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Opinions vs math on gear ratio

Armo325

By Armo325
in Engines & Drivetrain

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Donstar Senior Enthusiast

Donstar

Posted
Posted
16 minutes ago, Jacoby said:

Assuming one does nothing with their new blingy truck accept drive it, this is sound advice.

 

But for people that tow regularly and really use there trucks the above statement is not sound advice at all. 

As you say in your posts, this is your opinion and I respect your comments.  Op mentioned that he doesn't tow much..  Although I haven't experienced towing with a 5.3 V8 and 3.08 gear ratio, I towed considerable loads with my 4.9 L6 Fords with the 3.08. (100 + or - hp)   I currently tow a 22.5 ft. travel trailer with my V6 Sierra crew cab and find it has ample power for my needs..

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Jacoby Enthusiast

Jacoby

Posted
Posted
38 minutes ago, Donstar said:

As you say in your posts, this is your opinion and I respect your comments.  Op mentioned that he doesn't tow much..  Although I haven't experienced towing with a 5.3 V8 and 3.08 gear ratio, I towed considerable loads with my 4.9 L6 Fords with the 3.08. (100 + or - hp)   I currently tow a 22.5 ft. travel trailer with my V6 Sierra crew cab and find it has ample power for my needs..

The argument here is not about power.  You could pull your travel trailer with a ride on lawn mower if you had to.  Gearing is about efficiency.  Lift your V6 Sierra 6" and put 35" tires on it and get back to me on whether you think it has enough power or not.  The "power" is still there but you're using a bunch of it just trying to move your new blingy lifted truck.  Re-gear it and all of the sudden your new blingy truck tows as good if not better then before you lifted it and put big wheels on it.  You can say your truck has ample power for your needs but your truck isn't comparable to the OPs. 

Armo325 Enthusiast

Armo325

Posted
  • Author
Posted

I appreciate the commentary, fellas. This is all good stuff.

Going back to Grumpy’s comments earlier regarding Moment of Inertia, one question I conveyed rather poorly in the original post is this: Why don’t any of the countless online “gear ratio” calculators attempt to capture the requirement to compensate for the heavier/larger wheels and tires?

Every calculator I have seen appears to be a fairly simple one that simply tries to give you a gear ratio that will return you to the OEM relationship between driveshaft RPM and wheel RPM.

One reason for this might just be that the math is “too hard.” I reject that based on a (possibly erroneous) assumption that people willing to take the time to build something like that online are also going to give it their best effort.

Another possibility is that, in the grand scheme of things, the heavier wheel/tire’s net effect on the rear diff isn’t a major contributor when compared to the other forces acting on it.

Thoughts?

Donstar Senior Enthusiast

Donstar

Posted
Posted
1 hour ago, Jacoby said:

The argument here is not about power.  You could pull your travel trailer with a ride on lawn mower if you had to.  Gearing is about efficiency.  Lift your V6 Sierra 6" and put 35" tires on it and get back to me on whether you think it has enough power or not.  The "power" is still there but you're using a bunch of it just trying to move your new blingy lifted truck.  Re-gear it and all of the sudden your new blingy truck tows as good if not better then before you lifted it and put big wheels on it.  You can say your truck has ample power for your needs but your truck isn't comparable to the OPs. 

It appears you feel challenged and for this I apologize.  I was sharing  experiences.  Op said that with his 35" tires he estimates his gear ratio to be roughly equivalent to 3.17.  I am curious as to which would be a more capable tow vehicle - a 4.3 V6 with 3.42 or a 5.3 with a 3.17?  I personally believe they both have ample power.  I am not being sarcastic or funny.  I'm basing my comments on experience towing similar loads with a lot less power. (no lawnmowers however ;-))

Jacoby Enthusiast

Jacoby

Posted
Posted
25 minutes ago, Donstar said:

It appears you feel challenged and for this I apologize.  I was sharing  experiences.  Op said that with his 35" tires he estimates his gear ratio to be roughly equivalent to 3.17.  I am curious as to which would be a more capable tow vehicle - a 4.3 V6 with 3.42 or a 5.3 with a 3.17?  I personally believe they both have ample power.  I am not being sarcastic or funny.  I'm basing my comments on experience towing similar loads with a lot less power. (no lawnmowers however ;-))

It would appear you don’t know how public forums work. In them we discuss things, we debate things, we offer others advice so on and so forth. I don’t feel challenged but I do feel the need to help you understand because you yourself aren’t making any sense. 

