High RPM while Light Towing & Upgrade Transmission?

Hi everyone. I purchased a raft last year and I have a question about towing. As you will see from my question, I am inexperienced in this area. We live near Denver and head to western Colorado and Utah frequently. Because of this, we go up and over a lot of steep, long inclines on I-70 and other highways. You might be familiar with the Johnston and Iselhower tunnels. These could be 6 miles of an avg of of 6 or 7% grade. Towing weight maybe 1,500 lbs at most with a 14' trailer with 15" wheels and 3,500lb axels. My 2008 has low mileage (132,000) and otherwise runs and drives great.

My question is the sometimes higher RPM than I am used to running. The Yukon has a 5.3l and 4sp transmission. I received a suggestion to downshift to the 3rd gear at this point to help with the shifting. This has helped keep (usually) the RPMs down below 5,000 but I do like to keep momentum when possible and maybe drive on the higher end of speed, usually averaging about 80 mph on open flat'ish roads.

In the future if I keep the Yukon, would a 6-speed transmission be feasible or would this potentially or likely just cause more issues?

With the 4L60E I'd only use 'D' / 4th for cruising.
Otherwise, I'd try to protect the 4L60E by using '3' & Tow /Haul mode simultaneously.
Even if your Tahoe's lucky enough to have 4.10 or better axle gearing,
it's safer for the 4L60E to not work very hard in, 4th unless it has 4.10 or more gear to help it out.

For even better results, have your pcm tuned.

For the monetary cost + time involved in converting your Tahoe to use a 6L80E,
it'd be SO MUCH EASIER to just buy a younger GMT900 with a 6L80E already.

The above is pessimistic. Although I would never tow over 65MpH,
protecting 4th gear and using Tow / Haul mode should be plenty.

If you get a warning about overheating engine and/or transmission,
change BOTH as soon as possible afterward, not at your mere convenience.

Marky Dissod said:

With the 4L60E I'd only use 'D' / 4th for cruising.
Otherwise, I'd try to protect the 4L60E by using '3' & Tow /Haul mode simultaneously.
Even if your Tahoe's lucky enough to have 4.10 or better axle gearing,
it's safer for the 4L60E to not work very hard in, 4th unless it has 4.10 or more gear to help it out.

For even better results, have your pcm tuned.

For the monetary cost + time involved in converting your Tahoe to use a 6L80E,
it'd be SO MUCH EASIER to just buy a younger GMT900 with a 6L80E already.

The above is pessimistic. Although I would never tow over 65MpH,
protecting 4th gear and using Tow / Haul mode should be plenty.

If you get a warning about overheating engine and/or transmission,
change BOTH as soon as possible afterward, not at your mere convenience.

Click to expand...

Thanks. I have the 3.73 but with such a light weight, how much is it affecting the engine and transmission on flat roads? I also thought I read something that the tow mode only activates when you are at 75% or so of max capactity. Can you elaborate ont eh "change Both ASAP? Change what exactly? Thank you!!

I adamantly believe that nearly every V8 vehicle GM EVER made prior to the 10L80E was undergeared;
question is by how much. Every time I floor my Z71 w/ 3.73, I die a lil inside.
The 4L60E will ALWAYS be a '1500' transmission in our big@$$ trucks.
4.10 makes the rest of the powertrain stronger. Check your manual's tow ratings, 4.10 is best.

On flat roads, if you're not accelerating, 4th MAY? be OK? - but I'm a paranoid pessimist.
Anything other than flat roads, I'd use 3rd and Tow / Haul mode combined.

Tow / Haul mode is accessed by that lil recessed button on the shifter's tip.
It changes the 'personality' of the shift strategy.
Tow / Haul helps protect the powertrain from extra weight and aero drag.
"... tow mode only activates when you are at 75% or so of max capacity".
It activates when you press it. Nuff said.

Motor oil and ATF may have improved since the mid-'90s, but I still firmly believe that
neither were ever meant to last very long at those temps.
I guarantee, that if you see ATF temps 230F or hotter, somewhere in the transmission,
the ATF is much hotter than that. Pretty much same goes for motor oil.
Every time I've ever been unlucky enough for either my ATF or my motor oil to hit 230F,
as a paranoid pessimist, I change BOTH.

so I don't tow much and I don't rebuild automatic trannys so this is just a question but it seems to me if you tow much you'd want to gear your truck to be a decent rpm (maybe close to tq peak?) at the speed you want to cruise at in the 1:1 gear ratio, with the converter locked up.


oil Temps I'm good with 250deg..eh, I've hit 300 in a performance car many times, drove a 1000miles home after, changed it. sent off a sample and it came back fine, no extra engine wear and oil was fine, I agree it was probably hotter than that up around the exhaust ports in the head. the sensor is in the pan. over 100k on the engine and it's spotless inside, it's a rode hard put up wet ls3, no afm but still just an ls.

but auto tranny I'd want to keep cool, I don't buy into the over cooled stuff. my truck completely stock cooling runs around at 130deg to 150deg all day long around town on 90deg days. if over cooling was a thing, these hybrid tranny without a tq converter would fail early, but they tend to be the closest to a 300k mile automatic gm put in anything since the th350/400.(it's just a standard 4 speed clutch setup besides the electric motors) I also agree the tranny fluid coming out of the torque converter while it's slipping is probably higher than the sump temp where the sensor is on a normal auto.

to me an auto tranny is to much magic living on hopes and dreams to be playing with high Temps in it haha.

on a side note, gm even put tranny coolers on their manual trannys that run the same dex 6 as everything else does and there's no slipping or clutches to worry about in those. if the tranny fluid was truly fine at 300deg which I've seen posted about, I don't see them running coolers on manuals. I think it's just to thin at the Temps, even if the fluid itself didn't burn up. I like the idea of low atf Temps.

j91z28d1 said:

oil Temps I'm good with 250F.
eh, I've hit 300F in a performance car many times, drove a 1000miles home after, changed it.
sent off a sample and it came back fine, no extra engine wear and oil was fine, I agree it was probably hotter than that up around the exhaust ports in the head.
the sensor is in the pan. over 100k on the engine and it's spotless inside.
it's a rode hard put up wet ls3, no afm but still just an ls.

