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3.73s

M1ck3y

By M1ck3y
in Modifications, Wheels & Tires

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swathdiver Senior Enthusiast

swathdiver

Posted
Posted
7 hours ago, M1ck3y said:

 

Right, I’m wondering how much my top speed will drop after changing gears…

 

In my GMT900 trucks, both steel and aluminum driveshafts are limited to 5,000 rpms.  Same for the steel front driveshaft.

 

If we assume the same applies to your generation truck, your driveshaft would be turning 4,977 rpms at 142 mph in 7th and 8th gear with the tires currently on your truck worn down to 2/32nds of an inch.  At 143 mph, the driveshaft would be turning 5,013 rpms.

 

With 3.73 gears your top speed is now 123 mph with the shaft turning at 4,973 rpms.

 

With 4.10 gears your top speed is now 112 mph with the shaft turning at 4,982 rpms.

 

Now having said that, we've seen on this and other forums many photos of aluminum driveshafts twisted into pieces when fellas took their K2 generation trucks to the magic number of 112 mph.  That's the speed several reported going when their shafts self-destructed.

 

We also have photographic evidence of a stock tired 2012 Yukon XL Denali going 143 mph or so and his driveshaft would have been turning about 5,400 rpms.

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M1ck3y Senior Enthusiast

M1ck3y

Posted
  • Author
Posted (edited)
8 hours ago, GETGONE said:

Unless you're tuned it won't matter anyways since it's electronically limited to 112mph.


I drive a 17, and it is tuned. I posted here because I’m trying to get as many knowledgeable responses as possible.

 

The driveshafts on k2’s go out 120 - 130. And I’ve read that the max tow trucks came with a lower top speed around 90 or something (down from 100). So I assumed it must have dropped because of the 3.42 ratio. Which is a substantial drop for such a small change.

 

I was hoping someone would have specific information regarding my driveshaft. The shop I’m working with wasn’t much help.

 

 

 

Edited by M1ck3y
  • Like 1
GETGONE Enthusiast

GETGONE

Posted
Posted

I have a 21 with the 6.2 and have hit 112 NUMEROUS times so the driveshaft thing is BS on the T1 trucks. 

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KARNUT Senior Enthusiast

KARNUT

Posted
Posted

Until I joined here I always thought the limit was because of the tires. All my trucks were tuned at the minimum. All had limits removed or moved up. Never had a drive shaft problem or a hint of vibration. Speed limits are high here people routinely cruise at 80 MPH. It hard to believe the driveshaft is out of balance enough to be close to explosion. And not be felt at cruising speeds of 70-80 MPH. I rarely went that fast in my trucks running aggressive high ply tires. The drive shaft never entered my mind. 

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GETGONE Enthusiast

GETGONE

Posted
Posted

The limit IS due to the tires. That's why trucks with the 18's have a 98mph top speed and the trucks with 20's have 112mph because of the tire's speed rating. S vs T.

  • Like 1
davester Senior Enthusiast

davester

Posted
Posted

Like most rotating stuff that's balanced, it's balanced on a machine, to work up to a certain rpm.  Going just over that rpm doesn't mean it will vibrate, or fail, just that it is unknown as to what will happen.  As well, going faster also requires a lot more power that the driveshaft also has to handle, so it's both spinning faster than it was balanced for, as well as having a lot of torque applied to it.

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johnnyquick Enthusiast

johnnyquick

Posted
Posted
On 10/1/2025 at 8:00 PM, davester said:

Like most rotating stuff that's balanced, it's balanced on a machine, to work up to a certain rpm.  Going just over that rpm doesn't mean it will vibrate, or fail, just that it is unknown as to what will happen.  As well, going faster also requires a lot more power that the driveshaft also has to handle, so it's both spinning faster than it was balanced for, as well as having a lot of torque applied to it.

Doesn't balanced mean balanced?

I have not heard of anything being balanced by RPM etc, do you have an example of this?

I am happy to be educated on it if you can think of something.

TIA

davester Senior Enthusiast

davester

Posted
Posted

A balancing machine spins an object up to a certain rpm, and then uses sensors to figure out if/how much it is out of balance at that speed, and what weights need to be placed where to get it balanced within some tolerance.

 

So, an wheel or a crankshaft for an F1 car gets spun up to a much higher RPM vs the ones for regular cars, to balance them, because they get spun up IRL that much faster.  The parts are balanced for any rpm up to those rpms, but if you get them going faster than that rpm, then smaller and smaller weight differences will cause vibrations.

 

The tire balancing machine at shops is a common example, it gets the tires up to a certain rpm to balance them.  Going past that rpm in real life, you may experience vibrations from the tire.  If the tire guy is doing his job right, he'll balance a tire for a Chevette at a much lower rpm compared to a Corvette.

  • Like 3
Silverado4x4 Senior Enthusiast

Silverado4x4

Posted
Posted (edited)
12 hours ago, davester said:

A balancing machine spins an object up to a certain rpm, and then uses sensors to figure out if/how much it is out of balance at that speed, and what weights need to be placed where to get it balanced within some tolerance.

 

So, an wheel or a crankshaft for an F1 car gets spun up to a much higher RPM vs the ones for regular cars, to balance them, because they get spun up IRL that much faster.  The parts are balanced for any rpm up to those rpms, but if you get them going faster than that rpm, then smaller and smaller weight differences will cause vibrations.

 

The tire balancing machine at shops is a common example, it gets the tires up to a certain rpm to balance them.  Going past that rpm in real life, you may experience vibrations from the tire.  If the tire guy is doing his job right, he'll balance a tire for a Chevette at a much lower rpm compared to a Corvette.

Exactly, just an example, driveshaft balanced up to 6000rpm with 1oz weight over 6000rpm that same 1oz weight turns into around 150lbs of force anything higher in rpm now the shaft is out of balance and will start the vibration as rpm increases it gets worse. Same goes for tire balancing as davester said.

Edited by Silverado4x4
  • Like 2
  • 4 weeks later...
redwngr Senior Enthusiast

redwngr

Posted
Posted (edited)

GM might decide to limit speed below the trating of the tires (for whatever reason).

 

Hard to believe they would even think about accepting the liability of having the vehicle speed limiter above the rating of the tires they installed.

 

Will tires fail immediately if rated speed is exceeded?  --- of course not . 

 

 

image.thumb.png.2c77f78362e9355d1c782acd3907bc78.png

Edited by redwngr
  • Confused 1
  • 2 weeks later...
SIXFOUR Enthusiast

SIXFOUR

Posted
Posted

Well, unfortunately they don't make Z rated all terrain tires..

In central Texas we have SH130, where the posted speed limit is 85 mph. And if you're going that slow, you need to stay to the right lane!Screenshot_20251106_193457_Chrome.thumb.jpg.75061b0ac81f032e51eca8647285d632.jpg

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