Gear ratio (rear end) for towing

[bhayden]

Much has been said about the mystical magical 4:11 gears being the ultimate for towing. Our 2000 GMC Safari has the factory towing option with 3.7 gears and I've found that to be ideal. As it works out that gear ratio puts the engine right at it's max torque when driving ~60mph.

MPH = (Engine RPM * Tire Diameter * pi) / (Gear Ratio of gear for tranmission * Axle ration [final drive] * 1056)

Certainly lower gears, like 4:11 easy the work load on the drivetrain when accelerating from a stop. However, I'm not sure it's nearly as important with an automatic as it would be with a manual transmition. The 3.7 gears provide a "civilized" ride for general (i.e. non-towing) driving. A little more "peppy" performance than stock (3.4?) but not so severve as to have a major effect on gas mileage I suspect would be the case with 4:11.

I'm wondering what some of the more savey types out there think of the idea of matching gearing to the torque curve of the TV rather than the "lower is better" school of thought.

-Bernie-

[overlander63]

The smaller the engine, and the larger the tow vehicle and trailer, the higher the gear ratio needs to be. Our van has the 3.73 gears instead of the 4.11, and I would really like it to have the 4.11. This because our van is used almost excusively to tow our coach.

[scottanlily]

Hello towing folks,

Overlander 63 is absolutley right .Small engine lower gear ratio.Its physics
no matter what .IF you have a big block engine say 454 it has the torque needed to pull big trailers and the 3:73 ratio gears .That 2000 sierra has the 5.3 yes? its only a 327 cubic inch displacement ,not even a 350 .The rear ends like to grenade as well ,well documented fact .But if your towing a bambi new model or a vintage flying cloud that would be fine .By the way the automatic really needs the lower ratio ,not the manual ,reducing the rear axle load is important .All that being said ,Yes the 4:10 with a 3 speed auto would kill mileage ,but thats why the 4l60 E automatic overdrive is used .How did you calculate the torque curve to determine it is at the optimum RPM?
and what about towing in overdrive and what RPM is it at ? The lower gear is better ,if you towed your 24ftr with a 4:!0 geared sierra you know it .

Scott

[Frank S]

Hi bhayden--I have a 2001 Suburban 1/2 ton with a 5.3 ltr, a 4.10 rear, and 4-speed auto trans., towing a 1973 27' Overlander, with a wet weight of 7200 lbs. I always tow in 3rd gear (1 to 1 ratio), which keeps me in the sweet spot of the torque range at highway speeds. I have towed the same A/S with a 5.7 ltr, and a 3.73 rear which was less effective staying in the sweet spot.--Frank S

[Frank S]

Hi bhayden--Rereading my 12:53 PM post today, I noticed I really pushed up the weight of my 27' Overlander. SHOULD BE 6200 LBS.--Frank S

[Silvertwinkie]

Outside of Terry hitting it right on the head, I can only add this:

I have had both 3.73s and 4.10s on my Chevys.

This past weekend I was able to ride in the exact drivetrain setup with both 3.73s and compare it to my 4.10s, both towing similar size and weight Airstreams. The RPM difference was 300 at 62mph using tow/haul and driving in "D".

To me, it's a no brainer. If you have a smallblock and want to tow more than 6000lbs, carry passengers, fuel, cargo, etc, you should be seriously considering 4.10s.

As for fuel economy, again, we caravaned from the midwest rally to the Palaceds (about 200 miles) last weekend. The interesting thing was that both drivetrains (same engine, trans, etc) had very darn close to the exact same fuel consumption. When I was at 1/4 tank with my 4.10s, the 3.73 also towing a 25' Airstream was at the same exact mark from the half tanks we started from.

Now I realize these are by no means scientific results, but the do seem to indicate that any fuel hits between 4.10s and 3.73s are very small, yet the tow ratings they provide are fairly substantial, going by not only the manufacs websites, but also from my personal exp having had both gear sizes.

Now if you have a big block or diesel in particular, 4.10 might be nice, but are less important since the engines have more torque off the flywheel and don't need as much torque multiplication as a small block might benefit.