Click to expand...

Was this in a manual or auto?
The engines of cars with autos run hotter and take a lil bit longer to cool back down, compared to manual cars.

Also, corvettes don't (can't?) tow / haul 166% of their own weight for hundreds of miles / several hours at a time.

Some good motor oils may be able to handle 300F for up to 30min at a time, but without actual data to compare them,
I can only say that most motor oils, even synthetic ones, start to offgas at 260F.

As for ATF, I stand by: 230F is as hot as I ever want it to get, even if it can 'handle' hotter, it shouldn't need to.

oh yeah, I wouldn't run around with 300deg oil Temps all the time. but if you are say taking a holiday trip with the family once or twice a year, and change oil when you get back. I wouldn't worry about the oil side much. but the tranny, I would try to keep that cool on my personal vehicle. I don't disagree with the theory new dex 6 can handle higher Temps than old stuff, but still.

SunnyInCo said:

Hi everyone. I purchased a raft last year and I have a question about towing. As you will see from my question, I am inexperienced in this area. We live near Denver and head to western Colorado and Utah frequently. Because of this, we go up and over a lot of steep, long inclines on I-70 and other highways. You might be familiar with the Johnston and Iselhower tunnels. These could be 6 miles of an avg of of 6 or 7% grade. Towing weight maybe 1,500 lbs at most with a 14' trailer with 15" wheels and 3,500lb axels. My 2008 has low mileage (132,000) and otherwise runs and drives great.

My question is the sometimes higher RPM than I am used to running. The Yukon has a 5.3l and 4sp transmission. I received a suggestion to downshift to the 3rd gear at this point to help with the shifting. This has helped keep (usually) the RPMs down below 5,000 but I do like to keep momentum when possible and maybe drive on the higher end of speed, usually averaging about 80 mph on open flat'ish roads.

In the future if I keep the Yukon, would a 6-speed transmission be feasible or would this potentially or likely just cause more issues?

Click to expand...

So what is the total towing weight? I'm assuming you are within the GVWR and GCWR of your vehicle.

Towing in 3rd (on the 4Lxx transmissions) is the preferred gear when getting up there on the towing capacity. Driving in 3rd will basically be like you have a steeper gear ratio.

80 MPH towing speed? That sounds pretty darn fast to me. I would try to slow down, just for safety. But I know everyone drives really fast nowadays.

5,000 RPM is perfectly fine on the engine. That's the range where peak HP is made.

The 6Lxx is a double overdrive transmission; it splits the overdrive in two, one higher and one lower than your 4L transmission. I don't really "think" it makes a huge difference on the highway speeds, where it really benefits is the very steep first gear. Can you upgrade to the 6 speed on your truck? I know the later versions did get the 6 speed. I've never wondered that...

SunnyInCo said:

Hi everyone. I purchased a raft last year and I have a question about towing. As you will see from my question, I am inexperienced in this area. We live near Denver and head to western Colorado and Utah frequently. Because of this, we go up and over a lot of steep, long inclines on I-70 and other highways. You might be familiar with the Johnston and Iselhower tunnels. These could be 6 miles of an avg of of 6 or 7% grade. Towing weight maybe 1,500 lbs at most with a 14' trailer with 15" wheels and 3,500lb axels. My 2008 has low mileage (132,000) and otherwise runs and drives great.

My question is the sometimes higher RPM than I am used to running. The Yukon has a 5.3l and 4sp transmission. I received a suggestion to downshift to the 3rd gear at this point to help with the shifting. This has helped keep (usually) the RPMs down below 5,000 but I do like to keep momentum when possible and maybe drive on the higher end of speed, usually averaging about 80 mph on open flat'ish roads.

In the future if I keep the Yukon, would a 6-speed transmission be feasible or would this potentially or likely just cause more issues?

Click to expand...

It's major surgery to convert to the 6L80 transmission. Different cluster, shifter, computers, etc.

Tow Haul was designed to be effective when you are at 75% or more of your GCWR. You can tell if it's working or not while towing by how it shifts.

If I activate Tow/Haul unloaded, the transmission shifts harder and at higher rpms. It is noticeably less comfortable drive. With a proper load on in Tow/Haul, it almost feels like normal driving, smooth shifting but they are at higher rpms.

If you want to cruise at 80 while towing, use the overdrive. You'll certainly want to downshift or let the computer downshift while going up and over the Eisenhower Tunnel. These motors make peak torque around 4000-4400 rpms so that's where you'd want to keep the engine humming along on the steepest grades. So if you have stock tires and want to run the Eisenhower Tunnel at 60 mph, you would want to hold her in 2nd gear which would be about 3,986 rpms minus converter slip. 3rd gear at 2,453 and 4th at 1,707 rpms.

Little known or mostly ignored factoid: officially named the "Eisenhower–Edwin C. Johnson Memorial Tunnel," the Eisenhower part of the tunnel is the westbound bore, named after the U.S. President and opened in 1973; the eastbound tunnel is the Johnson bore, opened in 1979 and named after a former Colorado Governor and U.S. Senator.