Search the Community
Showing results for tags 'differential gear'.
I was going to post this as a follow-up in the 4.10 gear thread about re-calibrating the speedometer but it seemed like a bridge too far in terms of a thread jack so I thought I'd start a new thread. I suspect I'm going to cover ground that has already been covered in a few threads so I apologize but hopefully this helps someone. Also, if you don't want to waste your time with my nerd-ery throughout, just go to the last two paragraphs. Re-gearing for different tire sizes is a topic that I have followed for some time with keen interest. For one, I have a lifted truck with 35-inch tires (more on that later) still on the stock gears and am eagerly awaiting the moment I have enough cash to re-gear it. I hope to do so in the next couple months. Second, this topic is interesting to me because it seems to be loaded with opinions about the correct gear to choose, which is surprising since gear ratios and their effects are a known quantity. Full disclosure: I have never worked in the auto industry and I am NOT a certified auto technician. I do have a BS in Aerospace Engineering (I am NOT a practicing engineer) and a fair amount of technical experience so I consider myself at least partially qualified to discuss technical topics in general terms. As always, there are definitely people that frequent these forums that know more than I do. If you fit that description, please chime in and correct my errors. Pointing out that I'm wrong won't hurt my feelings. I will feel bad, however, if someone spends a bunch of money or makes a poor decision based on my bad information. Everyone has different desires for their truck's performance so I always cringe when I see a new thread with the topic "What gear ratio should choose?" The only thing that makes me cringe more is when someone posts in that same thread "You should get 4.56. That's what I did and I love it. You'll be happy." That's great but it's also totally useless feedback in my opinion. Without any explanation for the reasoning behind the decision and likely with no knowledge on what will make someone else happy that's a pretty big leap. I find myself frustrated when someone's emotions drive their recommendation for a specific gear ratio based on how they feel when they drive their own truck. I don't know anyone on this forum personally. I respect all of you for our common love for our own trucks but I will always be skeptical when a relative stranger gives me a recommendation based on their "feelings." On to my specific example: my own 2017 Silverado 1500 LT Z71. I bought the truck from the dealer with a 6" lift, 20-inch wheels and 35-inch tires already installed. I love my truck. What I didn't do, largely due to a lack of experience and research was get new gears installed right away. I wish I had. I plan to do so soon so the natural question is "What gear ratio should I get?" I don't tow much. When I do, it isn't especially heavy. I bought a 1500 for that reason. I love the bigger, stronger trucks but at this point in my life I just don't need one. So if you're like me and have 35-inch tires and the GM stock 3.42 differential gear (6-speed transmission), the determination on the new gear ratio now depends on what I want to do with the truck. In my case, I want to get back to stock performance/drive-ability, or as close as possible. To figure out how to do that, there is no opinion needed. It's a question of geometry and its associated math. If you Google the phrase "gear ratio tire size" you will be inundated with calculators that will tell you what you need. I picked the first two that popped up. One said my current effective gear ratio is 3.17 and I need to be a 3.98 to get back to stock. The other said my numbers were slightly different but not by much. The difference is likely because one calculator used a simple "tire diameter" figure and the other used actual tire sizes. With that info alone, I will round to the next lower ratio (numerically higher) and go with 4.10 gears. The common response to this line of reasoning that I see is "But, dude, what about the rolling mass of the new, bigger tires?!?" Great point. First off, I'm a nerd so what we should probably be referring to is "angular momentum" which is what is probably meant when people say "rolling mass." Every wheel/tire out there is rolling mass but it's the angular momentum of that wheel and tire that we're concerned about. Specifically accelerating when we're talking gears. First, a few relevant formulas: L=I*ω "L" is Angular Momentum "I" is moment of inertia "ω" is angular speed, aka how fast the thing is spinning I=r^2*m "r" is radius "m" is mass The second formula just states the obvious - a bigger/heavier wheel has a bigger moment of inertia than a smaller one. What might be less obvious is the impact each variable has. Moment of inertia increases linearly with an increase in mass but exponentially with an increase in radius. To make it relevant, if my 35s and your 37s, including wheels, are the same total weight you have a much bigger angular momentum problem to deal with than I do. You can see from this point already that accounting for angular momentum is difficult because it's based on speed. Turning your new big, heavy wheel/tire combo gets harder as you go faster so it's very difficult to account for that stress on your drive train since it changes as you accelerate. I would submit the bigger discussion here is the need for better brakes but that's a different topic. I haven't been able to find a good calculator to account for angular momentum differences with wheel/tire combos and haven't been willing to really dig down to generate an answer. Part of that is because of the speed variability. Another part is because I had had a surprisingly difficult time finding the actual weight of OEM wheels and tires. Finally, I'm not sure the difference in angular momentum makes a huge impact on the overall stress on the drive train. While The additional weight/radius of the tires is relevant, I don't know how much it matters when those wheels are still sitting under a truck that weighs over 5,000 lbs. This point is a bit speculative on my part and my instinct could be way off how in how much angular momentum affects the overall drive train forces. Real engineers, feel free to chime in here if I'm way off base. With all of that in mind, my conclusion is that changing gear ratios for new wheels and tires is hugely important because it allows you to match your differential ratio back to your transmission. The "system of systems" inside the truck were designed for a specific reason and matching your gear ratio to your new tires will get you back in that "sweet spot" the GM engineers found when they designed the truck. Using the online calculators to determine your new gear ratio based purely on the new radius or your tires is likely enough to answer the mail. Based on those calculators, I need to go to 4.10 to get back to stock. The guys that jump up and down saying 4.56 is the answer because they "like it" are potentially introducing new engineering problems in their truck by forcing all of the rotating internals to spin at a higher speed than originally intended for long periods of time. I'm not saying there aren't reasons to go to 4.56 but I would argue that gearing too low can be as risky as gearing too high if your goal is to have a safe, drive-able truck that lasts a long time. Again, I hope this is helpful and I welcome any feedback anyone has to make this info more useful.