How to Choose Your Axle Gear Ratio

Hey, this is Tyler for BleepinJeep.com. This week we're going to talk about axle gearing. I'm going to go over some ideas with you on how to select the best gear ratio based on the tire size that you're running on your four-wheel drive. Let me show you how to how to do that. Before we get into what ratio you should choose, what gear ratio you should run in your axles, let's get right down to the basics and talk about what exactly is your gear ratio. So in the differential, in your axle, there is a ring gear and a pinion gear. The pinion gear is attached, the driveshaft comes down and attaches to a yoke here. That pinion gear engages the ring gear and then your axles are attached to the ring gear through a carrier. So you'd have one axle coming out this way and one if you can imagine going down through the table. So this is your drive gear and this is your driven gear. So the pinion receives power from the engine through the transmission and transfer case that goes into the driveshaft. The driveshaft attaches to the pinion and then the pinion rotates the ring gear which in turn rotates your axles. Now gear ratio is very simply the number of teeths on the pinion gear compared to the number of teeth on the ring gear. So how you determine your gear ratio is you count up the number of teeth on your pinion. 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11. Now we count up the number of teeth on our ring gear. I'm going to make a mark so I can keep track of where I'm at. So 1, 2, 3, 4, 40, 41. 41. So I have 41 on my ring gear and 11 on my pinion. Now I just take the number of ring gear teeth, so 41, and I divide it by the number of pinion teeth which were 11 and that gives me 3.727272. Now gear ratios are always given in three-digit numbers. There'll be a number, a decimal point, and two digits. So we're going to round that up to 373 and that is our gear ratio for this gear set. One way to determine your gear ratio is if your rear end has a tag on it like this. This says 307. This is a 307 gear ratio. There's also a 43-14. That tells you there's 43 teeth on the ring gear, 14 teeth on the pinion. And if you divide 43 by 14 you'll get 307. Problem is this tag is oftentimes missing or so corroded that you can't read it. On most gear sets, but not all, the gear ratio is stamped right into the gear set. So this is a Dana 44-373, 373 gear set. In case that isn't stamped there, then the counting method is how you determine what your gear ratio is. Here you can also see it stamped into the ring Dana 44-3.73. Here's an example of an aftermarket gear set that does not have the gear ratio stamped into it. All it has is an RG and then a serial number. So the only way to figure out the gear ratio on this set is to count the teeth and divide it. This is a Dana 30 gear set and this is something you might see as well. 41-10. This is the number of teeth on the ring gear. This is the number of teeth on the pinion. So 41 divided by 10 is 4.1 or 410. So why do we even care about gear ratio? Well this is why it's important. When your Jeep was designed at the factory, the engineers had to take into account a whole bunch of stuff. What kind of engine was in it? What kind of transmission? What the gear ratios are in that transmission? What size the rear axle is? How big the tires are on the vehicle? And how all of those things are going to interact in such a way to keep the engine operating in the most ideal RPM range that would give you a good balance of power but also good fuel economy. So they take all those things into account when designing the vehicle. The only trouble is most of the time our four-wheel drives come from the factory with stupid tiny little tires and it doesn't take long to get tired of that poor ground clearance. So what do we do? We put a lift on the vehicle so we can fit bigger tires. Well that's great but when we put those big tires on we have now changed that entire equation and now suddenly our transmission, especially an automatic transmission, it doesn't shift at the correct RPM. Or when we're out on the highway cruising we can't ever get it to go into overdrive because it's trying to find gears between fourth and fifth and fourth and fifth. Or we can't ever get to a sufficient RPM range for the torque converter to lock up which causes the torque converter to overheat and wear out and all kinds of of economy, fuel economy, and performance problems. So gear ratio is a big deal and it's the simplest way to correct for going with bigger tires. So we've figured out why the gear ratio is important. Now let's figure out we've added big bigger tires to our Jeep. How do we determine what gear ratio to run based on the size of those tires to get us back within that good normal operating range for the transmission and the engine? I'm going to give you a formula here in just a second and don't worry I will I'll put a I'll put a graphic up in the video that you can pause and write down. But the way it works is you take your new tire size, you times it by the old gear ratio, and that's why we needed to know what the original gear ratio is. Then you divide all that by the old tire size and that gives you pretty darn close what your new gear ratio should be. Let me take you through one of these calculations just to show you how this works. Let's let's uh let's calculate for project tetanus what gear ratio I should run given the tire size that I want to run. So my plan is to run 33 inch tires so that's the new tire size. I need to times that by my my old gear ratio which I just showed you out there is 3.07 and then I need to divide that by the old tire size and it had metric tires on it but they come out to be about 29 inches tall and this is going to give us our ideal new gear ratio. So let's go ahead and do that 33 times 3.07 divided by the old tire size 29 inches and that equals 3.49. So I pick the closest gear ratio to 3.49 and that's a 355. 