OK this may be a dumb question since the allison has been out for 5 years now but here goes... I have been thinking of upgrading my tires and have been doing some research over the last year or so. On some towing sites they caution against running 285 or larger due to allison failures on this size tire. they say its due to the trans "not knowing" about the tire size change. They also say the tranny has trouble getting into 5th towing heavy loads. I was concerned about this then I found the follwing quotes on the allison web site:

NOTE: Changing the calibration to correct speedometer readout does not change the TCM calibration, and thus it does not compensate for the adverse impact on transmission operation.

Q. Speedometer / Tires - What effects will big tires have on transmission operation? A. The TCM calibration is based on factory tire size and rear axle ratio. Changing either of these parameters will have some adverse impact on transmission operation, including features like grade braking and shift stabilization. For instance, we have noted that when P285 tires are installed that the transmission generally does not shift to 5th range when towing heavy loads.



In spite of this there has been a lot of experience out there over the last 5 years and I am looking for someone to chime in. I tow about 7K lbs on about a monthly basis through the mountains of W.V. and I want 285s but I want my tranny to last and I want it to get into 5th gear. Can anyone shed some experinced light on the subject? Im already installing a deep pan kit but I dont want to have to go to a transgo kit or converter.

thanks-

lets just say that my friend has a 2002 D/A 4x4 with a 14 inch lift and 38's with 4.10 (texas truck) and his speedo hasnt been recalibrated and he has no problem towing a 28 ft enclosed, quickly.

I had 61,XXX miles on my stock tranny. 40,XXX of those miles with the 37's, 40,XXX miles with a power adder, around 15,XXX miles towing 8+ thousand pounds, and had to rebuild because I got the bug for more hp. I think if I left it stock or kept the Hypertech, the tranny would have lasted 100+ thousand miles. Don't worry about the transmission, it is one tough puppy. by the way, my tranny would shift and tow in 5th gear, even with the 37" tires.

Never had a issue running 285's.


I have to ask...Why would you be towing a heavy load without the overdrive not disengaged in the first place?

Tow haul should be on and no overdrive is what were suppose to do right?

285's have been on both my trucks since they were new. My 2001 LB7 has 123,000 miles on it and my 04 LLY has 60,000 on it. Botch shift nicely and have no problem towing in 5th gear.

Talk to people that run 315's... im sure they could set everyone straight :lol:

Never had a issue running 285's.

I have to ask...Why would you be towing a heavy load without the overdrive not disengaged in the first place?

Tow haul should be on and no overdrive is what were suppose to do right?

You can leave overdrive on while towing. You only want to turn it off if it's going in and out of overdrive frequently.

I leave mine on all the time. Transmission knows what gear to be in.

I also have stock tires.

Never had a issue running 285's.


I have to ask...Why would you be towing a heavy load without the overdrive not disengaged in the first place?

Tow haul should be on and no overdrive is what were suppose to do right?

Tow haul locks up the torque converter at 25 MPH and keeps it locked through the remainder of the gears, it also changes the shifting patterns to allow holding the gear longer in each gear. I generally tow in tow haul and depending the time of year, i.e. not summer, i put it in drive and tow away. The Allison will determine if it is ok to shift into the next highest gear and if it not then it will stay in that gear until then. My truck will pull in 5th at 70 MPH just fine, but in the summer I don't like to tow that fast due to the heat and stress that it causes on tires, transmissions, cooling systems especially when it is 110* outside. In the winter, when I tow the most, it is hammer down. At 70 or above, that keeps my RPM's at around 1850, which seems to keep the EGT's in check and isn't lugging the engine. If I come to a situation where it starts to hunt between 4th and 5th, I will hit the OD lockout button and leave it there until the road is flat. All of this is stock 3:73 gears, 37x14R20 Toyo tire that measures 36" tall and 8" lift, towing 28' toyhauler @ approx. 10,000#'s.

