JeepinSoldier Posted December 6, 2011 Share Posted December 6, 2011 I have recently moved to Colorado for a new job after retirement from the Army. My Comanche is a 1990 with the 4.0, AX 15 5 speed, and 3.07 gears. The truck has 30" tires on it now and I may move up to 31's. It will never have larger than 31's, as its a longbed and my DD. I have a Wrangler on 35's for wheeling. The combination of altitude and poor gear ratio is making the truck a dog in the mountians, and I'm pretty sure the front axle carrier/ring/pinion are in the process of self destructing. No problem as I wanted to swap the axle for a non disconnect any way. Will a 3.55 be OK in the mountians, or do I need 3.73 or 4.10? Link to comment Share on other sites More sharing options...
Eagle Posted December 6, 2011 Share Posted December 6, 2011 3.54s will be "okay" with 30" tires, but 3.73 would be better. And if 31s are on the horizon, you really want 4.10s. When I picked up my '88 MJ I regeared with a set of 3.73s I had bought for my Cherokee, with the intention of running them with 30" tires.The MJ came to me with a 4" lift so I threw on a set of used 31" tires and the 3.73 gears. The overall final drive ratio, as indicated by MPH per 1000 RPM, came out to be exactly the same as a stock XJ or MJ with 3.54 gears. I have now removed the lift and the MJ is on 225/75-15 tires, and the 3.73 gears with the 5-speed are still a good combination. In mountain terrain, I think 4.10s would be better for you with either 30s or 31s, and it's a lot easier to find a 4-banger XJ donor vehicle with 4.10s than it will be to find 3.73s. Link to comment Share on other sites More sharing options...
Jeepman Posted December 6, 2011 Share Posted December 6, 2011 Here is a nice Gear calculator To help you figure out the RPM's the engine will be turning with each different gear ratio then you can make a better choice based on the RPM's you want to be running... http://www.grimmjeeper.com/gears.html Link to comment Share on other sites More sharing options...
87Warrior Posted December 6, 2011 Share Posted December 6, 2011 You shouldn't bother your self running gear calculators at all, unless you are trying to figure out what what speedometer gear to run. I think you would be happy with 4.10s and 31's. Stock for stock, I enjoyed driving my TJ Rubi (5spd) in the mountains much more than our XJ (auto). The Rubi had 4.10s and 30's and was heard just about perfect for the mountains while the XJ with 3.55's was constantly down shifting to try and find the "sweet spot". I know it is hard to draw parallels between the 5spd and auto, but the 4.0 has its powerband which is actually quite good once you are in there. You should not be afraid to let your engine tach up and use the power it can make, it is good for them. Link to comment Share on other sites More sharing options...
Jeepman Posted December 6, 2011 Share Posted December 6, 2011 You shouldn't bother your self running gear calculators at all, unless you are trying to figure out what what speedometer gear to run. The Gear Calculator has nothing to do with speedometer? it is to simulate different gear ratio's and tire sizes etc, to figure out the RPM range you should expect... Link to comment Share on other sites More sharing options...
87Warrior Posted December 6, 2011 Share Posted December 6, 2011 The Gear Calculator has nothing to do with speedometer? it is to simulate different gear ratio's and tire sizes etc, to figure out the RPM range you should expect... :doh: Whoops. I was thinking about those gear/tire size/speedometer tooth charts. Link to comment Share on other sites More sharing options...
Jeepman Posted December 6, 2011 Share Posted December 6, 2011 Ha ha :cheers: Now I can see where you were going... Link to comment Share on other sites More sharing options...
Eagle Posted December 7, 2011 Share Posted December 7, 2011 Here is a nice Gear calculator To help you figure out the RPM's the engine will be turning with each different gear ratio then you can make a better choice based on the RPM's you want to be running... http://www.grimmjeeper.com/gears.html That's a very nice calculator, except for the fact it's wrong. Aside from the fact that they show the 5th gear ratio for the BA 10/5 as 0.79:1 and the factory manual shows it as 0.72:1, even checking their fourth gear speeds to RPMs they're off. I think their calculations (like every other on-line calculator I've seen) use mathematical formulas based on tire dimensions. What these don't take into account is that sidewall "squat" results in a real-world rolling radius that's less than half the static tire diameter. When I made up my spreadsheet, I researched tire manufacturers' data and used published numbers for revolutions-per-mile as the basis for my formulas. The result is that, for the final section of this calculator, they show lower RPMs for each speed than they should. For 225/75-15 tires and 3.07 gears, 65 MPH in fourth gear should be 2395 RPM. They came up with 2370. For 30" tires and 3.73 gears (and I used a tire diameter of 29.5 in their calculator, not the nominal 30) the actual RPM in fourth gear at 65 MPH should be 2849. They came up with 2761. The differences aren't horrible, and since they're wrong across the board the calculatpor is still useful for predicting how much change you'll get from regearing or swapping tires. But the numbers aren't accurate. Link to comment Share on other sites More sharing options...