355 gears would give me the ideal rpm operating range given the new tire size. Now this number is not set in stone it's more of it's it's a guideline. Now with project tetanus I'm actually going to be running 373 gears and the reason for that is I happen to have a set of 373 gears for the Dana 44 in the rear. It's close enough to this ratio that I'm not super concerned. It's going to give me more low-end torque and low-end power because the higher the number is the lower the gearing but this should still give me decent on-road gas mileage and on-road manners given that it's going to be a daily driver. The other thing to consider is I'm going to be running a manual transmission not an automatic transmission so I'm not as concerned about an automatic trying to shift within certain rpm ranges. That's not going to be an issue for me with a manual transmission. So this is a guideline it's not a chain to chain you down with but if you want your vehicle to operate within the the closest parameters that it that it was engineered to operate in you need to get pretty close to this number. Let's just do one more as hypothetical. Let's take my YJ. My YJ it currently has 33 inch tires but I wanted or I planned my plan when when building it was to have 35 inch tires. Its gear ratio originally was 410 and it originally had 29 inch tires. So if we take this and we say 35 times the old gear ratio of 410 4.10 divided by 29 that equals 495. So the closest gear ratio would probably be 488 which is exactly what I'm running. The problem is I don't have 35 inch tires I have 33 inch tires because I just got a really good deal on some used tires and that's what I ended up with and because of those those 33 inch tires I run at a much higher rpm rate than I should and I've got really really good low speed crawling torque and really good low speed power but when I get up to about 55 miles per hour I'm pushing 3000 rpm which is really a lot faster than that motor should be running down the highway and really really hurts my gas mileage. So let's figure out what gear ratio I should be running with 33 inch tires. So 33 times 410 which was my original gear ratio divided by 29. 33 times 410 divided by 29. That equals 466. The next closest gear ratio would be 456. So if I wanted to run 33 inch tires I should have went with 456 instead of 488. But again my original intention was to run 35 inch tires and eventually I will run 35 inch tires. That should get me back into the rpm range that I want to be in where I can have good gas mileage and still have this low end power. All right I hope those examples were helpful to you. Now there's a couple of odds and ends we need to go over before we quit here. The first thing is gear sets are machined pairs. When they are manufactured they are they are precision machined for this pinion to line up with this ring gear. You cannot interchange them. You can't take a pinion from one set and try to set it up with a ring gear of another set to create some kind of a weird ratio. They just they won't run. They'll be noisy. They'll be excessive wear and the gear set will eventually fail. So when buying used gears you need to make sure that you get a matched set. The pinion has to stay with that specific ring. Let me clear up one other thing too just to prevent some confusion. The lower the number the higher the higher the gears are. So let's go 410, 456, 488. So the numerically larger the number the lower your gearing is. So this is going to give you really good gas mileage but you're not going to have any power off the line and you're crawling out out crawling and four-wheel driving. This ratio sucks. This is going to give you really good power right off the line. It's going to be awesome low gearing when you're crawling. Out on the highway your gas mileage is going to suck. So the bigger the number the lower your gearing. Going right along with this I wanted to explain something called a carrier brake. Now all of your Dana axles and that's your Dana 30, Dana 35, Dana 44, even the Dana 60. So all these axles that came in your jeeps have what's called a carrier brake. And all that means is this, this is the carrier. Your ring gear attaches to the carrier. Carriers are designed, the Dana carriers are designed to only accommodate gears up to a certain point. The carrier brake for a Dana 30 is right here at 355. So if you have factory gears that are 355 or numerically lower then 355 is the the deepest gear that you can get. If your factory Dana 30 came with 373 gears or numerically higher then it can accommodate all of these lower gears. But that's the carrier brake right there. Why that's important is if you go to order a traction device say like a locker when you're doing your gear set you need to make sure that you get the correct size of locker, the correct size of carrier for the gear set that you're ordering. If you're going low gears like a 488 you need to make sure you get the correct carrier or you will not be able to get the gear set up properly in the differential because this part of the carrier right here will be too thin. You won't be able to get that ring gear far enough or close enough to the pinion to be able to set the gear up right. So that's something you need to be aware of, carrier brakes. Another thing I wanted to talk about is reverse cut gears. This is a Dana 30 gear set laid on top of a Ford 9-inch gear set. You'll notice see how all the teeth on the Ford 9-inch curve this way but all the teeth on that Dana 30 curve that way. That's because this is a reverse cut. The reason this is a reverse cut is because it is a high pinion front axle. High pinion meaning if this is the center line of the axle this pinion is going to come in above the center line of that axle. So the pinion is going to come in and mesh above the center line. If it was a low pinion then it's going to mesh below the center line of the axle. So if you have a high pinion front axle which some some of the Cherokees have them some of them don't, then you need to make sure you get reverse cut gears. The next thing that I wanted to talk to you about is what I call gear fudging. And that's just simply that not all gear ratios are available in the exact same number for all axles. I'll give you an example. My YJ has a Dana 30 in the front and I have a 488 gear set. My rear is a Ford 9-inch. Well you can't find a 488 gear set for a 9-inch but you can find a 486. Now all gear ratios are a three-digit number like this. A number, a decimal point, and then two more. If you are this close, you're within two thousandths, you're going to be okay. Another example, this gear set sitting over here is a 430 gear set for the 9-inch. Well you can't get a 430 gear set for the Dana 30 front axle but you can get a 427. Well these are numerically close enough that you're going to be okay. What you don't want to have is say a 486 in the rear and a 456 in the front. That's a recipe for disaster. Anyways that's what I call gear fudging. The numbers, they need to be within about two or three thousandths but you do have a little bit of leeway. Well I think that just about wraps it up for axle gearing. I hope that this has been helpful for you guys. I really hope that this helps you get a gear set or choose a gear set that you're going to be happy with that will give you the performance that you want. As always, please like, share, and subscribe so we can keep the videos coming. We appreciate your support of the channel. Go check out bleepinjeep.com for a ton of just awesome off-road how-to videos and there's some pretty cool stuff on there too. T-shirts, some other things that Matt's got. We appreciate your support and I will see you guys next week.