Towing: 4-speed Vs 6-speed With 5.3l

music · April 16, 2008

I'm currently in the market for a new truck, and have narrowed it down to the Tundra and the Sierra/silverado crew cabs. Prefer the GM truck so far and currently drive a 99 Sierra 5.3. The upcoming 6-speed mated to the 5.3L raised my attention. I tow 4 to 6k pounds occasionally (sometimes long haul), otherwise just a daily driver. I have found that the 4-speed works very well for most of what I do, and am unsure about what the 6-speed will really offer. My wife's minivan has a 5-speed, and it drives me nuts on hilly interstate with the constant hunting from 4 to 5.

Thoughts on the GMC Options for 2009

4-SPEED: Third gear (with a 1.0 ratio) on the 4-speed is a nice pulling gear, which when mated to a 3.73 rear diff axle gives 3.73 rpms per axle revolution. Forth gear on the 4-speed, mated to 3.73 axle, gives 2.6 rpm per axle rev. I've sometimes wished for an "in between" ratio, but not very often. In almost no cases that I've ever experienced will the 5.3L pull 4k to 5k pounds in 4th on anything but the flattest of ground.

6-SPEED: The new six-speed has a 1.15 (4th) and a 0.85 (5th) ratio plus the additional 0.67 (6th), giving overall ratios of 3.93 (4th) and 2.91 (5th) when mated to a 3.42 rear axle. The overall ratio for the six-speed would be 4.3 (4th) and 3.2 (5th) rpm per tire revolution when mated to a 3.73 axle.

Question

The new six-speed will divide the overall drive ratio into more "bands", but the question for me is whether or not the 5th gear ratio will be "usable" for towing with the 5.3L, or if it is going to hunt back and forth everytime you touch an incline. Will an overall ratio of 2.91 (3.42 axle) or 3.2 (3.73 axle) "pull" adequately, or will it mostly want to spend it's time in 4th (at a HIGHER engine rpm than the 4-speed runs at)? I imagine that the extra gears work out well for the 6.0 and 6.2L engines. Any thoughts about the 5.3L would be appreciated.

dreisenauer · April 16, 2008

I think the 6-speed will be better. I pull 2-4K lb trailers long distances at 80MPH with a 5.3 4 speed. When it's windy, 4 isn't enough and 3 is more power than needed. A 6 speed will still probably kick down to 4 up a grade, but it should be less often.

SilveradoCJ7 · April 17, 2008

I always wish I had a gear in between the 4th and 3rd when towing. The 6 speed should fill that void. Also with a 6 speed if it shifts on hills it wont be such a dramatic change in rpms. Much smoother acceleration as well. If you can wait then get an 09 when they come out. If you cant wait the new 5.3L's are quite a bit more powerful then your 99. I think the 99's were around 275 HP. The new 5.3L are 315 HP. Should help a little.

s-ten · April 17, 2008

The 6-speed should be a little bit better then the 4. One of the biggest problems with the LS based engines for towing is that they have a very high torque band, this causes the tranny to down shift in order for the engine to have enough torque and power to pull the load. As long as engines are built this way they will need to change gears more often to stay in the powerband. The 6-speed should alleviate the huge surges that the 4 speeds have when they shift from 4 to 3, but it will not eliminate the shifting problem all together.

music · April 17, 2008

Thanks for the replies. Any thoughts on the fact that the new 6-speed doesn't have a one-to-one pass through? All gears use a step up/down power transfer, whereas the 3rd gear in the 4-speed is a 1-to-1 direct ratio. Could this pose a trouble spot for towing applications?

I don't "need" to buy a truck now, but I'm warming to the idea. I keep vehicles a long time Current incentives are pretty good and I'm comfortable with the existing setup, since it's been around so long. If the six-speed is worth the wait, then I can wait, but I don't want to pay much of a premium for it.

svensk · April 17, 2008

seriously, keep the 99 until the 6spd comes out for the 5.3. Unless they're tuned an 08 with 4spd isn't any more powerful. The drive by wire w/ nannies makes the trucks seem really sluggish.

ChrisNJ · April 17, 2008

I think Musicman raises a very good question. My 6.2/6sp stays in 5th while towing and never hits 6th, I think it actually may be locked out in tow/haul mode, as I have never seen it. I tow 8K TT. It goes to 4th on even mild hills, but sounds good, not struggling, I think around 3500rpm, but right back to 5th on the other side and level highways. Now if I baby it, and allow the mph to slow up the hill, it won't go to 4th, but I lose speed, how much I allow depends on traffic, and what's in front of me (hill continues, gets bigger/smaller, etc...). The hills I am refering to are the Jersey Turnpike (mild), to I-80 through NJ-PA (mild to mid-grades), a few big ones that it stays in 4th for sure but does not lose speed.

He raises a very good point, that it is highly possible that with a 5.3, it could stay in 4th the whole time, which no one would like. You are towing a lot less than me, and maybe don't have a TT that has a large front end. I don't see an answer to your question, except waiting and getting someones experience with similiar situation.

What might shed some light is actually printing out the torque curves of the 6.2 and 5.3 and comparing them at different rpms to see if there is a huge difference. Mine runs around 2300rpm @ 65 in 5th and 3500 in 4th ( I think, I'm towing again next weekend and will look again if you really need it.)

OR, just splurge on the 6.2, and you won't have to worry about it, but I understand it will be a lot more $ only avail. in the SLT/LTZ and CC.

