and transmission cooler.15.6 mpg boat towing average over 30K+ miles, 20 mpg w/o the boat. That's including towing through the Smokey Mountains on at least 1/2 the long tow trips and tow in OD w/ cruise control except for long steep hills when I go in drive Drive.
52263

russlars
09-30-2009, 05:48 PM
Towing with a 4.3 liter, 160 hp '88 Chevrolet Astro Van with 3:73 locking differential and factory towing package, including engine oil and transmission cooler.15.6 mpg boat towing average over 30K+ miles, 20 mpg w/o the boat. That's including towing through the Smokey Mountains on at least 1/2 the long tow trips and tow in OD w/ cruise control except for long steep hills when I go in drive Drive.
52263
Thanks No 1. I am most impressed with the fact that your '88 Astro Van is still on the road!:) This obviously is the result of your fastidious care and maintenance. I've never owned one, but it seems that I have heard that they were not the most reliable vehicles and I'm sure that a good share of that vintage are now being used for other purposes, like rebar or something.;)

jraben8
09-30-2009, 05:50 PM
Jim, wouldn't it be easier to pedal up-hill if the gear on the crank is larger, not smaller? I thought it would be harder to pedal up-hill if the crank is smaller and the wheel gear is larger?

Think of it this way. The distance around the large crank gear on a standard mountain bike is 44 chain teeth. The small crank gear is normally 22 chain teeth. With the large crank gear you're moving the chain 2x the distance with each revolution keeping the rear gear constant.

Now the definition of "harder" is relative in this scenario since when in the small crank gear you are having to pedal 2x as many times to cover the same distance. If you are going uphill it would be easier to take many little steps vs a few large strides.

But in the rear gears scenario, the smallest chain rings are normally 11 or 12 teeth around which means that the chain moves very little to make the wheel turn a full revolution but requires much more power to move it . When in the large rear ring which is normally a 32 or 34t, the chain travels much further per revolution of the rear wheel. The crank will spin more (rpm's) but not have to work as hard.

Did this help?

TMCNo1
09-30-2009, 06:39 PM
Thanks No 1. I am most impressed with the fact that your '88 Astro Van is still on the road!:) This obviously is the result of your fastidious care and maintenance. I've never owned one, but it seems that I have heard that they were not the most reliable vehicles and I'm sure that a good share of that vintage are now being used for other purposes, like rebar or something.;)


I had tried since 1978 to make chicken salad out of chicken manure by trying to make a tow vehicle out of something that wasn't really up to par for the job, so in 1988 just before buying the '89 MC, I chose to get something dedicated to towing a boat with a 6000 lb tow rating and equipped to do so. It is about to turn 60K and the near 30K miles on it other than towing the boat has been added on trips with the family, because of the additional room and comfort. It's one of the best driving/performing vehicles we've ever had, especially loaded and it rides like a Caddy, so the OTD $15.8K twenty one years ago has gone a long way.
Thanks about the maintenance comment, but since it's not our daily driver, I can baby it some for the more important boat duties.

airdrew99
10-01-2009, 12:04 PM
I highly recommend a 3.73:1 gear ratio. It is the best all around for daily driving and boat hauling. Anything above a 4:1 ratio is horrible on fuel mileage. Some of my friends in the past have had around 3:23:1 gears. If you don't tow at all, these would work great. But as soon as you start to tow a boat, vehicle has to have tons of horsepower to maintain speed on the highway and to accelerate. With the lower 3:1 ratio gears, you lose the mechanical advantage you have with a mid to high 3:1.


Drew

Footin
10-01-2009, 08:39 PM
I would go with the 3.42.

My trailblazer has the 3.73 and when I tow I do not use overdrive, while towing in 3rd gear the RPM at 70 are about 3k, at least when towing with a 3.42 gear it would lower this a little and get better MPG when not towing.

Thrall
10-02-2009, 01:55 AM
russlars,
The trucks are going to have the same trans, shift points, etc.
Since it's a gasser and you live where you'll presumably be driving at altitude thru the mtns, get the lowest geared (highest numerical) axle ratio you can. You're only going to see a couple hundred rpms difference between each available gear split, with plenty of rpms to spare on the top end.
My 08 F150, 5.4, 3.73 is pretty much a turd towing anything at altitude. If you tow alot, get 4.10 gears, or whatever teh lowest gearing available is.
If the 09 has more gears or a deeper OD than my 08, you won't even notice lower final gearing form a rpm or fuel mileage standpoint.
I've owned several 2nd gen Dodge Ram 1500's. All 360ci, auto, 4x4. Had 3.54's, 3.90's and 4.10's. All got the same cr@p mileage (12-13mpg avg), as does my F150. Never noticed better mileage with the higher geared rear ends, but the 4.10 geared truck towed better than the 3.54 truck.
Ford Triton motors don't have much low end torque, IMO, so again, get the lower gears to keep it wound up a little more. Should save you a few downshifts in the hills.

Conversely, looking at diesels, I'd reccomend getting the highest geared diff available unless towing HEAVY (over 10klbs, all the time). My only complaint with my 07 Dodge diesel is that with the stock 265 tires, it needs a higher rear end ratio or another gear. I've remedied the high hwy rpms a bit by running 285 tires. Drops the rpms close to 200 rpms at hiway speeds.
Those extra rpms with the stock donuts don't seem to affect fuel milage though, just nice to not be winding out at 2600rpms at 80mph!