In my 06 LBZ, i leave it in tow haul mode and "D" and I often times find the transmission in 6th gear towing the same toy hauler as above. The Allison was built to tow in OD. This truck is stock 245's on PYO's, 4" downpipe back exhaust, no cat.

Never had a issue running 285's.


I have to ask...Why would you be towing a heavy load without the overdrive not disengaged in the first place?

Tow haul should be on and no overdrive is what were suppose to do right?

SSTAT sig shows he has an 2002, the 2002 cannot turn off the overdrive without having the od lock out kit installed.
I have an 02 with 285's I can pull 7000lbs, with 4.10 going up a decent sized hill with the speed limit at 55mph, and I can keep the truck at 60 without it shifting out of 5th. Talking about one hill in particular which I drive alot pulling a boat. They turned it into a three lane highway each way for the slower trucks to use the outside lane, it is about 3/4 mile long. I don't think there will be a problem there for you.

Thanks for all the replies. I figured people out there must have been doing this long enough to know if there would be a problem. - Pat

SSTAT sig shows he has an 2002, the 2002 cannot turn off the overdrive without having the od lock out kit installed.
I have an 02 with 285's I can pull 7000lbs, with 4.10 going up a decent sized hill with the speed limit at 55mph, and I can keep the truck at 60 without it shifting out of 5th. Talking about one hill in particular which I drive alot pulling a boat. They turned it into a three lane highway each way for the slower trucks to use the outside lane, it is about 3/4 mile long. I don't think there will be a problem there for you.

Maybe this is a stupid question but where do you get one of these OD lockout kits and is it needed?? Does T/H not lock OD out?? Havn't had my truck long and trying to learn!

I installed the O/D lockout switch on my '01. There are a few parts you have to buy, install is fairly simple and then you have to take it to the dealer to have the computer 'learn' the new switch.

Here is the Service Bulletin on the issue:

A/T - Shifts IN and OUT of 4th and 5th Gear

File In Section: 07 - Transmission/Transaxle
Bulletin No.: 02-07-30-051A
Date: February, 2003
TECHNICAL
Subject:
Transmission Shifting In and Out of 4th and 5th Gear (Hunting) When Pulling/carrying a Load, Unable to Manually Select 4th Gear (Install 5th Gear Inhibit Switch (http://www.alldatadiy.com/alldata/AFI~V7198278~C35582~R0~OD~N/0/80851247/83211660/83214935/83214941/34853741/34860071/34861128/97116948))

Models:
2001-2002 Chevrolet Silverado HD2500/HD3500 Series
2001-2002 GMC Sierra HD2500/HD3500 Series with 6.6L Diesel or 8.1L Gas Engine (VINs 1, G - RPOs LB7, L18) and Allison(R) Automatic Transmission (RPO M74)