Jeepman Posted December 7, 2011 Share Posted December 7, 2011 That's a very nice calculator, except for the fact it's wrong. Aside from the fact that they show the 5th gear ratio for the BA 10/5 as 0.79:1 and the factory manual shows it as 0.72:1, even checking their fourth gear speeds to RPMs they're off. I think their calculations (like every other on-line calculator I've seen) use mathematical formulas based on tire dimensions. What these don't take into account is that sidewall "squat" results in a real-world rolling radius that's less than half the static tire diameter. When I made up my spreadsheet, I researched tire manufacturers' data and used published numbers for revolutions-per-mile as the basis for my formulas. The result is that, for the final section of this calculator, they show lower RPMs for each speed than they should. For 225/75-15 tires and 3.07 gears, 65 MPH in fourth gear should be 2395 RPM. They came up with 2370. For 30" tires and 3.73 gears (and I used a tire diameter of 29.5 in their calculator, not the nominal 30) the actual RPM in fourth gear at 65 MPH should be 2849. They came up with 2761. The differences aren't horrible, and since they're wrong across the board the calculatpor is still useful for predicting how much change you'll get from regearing or swapping tires. But the numbers aren't accurate. You should email the host of the calculator so he can correct the mistake if there is any so it can be updated as needed... Now it is an online calculator and NO calculator should be taken as 100% acurate since they do not cover all variables such as wind and hills etc... It is just meant to be a tool to simulate RPM's rather then a trial and error actually changing gear ratio's to find that perfect Number... Link to comment Share on other sites More sharing options...
Justin_k Posted December 7, 2011 Share Posted December 7, 2011 Good formula: New tire size x current gears divided by factory tire size Link to comment Share on other sites More sharing options...
jeepcoma Posted December 7, 2011 Share Posted December 7, 2011 I would for sure go with the lowest gearing you can (highest number). I'm running 3.54s with my ax-15 on stock size tires and it's way nicer to drive than it ever was with the 3.07s (I can't even remember how horrible that was). If I ever need to regear, I'm going with 4.10s. I would not be afraid of overgearing this thing, even for a DD (which mine is). It's far nicer on the highway at speed with the RPMs north of 2k than dogging along, and probably more fuel efficient too unless you live on totally flat roads with no elevation changes. Link to comment Share on other sites More sharing options...
johnj92131 Posted December 8, 2011 Share Posted December 8, 2011 If you check hornbrod's posts you will see he went from 3.54 to 4.10 and picked up a couple of mpg on the highway. Don is running a Hesco stroker and AW4 automatic. I am running 235/75 x 15 tires with the stock 3.54 gears and an AW4 automatic. But I am at sea level most of the time. As for tire calculators, just use the tire makers number of revs/mile and multiply by .98 to account for sidewall flex down the road. Did that with my Passat TDI then checked it over a measured 160 miles. Dead nuts on with the calculated numbers! Link to comment Share on other sites More sharing options...
Geonovast Posted December 8, 2011 Share Posted December 8, 2011 That's a very nice calculator, except for the fact it's wrong. Aside from the fact that they show the 5th gear ratio for the BA 10/5 as 0.79:1 and the factory manual shows it as 0.72:1, even checking their fourth gear speeds to RPMs they're off. I've been poking around a bit and it appears the BA 10/5 came with two different ratios for 5th. Input : 26 teeth, Layshaft Input : 37 teeth, Layshaft 5th : 53 teeth, Output 5th : 27 teeth, Ratio : 0.72496 Input : 26 teeth, Layshaft Input : 37 teeth, Layshaft 5th : 51 teeth, Output 5th : 28 teeth, Ratio : 0.78129 :dunno: Link to comment Share on other sites More sharing options...
carnuck Posted December 8, 2011 Share Posted December 8, 2011 With my BA 10/5 I was running 33" tires with 3.73 ratio. (originally 3.08 with stock tires) It was almost an exact match and I had plenty of power/top speed. I switched to 4.10s and lost ALL the top speed/power because the Renix is a low end torque maker. Link to comment Share on other sites More sharing options...
Eagle Posted December 8, 2011 Share Posted December 8, 2011 Now it is an online calculator and NO calculator should be taken as 100% acurate since they do not cover all variables lsuch as wind and hills etc... Wind and hills have exactly zero effect on the relationship between engine speed and road speed. Unless you have an automatic that's slipping very badly, the same engine RPM will always push the vehicle down the road at the exact same speed. It is just meant to be a tool to simulate RPM's rather then a trial and error actually changing gear ratio's to find that perfect Number... I realize that -- and for comparison purposes it's a better calculator than any other on-line calculator I've found. But, unfortunately, it isn't suitable for helping figure out if your speedometer is correct after a gear swap and/or tire size change, and that's something we all should be aware of, too. Link to comment Share on other sites More sharing options...
