How to Choose Your Axle Gear Ratio

hey this Tyler for bleep in cheap calm this week we're going to talk about Axl Geary 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 so 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 let's get right down to the basics and talk about what exactly is your gear ratio so in your the differential and your axle there is a ring gear and opinion gear the pinion gear is attached the drive shaft 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 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 drive shaft the drive shaft attach to the pinion and then the pinion rotates the ring gear which in turn rotates your axles gear ratio is very simply the number of teeth 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 out the count of the number of teeth on our ring gear I'm gonna 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 opinion 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 point 7 - 7 - 7 - how gear ratios are always given in three-digit numbers they'll be a number a decimal point and 2 digits so we're going to round that up to 373 and that is our gear ratio for this for this gear set one way to determine your gear ratio is if your rear end has a tag on it like this this is 3:07 this is a 307 gear ratio there's also a 43 - 14 that tells you there's 43 teeth on the pinion around 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 but 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 gear 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 40 110 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 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 are our four-wheel drives come from the factory with stupid tiny little tires and it doesn't take long to get tired of those of that poor ground clearance so what do we do we 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 lockup which causes the torque converter to overheat and wear out and all kinds 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 we 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 up 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 it 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 Bob don't um Bob let me take you through one of these calculations just to show you how this works let's let's 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 three point zero seven and then I need to divide that by the old tire size 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 3.0 seven divided by the old tire size 29 inches and that equals three point four nine so I pick the closest gear ratio to three point four nine 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 a hypothetical let's take my YJ my YJ it currently has 33 inch tires but I wanted or I plan my plan when when building it was to have thirty five 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 thirty five inch tires I have thirty three inch tires because of a 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 3,000 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 opinion 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 for 10 456 488 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 crawling in four-wheel driving this ratio sucks this is going to give you a really good power right off the line 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 the carrier break 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 this is the correct size of carrier for the gear set that you're ordering if you're going low gear is 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 nine-inch gear set you'll notice see how all the teeth on the Ford nine-inch curved this way all the teeth on that Dana 30 curved 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 centerline 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 centerline if it was a low pinion then it's going to mesh below the centerline 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 now 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 nine-inch well you can't find a 488 gear set for a nine-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 2000 s you're going to be okay another example this gear set sitting over here is a 430 gear set for the nine-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 bleepin Jeep calm for a ton of 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

Channel: BleepinJeep

Views: 1,606,323

Keywords: grand cherokee, regearing, cherokee, jeep wrangler, chrysler 8.25, how to regear, bleepinjeep, gears, dana 35, dana 30, lockers, wheeling, wrangler jeep, XJ, yj, project tetanus, axle gears, rear axle, jeeping, wrangler, jeep grand cherokee, off roading, mj, lifting, dana 60, jeep cherokee, do it yourself, axle shaft, offroad, cj, tires, axle, dana 44, xj, zj, jeep, 4x4

Id: oDVpyhKEbaw

Length: 19min 15sec (1155 seconds)

Published: Sat Dec 26 2015