This bulletin is being revised to modify the service procedure. Please discard Corporate Bulletin Number 02-07-30-051 (Section 07 - Transmission/Transaxle).
Condition
Some customers may comment that when pulling a load, the transmission shifts in and out of fifth gear too frequently, or they would like to be able to select 4th gear manually.
Advise the customer:
A customer who requests the fifth gear inhibit switch (http://www.alldatadiy.com/alldata/AFI~V7198278~C35582~R0~OD~N/0/80851247/83211660/83214935/83214941/34853741/34860071/34861128/97116948) must understand fueling systems of diesels and the resulting noise that may be heard. This noise might be objectionable to some drivers. The following is provided in an attempt to enlighten the driver/operator of the vehicle.
Diesel engine fuel injection systems that use a common rail system, as the Duramax(TM) diesel does, may or may not have what is referred to as pilot fuel injection at the lower engine speeds. Pilot fuel injection is defined as a pre-injection of a small quantity of fuel 5 to 20 crankshaft degrees before the main fuel injection. This pre-injection softens the main injection pressure rise and reduces combustion noise. At engine speeds above 2600 RPM, and a fuel quantity supply greater than 35 cu mm, it is necessary to eliminate pilot injection to meet emission requirements.
When driving at or above these speeds, main injection combustion noise may be noticeable. It is normal and not harmful to the engine.
When traveling in 5th gear, the pilot injection to main injection transition is typically in the 136 km/h (85 mph) range, however it may also be heard under heavy acceleration and when pulling loads up a hill during shifts at or above 2600 RPM. Customers that install the fifth gear inhibit switch (http://www.alldatadiy.com/alldata/AFI~V7198278~C35582~R0~OD~N/0/80851247/83211660/83214935/83214941/34853741/34860071/34861128/97116948) may encounter the fuel knocking of main fuel injection at speeds around 107 km/h (67 mph) in fourth gear between 2600 and 2700 RPM. This sound is considered normal and is to be expected.
Correction
Install the 5th gear inhibit switch (http://www.alldatadiy.com/alldata/AFI~V7198278~C35582~R0~OD~N/0/80851247/83211660/83214935/83214941/34853741/34860071/34861128/97116948) using the following procedure.
1.Open the hood and disconnect the negative battery cable from all batteries.
2.Remove the heat shield over the TCM.
3.Disconnect the TCM wiring connectors.
4.On vehicles with the L18 gasoline engine, remove the three retainers that retain the Throttle Actuator Control module to the engine side of the cowl, just under and behind the fender to cowl brace, and position the module to the side.
5.Remove the sound insulator retainer from the left upper most end of the dash insulator and carefully move the insulator to the side.
6.Approximately 76 mm (3 in) to the right of the stud for the sound insulator retainer, there is a small inward dimple in the dash. This is the pilot for the drill. Drill a 14 or 15 mm (37/64 or 19/32 in) hole through the dash panel.
7.Place a thin bead of sealer around the retainer. P/N 22573692 and insert the retainer into the dash panel.
8.Insert a pocket screwdriver into the center of the retainer to open the retainer.
9.Remove the instrument panel trim panel from around the IP cluster.
^For vehicles with an air bag selector switch. remove the accessory housing panel with the air bag selector switch and disconnect the wires from the air bag selector switch. Go to Step 10.
^For vehicles without the air bag selector switch, remove the IP bin from the center IP dash area. Insert one P/N 15040483 housing. Go to Step 10.
10.Remove the blank panel from the housing (if applicable) and install the switch (http://www.alldatadiy.com/alldata/AFI~V7198278~C35582~R0~OD~N/0/80851247/83211660/83214935/83214941/34853741/34860071/34861128/97116948) and wire assembly, P/N 15174508.
11.To the short white wire from the switch (http://www.alldatadiy.com/alldata/AFI~V7198278~C35582~R0~OD~N/0/80851247/83211660/83214935/83214941/34853741/34860071/34861128/97116948), properly crimp and solder a terminal, P/N 12110844.
12.To the yellow, green and blue wires from the switch (http://www.alldatadiy.com/alldata/AFI~V7198278~C35582~R0~OD~N/0/80851247/83211660/83214935/83214941/34853741/34860071/34861128/97116948), properly crimp and solder a terminal, P/N 12084912.
13.Position a piece of mechanics wire into the back side of the dash in order to pull the wiring through the opening in the dash, routing the wires under the HVAC control and steering column.
14.Insert the three long wires through the previously installed dash pass through.
15.Pull the three wires completely through the dash pass through.
16.Seal the wires to the dash pass through with sealer.
17.Cut off a 1524 mm (5 ft) section of the wiring conduit.
18.Route the three wires into/through the conduit.
19.Position the conduit so it drops down from the dash pass through. Reposition the insulator blanket and reinstall its retainer.
20.Reposition the Throttle Actuator Module and install the retainers (If applicable).
21.Route the newly created wiring harness along the vehicle's existing wiring to the TCM area using 3 to 4 plastic tie straps to secure it in place.
22.Open the gray/clear TCM connector body cover.
^Insert the green wire into cavity 20.
^Insert the blue wire into cavity 12.
^Insert the yellow wire into cavity 28.
23.Properly close the TCM connector, re-connect the connectors to the TCM, reposition the TCM and install the TCM cover.
24.Locate the Relay Block-Body to the right and up from the parking brake mechanism. Inspect for a cream colored connector plugged into the Relay Block-Body, usually in the top right hand corner of the Relay Block-Body.
^If there is no cream colored connector in the Relay Block-Body, obtain P/N 15306105 and perform the following steps.
-Remove all terminals and wires from P/N 15036105 connector.
-Insert the white wire from the new switch (http://www.alldatadiy.com/alldata/AFI~V7198278~C35582~R0~OD~N/0/80851247/83211660/83214935/83214941/34853741/34860071/34861128/97116948) assembly into cavity "E" of the connector.
-Place the connector into the top right hand corner of the Relay Block-Body.
^If there is a cream colored connector in the Relay Block-Body, perform the following steps.
-Disconnect the connector from the Relay Block-Body.
-Insert the white wire from the new switch (http://www.alldatadiy.com/alldata/AFI~V7198278~C35582~R0~OD~N/0/80851247/83211660/83214935/83214941/34853741/34860071/34861128/97116948) assembly into cavity "E" of the connector.
-Reinstall the connector into the Relay Block-Body.
25.Use a couple of tie straps and tie the newly installed wiring to the underdash wiring.
26.Reinstall the IP trim bezel.
27.Reconnect both batteries.
28.Use normal SPS reprogramming procedures to reprogram the TCM.
29.Follow the procedure outlined in Corporate Bulletin Number 02-07-30-032 and complete the FastLearn procedure.
30.Reset the customer clock.
31.Road test the vehicle in the normal driving mode at 72 km/h (45 mph), 5th gear. When the speed is stable and road conditions are favorable, depress the newly installed 5th gear lockout switch (http://www.alldatadiy.com/alldata/AFI~V7198278~C35582~R0~OD~N/0/80851247/83211660/83214935/83214941/34853741/34860071/34861128/97116948). The LED should illuminate and the transmission should downshift to 4th gear. The transmission then should only have 1st, 2nd, 3rd, and 4th gear operation as long as the switch LED is illuminated.
32.Depressing the switch (http://www.alldatadiy.com/alldata/AFI~V7198278~C35582~R0~OD~N/0/80851247/83211660/83214935/83214941/34853741/34860071/34861128/97116948) will allow 5th gear operation to resume. Fifth gear operation will also be reestablished any time that the key is cycled to the "OFF" position. This is normal.