Nice rig · April 17, 2008

I think the 6-speed will be better. I pull 2-4K lb trailers long distances at 80MPH with a 5.3 4 speed. When it's windy, 4 isn't enough and 3 is more power than needed. A 6 speed will still probably kick down to 4 up a grade, but it should be less often.

WTF would you tow 4k at 80 mph?????? Let me know what road you are on and I'll be sure to go another route.

music · April 17, 2008

Chris: Thanks for the feedback with your experience on the six-speed, and to everyone with an opinion about this topic. More data is always good!

I ran some quick calculations just now to think this out a bit better. I've attached a table summarizing the overall ratios, etc. between the two. It's interesting, since it seems that for "normal" towing speeds, the four speed hits right into the sweet spot of the 5.3L torque band, whereas the six-speed seems to "bracket" below/above it a wee bit. I'm going to guess that pulling 5k pounds with the 5.3L and the six speed would run in 5th on flat ground, but would jump to 4th for any hills at all, since 2260 rpm (with the 3.73 axle) @ 65 mph, on an incline, would be a bit sluggish. I don't have a torque curve for the 5.3, so I'm only going by personal experience on that one. (Please provide feedback about the calculations if something seems "off". I didn't spend much time on it.)

My personal opinion, for what its' worth, is that the typical user will "wish" for the other on different occasions, but that neither is really superior in all instances. My bigger concern is that the six-speed doesn't have a 1-to-1 power pass through gear, meaning that extra heat is always being generated by the step up/down conversion. The standard "tow" gear in the 4-speed is a 1-to-1 ratio from engine to drive shaft. Does the six-speed in the 2500 series have a 1-to-1 gear? Need to check that out... since I guess that GM engineers have thought this through quite a bit. I recall noticing on the 09 spec sheets for the 1500 that extra cooling capacity will be added for "towing". Maybe for the transmission?

Axle_ratios_5_3L_6_vs_4_speed__rc.pdf

vranasaurus · April 17, 2008

Does the six-speed in the 2500 series have a 1-to-1 gear?

If I'm not mistaken the gear ratios in the 6L90 and the 6L80 are the same.

SilveradoCJ7 · April 17, 2008

The 6 speed improves towing. There was a shoot out towing a year ago. Chevy 2500 6.0L w/ 6 speed vs. Ford F250 V10 4 speed vs. Dodge 2500 5.7L 4 speed. The Chevy won in almost every catagory of towing on hills, flat, 0-60, 40-70 and so on. The biggest difference was the 6 speed trans.

MPG should go up 1-2mpg depending on driving as well.

music · April 17, 2008

I agree that the six-speed benefits towing with the 6.0L and the 6.2L engines. They have the low-end grunt to make use of the new gearing. Does the 5.3L, with what I'm presuming to be a higher rpm-to-torque band, benefit similarly, or will it "hunt" a lot for the right gear when under load? The new Tundra 5.7L does great with a 6-speed, but to a large degree that is due to the 400 ft-lbs of torque across a very wide power band. Toyota kept the 5-speed with the 4.7L. I'm guessing, but I bet that the six-speed mated to the 4.7L wouldn't work nearly as well.

p.s. I updated the table above to include info on the Tundra 5.7L/6-speed combo.

vranasaurus · April 17, 2008

The 6 speed improves towing. There was a shoot out towing a year ago. Chevy 2500 6.0L w/ 6 speed vs. Ford F250 V10 4 speed vs. Dodge 2500 5.7L 4 speed. The Chevy won in almost every catagory of towing on hills, flat, 0-60, 40-70 and so on. The biggest difference was the 6 speed trans.

MPG should go up 1-2mpg depending on driving as well.

The MPG may go up while towing but when empty the OD gear is close enough to the 4 speed that it won't make any difference. I don't know about 1-2 MPG though unless you are comparing a 4 speed running in 3rd to a 6 speed running in 5th. It would also depend on what you were towing. If the 4 speed and 6 speed stayed in OD ( 4th and 6th respectively) there would really be no difference even when towing.

Towing is where the real advantage of the 6 speed is. The low 1st gear and the extra OD gear are only useful when towing. Empty you could take out 1st and 5th and it would probably perform fine.

vranasaurus · April 17, 2008

I agree that the six-speed benefits towing with the 6.0L and the 6.2L engines. They have the low-end grunt to make use of the new gearing. Does the 5.3L, with what I'm presuming to be a higher rpm-to-torque band, benefit similarly, or will it "hunt" a lot for the right gear when under load? The new Tundra 5.7L does great with a 6-speed, but to a large degree that is due to the 400 ft-lbs of torque across a very wide power band. Toyota kept the 5-speed with the 4.7L. I'm guessing, but I bet that the six-speed mated to the 4.7L wouldn't work nearly as well.

p.s. I updated the table above to include info on the Tundra 5.7L/6-speed combo.

Is the new 5.3 VVT? If it was it would go a long way to increasing the length of the power band and probably make it work with the 6 speed better.

music · April 18, 2008

Is the new 5.3 VVT? If it was it would go a long way to increasing the length of the power band and probably make it work with the 6 speed better.

I'm not sure, but I've not seen any mention of a 5.3L with VVT. I've searched for information on the torque/power curves of the 5.3L before without much success (although I didn't try very hard). I have seen references that it puts out over 300 ft-lb from ~1600 rpm through 4000 rpm without VVT, but the HP curve is a lot more steep I think.