You keep mentioning the OP has a 5.3l with 3.08 gears so at 3.17 he’s “doing better than stock”. Well the OP has 3.42 gears so at 3.17 he’s actually doing a lot worse than stock and to get back would need to re gear to 3.98 or 4.10 since there isn’t a 3.98 gear. 

You keep mentioning both trucks make ample power and I agree they do. Let me say it again. It’s not about power made it’s about how the power gets put to the ground. It doesn’t take an engineer (since there’s no rocket scientist in this thread) to determine a 5.3 with a 3.17 gear is a more capable tow vehicle over a 4.3 with 3.42s. It makes damn near 100 more hp & tq. That is more than enough to make up for the shortcomings of a lower gear. Point is OP still isn’t running as efficiently as he would be with stock gearing ratio. At 3.17 he’s way off the powerband and the truck is a dog. With a taller gear he’ll gain back performance because it won’t take as much power to get things moving and his gas mileage will go up and if he ever does tow something heavy it will do it with a lot less effort. When it comes to the OPs truck there are no negatives to re gearing. 

 

Ive enjoyed this discussion 

Grumpy Bear Senior Enthusiast

Grumpy Bear

Posted
Posted
48 minutes ago, Armo325 said:

Going back to Grumpy’s comments earlier regarding Moment of Inertia, one question I conveyed rather poorly in the original post is this: Why don’t any of the countless online “gear ratio” calculators attempt to capture the requirement to compensate for the heavier/larger wheels and tires? Thoughts?

Best GUESS? It would be very hard to calculate the inertia moment. It isn't a simple flywheel with a known mass distribution. What could be done is build a spin fixture and directly measure it. Not cheap, not easy...only a few care. I'll bet some private race shop on the continent has one though. Someone like Allen Johnson or Force Racing. Maybe one of the off road teams. They spin everything else for this purpose. An Engineering University like MIT or CSM? 

Donstar Senior Enthusiast

Donstar

Posted
Posted
3 hours ago, Jacoby said:

 It doesn’t take an engineer (since there’s no rocket scientist in this thread) to determine a 5.3 with a 3.17 gear is a more capable tow vehicle over a 4.3 with 3.42s. It makes damn near 100 more hp & tq. That is more than enough to make up for the shortcomings of a lower gear.

 

Ive enjoyed this discussion 

I also enjoy a good, respectful discussion.  I wonder if the 70 hp. difference between the 4.3 and 5.3 is more than enough.  My 4.3 (3.42) is rated to tow 7,000lbs.  The same truck with a 5.3 (3.08) is rated to tow 6100 lbs.  I don't have data on what a 5.3 (3.17) would be rated to tow but logic would suggest it would be  higher than the 5.3 (3.08).  However, I would need more information before concluding that it is more capable than the 4.3 (3.42).  My guess is that they would be reasonably comparable.  

JimCost2014 Senior Enthusiast

JimCost2014

Posted
Posted
31 minutes ago, SS502 said:

We’re gonna need a bigger boat....

Territorial pissing never gets old, but I am offended by the rocket scientist remark, seeing how I consider myself to be a "brain scientist", or depending on the day a sort of "rocket surgeon"...….  

Darksky Enthusiast

Darksky

Posted
Posted

Here it s a screen grab of the numbers I researched for my edification, when I was looking at gearing.

BF4BDB7C-F06F-44BD-A5A5-51EB7E955C26.jpeg

Grumpy Bear Senior Enthusiast

Grumpy Bear

Posted
Posted
3 hours ago, Darksky said:

Here it s a screen grab of the numbers I researched for my edification, when I was looking at gearing.

 

Question. At the bottom you mention tire weights, is that just the tire or tire and rim? Thanks.

Armo325 Enthusiast

Armo325

Posted
  • Author
Posted
2 hours ago, Grumpy Bear said:

Question. At the bottom you mention tire weights, is that just the tire or tire and rim? Thanks.

If anyone wants another data point, I did some digging on this about a year ago and just found my old numbers.  I didn't personally weigh any of these but rather used numbers I could find online so consider the source.  The aftermarket numbers were easy to find from the manufacturers.  The OEM numbers were harder to find and I would consider them less reliable.

 

OEM 2017 Z71 wheel & tire (265/65/R18) combined weight: 65 lbs.

20x10 Fuel Krank & 325/60/R20 tire combined weight: 111 lbs.

 

That's obviously only part of the story since the radius and mass distribution plays a huge role but, as you can see, just switching wheels and tires adds 46 lbs. per wheel.

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