[bhayden]

Originally Posted by Frank S

Hi bhayden--I have a 2001 Suburban 1/2 ton ... I always tow in 3rd gear (1 to 1 ratio), which keeps me in the sweet spot of the torque range at highway speeds. --Frank S

Ah, I had made the incorrect assumption that 4th gear was 1:1 when in fact the gears ratios for the 4L60-E are:

Automatic four-speed transmission with mode select, lock-up, electronic control, shift lever on column, 3.06:1 first gear ratio, 1.63:1 second gear ratio, 1:1 third gear ratio, 0.7:1 fourth gear ratio and 2.29:1 reverse gear ratio

Direct drive 1:1 should be fairly efficient, generate less heat and most certainly will reduce drivetrain loading. I guess I'll be locking it out of OD unless it's dead flat. It'll be interesting to see what affect this has on the "instantaneous milage" according to the display on the overhead console. Also talked to the local transmission shop and they recommended going to a full synthetic transmission fluid ($200-250 for changing the filter, flush and replace with synthetic fluid).

-Bernie

[Beginner]

Towing with the OD engaged is going to cost you a transmission overhaul. The overdrive is not sufficently heavy duty to withstand the riggors of towing. Tow in Drive.
The heat generating device in a transmission is the torque converter. When the torque converter locks up heat generation all but stop. This is good. Towing in drive still allows the torque convertor to lock up and leaves you some power should you need it. In some instances the fuel economy may increase.
Remember all the years before these lock-up OD transmissions?
Sixth Five MPH left the engine screaming. At least with the Lock-Up the RPMs should be down around 2700 or less.
You still need a Transmission cooler though, a good one.
Beginner

[Alumaholic]

Originally Posted by Frank S

Hi bhayden--I have a 2001 Suburban 1/2 ton with a 5.3 ltr, a 4.10 rear, and 4-speed auto trans., towing a 1973 27' Overlander, with a wet weight of 7200 lbs. I always tow in 3rd gear (1 to 1 ratio), which keeps me in the sweet spot of the torque range at highway speeds. I have towed the same A/S with a 5.7 ltr, and a 3.73 rear which was less effective staying in the sweet spot.--Frank S

Frank, You have the tow vehicle I tried for years to find/order. That engine/rear axle combination is very difficult to find in a half ton Suburban, but it's just what you need with the 5.3 with it's max torque coming at a higher RPM than you can maintain at highway speed with the 5.3/3:73 combo.
I ordered a 1500 Suburban with the 4:10 in 2001, but the dealer wouldn't make good on the deal his salesman promised, and I never took delivery.

[Silvertwinkie]

I've towed with my old tow vehicle, with great success and a temp gauge, in drive, and the vehicle did not have a tow/haul feature. It also had 3.73s.

The torque converter is the main heat generator. That said, I had my old car's powertrain control module re-flashed to lock the torque converter sooner than later. The car towed well in terms of the transmission, engine and gears. It has the power and the torque to do the job and the drivetrain in general from front to back was more than stout enough for most likely up to a 28'. Even when it hunted (which I didn't let it do too often), with the addition of the deep pan for the 4L60e and a ram air type torque converter cover, it was rare to see the trans get near 200 degrees. In retrospect, it wasn't a great setup in general in terms of wheelbase, etc, but if you are talking about a longer wheelbase truck might not be too bad with mods, however, if you do it correctly the 4L60e can tow in drive and is a strong trans....not to be discounted. Of course it's no 4L80e, but then again, it was never designed to be.

Looking at the GM site, it doesn't look like GM since at least the 2006 model year has a 4.10 option for the 1/2 ton Burb...maybe trucks...I'll have to look.....


....yea, it looks like you can get a 6.0L and 4.10s with even the non HD model pickups.

[scottanlily]

That 4l60E is a very good transmission and I service a fleet of 1 ton gm stake bed commercial dually trucks ,these drivers are not easy on them ,Out of 10 trucks ,3 I think had a new trans at 250,000 miles or more, they were not serviced as good as they should have been as this company for the longest time called me in for repairs only when somthing broke down which alot of times is typical . convinced them otherwise finally and service is now done routinely .The gas mileage isn't always worse with the 4:10 gears due to the engine making the most efficient power at the RPM its running on the road
less throttle required to maintain the given speed .

Scott

[16595]

So, I am towing an '05 31' Classic (8100# loaded) with a 1999 Suburban 1500 with the 5.7 vortec. How long will it last before rear axel etc. start to give me problems?

We are planning a 3 week trip around the Great Lakes area in August. And I am getting nervous about the tv.

Would the Chevy 1500HD (6.0) be a good choice?

Dwight

[Alumaholic]

Originally Posted by 16595

So, I am towing an '05 31' Classic (8100# loaded) with a 1999 Suburban 1500 with the 5.7 vortec. How long will it last before rear axel etc. start to give me problems?