Jeepman Posted December 9, 2011 Share Posted December 9, 2011 But, unfortunately, it isn't suitable for helping figure out if your speedometer is correct after a gear swap and/or tire size change, and that's something we all should be aware of, too. No one said the calculator had anything to do with correcting the speedometer gear it is mearly to simulate engine RPMs by axle ratio and tire size.. Wind and hills have exactly zero effect on the relationship between engine speed and road speed You must live in a perfect world with no hills and no wind... Link to comment Share on other sites More sharing options...
jeepcoma Posted December 9, 2011 Share Posted December 9, 2011 Wind and hills have exactly zero effect on the relationship between engine speed and road speed You must live in a perfect world with no hills and no wind... How so? With an automatic that has a locking torque converter (which is all of them, unless you bought and installed a non-lockup for drag racing) or a manual that's not slipping the clutch, there's a linear relationship between road speed and engine speed for any given gear/tire size combination. 50 mph in 5th gear will always be the same RPM, no matter what. It might take more or less throttle to hold that speed depending on wind and incline, but you'll never have a different RPM at that speed unless you shift gears or change the size of your tires. Link to comment Share on other sites More sharing options...
Jeepman Posted December 9, 2011 Share Posted December 9, 2011 THE POINT IS THERE IS NO PERFECT WORLD... Going uphill or fighting Wind will always cause the engine to work harder maintaining a set speed otherwise you slow down Therfore there are always variables that can not be calculated and we are not talking about thousands of RPM's here... Link to comment Share on other sites More sharing options...
Eagle Posted December 9, 2011 Share Posted December 9, 2011 THE POINT IS THERE IS NO PERFECT WORLD... Going uphill or fighting Wind will always cause the engine to work harder maintaining a set speed otherwise you slow down Therfore there are always variables that can not be calculated and we are not talking about thousands of RPM's here... But, as Jeepcoma tried to explain, and as I tried to explain, unless you have an automatic and the torque converter lock isn't working, the engine is mechanically connected to the rear wheels through the transmission, transfer case and drive shaft. The road speed simply cannot change without a matching change in engine speed, and the engine speed cannot change without the road speed changing. The Speed-to-RPM relationship is fixed. Yes, the vehicle may slow down on long or steep upgrades, and a 4-banger may even slow down when buffeted by a headwind -- but both the engine speed and the wheel speed will slow down in direct proportion to the gear ratios involved. Or, again as Jeepcoma stated, you may need to give it more gas to maintain speed (which is proof that the engine is working harder), but if the road speed doesn't change, the engine speed doesn't change. It just uses more gas to maintain that same speed. Unless you shift gears. Link to comment Share on other sites More sharing options...
Jeepman Posted December 10, 2011 Share Posted December 10, 2011 THE POINT IS THERE IS NO PERFECT WORLD... Link to comment Share on other sites More sharing options...
Eagle Posted December 10, 2011 Share Posted December 10, 2011 THE POINT IS THERE IS NO PERFECT WORLD... Okay, you win. In the United States, there is a fixed relationship between the engine speed and the wheel speed. In Canada the laws of Physics and Mechanics do not apply. Now we can both be right. Link to comment Share on other sites More sharing options...
Jeep Driver Posted December 10, 2011 Share Posted December 10, 2011 THE POINT IS THERE IS NO PERFECT WORLD... Okay, you win. In the United States, there is a fixed relationship between the engine speed and the wheel speed. In Canada the laws of Physics and Mechanics do not apply. Now we can both be right. I would take it a step further...........but I won't. At least it was a mod that said it. :P Link to comment Share on other sites More sharing options...
Jeepman Posted December 10, 2011 Share Posted December 10, 2011 THE POINT IS THERE IS NO PERFECT WORLD... Okay, you win. In the United States, there is a fixed relationship between the engine speed and the wheel speed. In Canada the laws of Physics and Mechanics do not apply. Now we can both be right. I would take it a step further...........but I won't. At least it was a mod that said it. :P Can you be a bigger douche... It is not a matter of right or wrong, it is not a matter of USA or Canada but always an ignorant American has to make a US/Canada remark.... I never said You are wrong, but Your not perfect and Nothing else is either including RPM's... Link to comment Share on other sites More sharing options...
jeepcoma Posted December 10, 2011 Share Posted December 10, 2011 It is not a matter of right or wrong But it IS. Quite simply, you are wrong. Perfection has nothing to do with it, it's physics. Do you have personal experience to the contrary? Most days 65mph in OD is 2000 RPM and other days it's not? You may notice that you push the pedal down and the torque converter will unlock and/or it will downshift, but that's not what we're talking about. Link to comment Share on other sites More sharing options...
87Warrior Posted December 10, 2011 Share Posted December 10, 2011 I think the point Jeepman is trying to make is that there is no perfect/best gear selection for all driving conditions. And yes, we have the ability to choose gears and tires so that the motor will run the same RPM as stock for any given speed. I ask, do you want it to run like it was stock with the extra wind drag due to a lift, tire weight and winch bumper attached to your rig? I think there are two slightly different arguments going on at the same time in this thread. Link to comment Share on other sites More sharing options...
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