Parts List:

http://photos.imageevent.com/pharcydecustoms/jeffstruck/OD%20Switch%20Parts.gif

I ordered all of my parts from GMPartsDirect.com for quite a bit less than the dealer wanted.


Here is a pic of where the switch mounts up:

http://photos.imageevent.com/pharcydecustoms/jeffstruck/websize/DSC02976%20_Small_.JPG


Let me know if you need anything else!

I see most of the replies are for the 5 speeds, I have a 06 LBZ w/ 6 speed. Does the same apply for these also? I have read that the 285s will drop your RPMs enough to take you out of the "power band" on the highway, causing the hunting between 5th and 6th and a big lose of gas mileage, and power when towing... thoughts


PS I'm new to the forum, and have found it to be the best place for useful info, thanks!

I have seen no ill effects from running 285s with my 06. Even stock it doesn't downshift out of 6th much towing. Now with the Edge on level 2, with an 8k lbs or less trailer, I have never had it downshift out of 6th while towing. I went on a trip to southern Utah a few weeks ago, grossing 18.5k truck and trailer and it left 6th maybe 5 times, and there are some pretty good size hills you have to go over the way I went. It never went below 5th. I don't know how it would have done with stock power though.

Your speedometer and your speed sensor input to your tany will be off an amount equal to the % differance of the size of the tires you are changing to. If you are going to a 29" form a 26" diameter you are going up a little over 10%. This will be noticeablewhen shifting. There are devices on the market that alter the speed sensor signal to compensate for the change and this will also correct the speedometer. I've forgotten the name of them but someone will chime in I'm sure. You can get by without it but will be better off with one.