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djkslc 05-12-2013 09:40 AM

Disappointed in towing
I traded in my wife's 08 Tahoe 5.3 motor with 100k miles for a 12 gmc Yukon 5.3 l with 15k miles. I towed my 02 xstar yesterday for the first time yesterday. The tow includes a large climb over parleys summit. This gmc would not down shift the way my 08 Tahoe did. I was in tow/haul mode and had the gas pedal down all the way and the down shift took forever. I could do 65 or so up and over the summit in my 08 and I was struggling to be at 50 in the gmc due to the fact it didn't down shift to keep the speed consistent. What am I missing

BrianM 05-12-2013 10:07 AM

Different rear end. When they went to the 6 speed they also put substantially higher gears in the rear ends for the sake of economy.

1redTA 05-12-2013 10:28 AM

Yep the solution is too find a 3.73 to go with the 6 speed like what was probably in your 08 with a 4 speed. I think the 6 speed uses a 3.08 or 3.23. Also, you can tow in 5th gear to help

mustangtexas 05-12-2013 10:47 AM

Your GMC is under warranty. Take it in and have it checked out. Even with a higher rear differential ratio the lower ratios of the first 4 sets of the 6 speed auto give you a better low end final drive ratio for towing. The sixth gear set is to give the extra miles per gallon rating on highway and all six allows for additional city MPG as under normal driving shifts come quicker reducing rpm. Results help raise the all important CAFE rating. At first glance your issue sounds like a programming issue or a speed sensor.

nickespi 05-12-2013 11:47 AM

Something is going on. The 4 speed and 6 speed have similar ratios in the 3/4/5 gear range. Definitely have it looked at.

TxsRiverRat 05-12-2013 12:03 PM

Is there an OD button and was it turned off?

gsxrjtt 05-12-2013 12:07 PM

Some times you can look into a .. Super chips, or bully dog system.. to reprogram your truck and in tow modes change shift points

Indyxc 05-12-2013 05:51 PM

Something is wrong...

I towed my last boat from Florida to Michigan with a 5.3 Tahoe with a 6 spd, and through the mountain of Kentucky/Tennessee the trans was shifting into 4th to maintain 75 mph.

pap 05-12-2013 07:11 PM

tow/haul generally limits shifting, but that sounds extreme. The different rear end would explain more frequent downshifts, not what he describes.

Either your tranny is not programmed correctly, or perhaps that particular model has a real slow downshift mode in tow/haul.

You can always manually drop it down, yes?

Miss Rita 05-12-2013 10:23 PM


through the mountain of Kentucky/Tennessee the trans was shifting into 4th to maintain 75 mph.
Why would you stress your Tahoe to maintain 75 mph uphill? You must have had it floored; seems like you're flogging it a little. Not to mention that ST tires are rated to 65 mph.

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