We are planning a 3 week trip around the Great Lakes area in August. And I am getting nervous about the tv.

Would the Chevy 1500HD (6.0) be a good choice?

Dwight

I doubt that you'll have any mechanical problems if you keep your TV properly maintained. That's an extremely tough drive train, even though barely adequate for the length of your trailer. The inadequacy will show up on the mountain grades, but it doesn't mean your headed for a breakdown as long as you stay out of overdrive and let the truck negotiate the mountain grades at a comfortable RPM.

The 1500HD 6.0L would be a very good choice; a fantastic choice if you can find one with a 4:10 rear axle. If you don't otherwise need a 4X4, a two wheel drive makes a better tow vehicle and gives you 500 extra lbs. of towing capacity. I always get one with a limited slip differential and only buy 4X4s because I need it too drag my hunting trailer in and out of the mountains.

[scottanlily]

Hello 16595,

The most important thing NOT to do is have your truck in 4 wheel drive on any hard dry pavement ,that will break the rear axle carrier due to the binding that occurs between the front and rear differentials as the tires rotate.There must be some tire slippage such as on snowy ,muddy,or dirt roads .It could also break the transfer case or front diff .You cannot worry about the rear diff under normal conditions ,nor should you .Make sure the gear oil level is full and serviced very regularly when towing alot .I strongly recommend the manafacturers gear oil requirements of the correct oil and limited slip additive
that is needed with a limited slip rear differential .Most of those GM trucks had a limited slip rear axle assy.Others have success with synthetic lubricants
but ,OEM specs you cannot go wrong ,nor can the gear lube be blamed if somthing breaks .

Scott

[16595]

Thanks, klevan and scottanlily for the advice. I only use 4wd when the snow is deep.

I have learned here today that I don't want to use the overdrive, which I have been doing unless there was a great amount of downshifting. Will stop that forthwith.

I have been getting about 10mpg when towing 62-65mph, with O/D. That may drop a bit.

I plan on doing some shopping this week for tv. Should be a good time to buy a Chev/GMC pickup with revised models coming down the pipe. Would love the Duramax/Allison option, but don't want to spend that kind of bucks.

Keep the good info flowing. This is a great place to learn. Our family (parents & us) have been towing Airstreams since '66, but can always learn more.

Dwight

[Frank S]

Hi klevan--It's hard to find a Suburban on the lot with a 4.10 rear. I always enter a factory order, so I get exaqctly what I want. Takes a little longer, dealer really wants to sell me what he has, but I get what I want. I also don't get loaded up with a hole in the roof, leather seats, bucket seats, satellite radio, and other things I really don't want, even if they throw them in for free.--Frank S

[Frank S]

Hi 16595--Our first trip in our 27' Overlander in 1988 was around lake Michigan, with a Chevy van with a 5.7 and a 3.73 rear. At that time I didn't even have an equilizer hitch. Did just fine. All the roads around the lake were secondary, so my speed seldom exceeded 55mph. It's also pretty flat. Go for it, you'll have a ball.--Frank S

[bhayden]

Did our second long trip this weekend and locked it down to 3rd, no overdrive. Seattle to Yakima pulled down 11. About what I'd have expected using OD. Coming home the milage was just over 10mpg. Could very likely have been headwinds; doesn't take much to make a big differnence. Experimented a little with the rig in OD and 3rd on downhill sections. Didn't appear to make any difference according to the computer for "instantanous milage".

The big thing I noticed, on the way over especially, is that with the engine locked in 3rd the coolant temp (no tranny temp gauge, yet) was noticably cooler. It was showing ~170 instead of the usual 190 degrees.

[scottanlily]

Hello bhayden,

your temperature point is well noted .Your truck isn't lugging as it does in overdrive ,although it may not seem like it is .Usually mileage will be close to the same and the reduction in the engine temp is a big bonus!That by itself helps the trans run cooler and your stress level !

Scott

[Silvertwinkie]

I agree. There will most likely not be any type of mechanical meltdown if it's properly maintained and you don't drive it like you stole it. .

The 6.0L is a great engine and I was shocked how much better it was from the 5.7 which I considered the gold standard, even after the 6.0L first came out.

I can't agree more about the limited slip. I, to this day can't figure out why they just don't make it standard instead of having it as an option. In some circumstances, even without 4x4, the limited slip can really get you out of a few places....of course if you boondock, 4x4 is a great tool to have, in addition to the limited slip.