Philistine Posted December 20, 2009 Share Posted December 20, 2009 What was the stock tire size for an '86 MJ? I'm asking because I'm trying to figure out where the power band is in the RPMs. The PO did a lift and put on some 35's without regearing the axles. I'm thinking going with 4.56s or 4.88s as I want more torque for rock crawling. But I still need to drive it to the trails so I don't want to overtax the engine on the highway. Link to comment Share on other sites More sharing options...
Pete M Posted December 20, 2009 Share Posted December 20, 2009 all depended on the package, but they were small. 225s were the biggest available and they went all the way down to 195s. You also might want to upgrade some other things while you're at it, like front axle U-joints and that spindly front driveshaft if it's still there. Link to comment Share on other sites More sharing options...
mvusse Posted December 20, 2009 Share Posted December 20, 2009 I believe you have 4.10 axles already, and stock tires I believe were most commonly 205/75R15, which is a 27" tire. 225/75R15 usually came with the metric tonne package. You'd need ~5.30 gears just to get back to the stock drive ratio. I believe the highest gears available for stock axles are 4.88. 4.88 gears with 32" tires would be same as 4.10 with stock tires. Only way to get good ratio with 35" tires and a 4 cylinder is using different axles with higher gear ratios available, or putting an underdrive unit between the transmission and transfer case. Neither choice is inexpensive, and both are a lot of work. Best bang for the buck is to swap in a 4.0, but even that is quite a bit of work in an 86. A 4.0 with 4.10 gears pushing 35s would be worlds better than the 2.5, and with 4.56 would be almost stock final drive ratio, and Eagle would say 4.88 is perfect. Edit: to add the AMC20 might mean you have the metric tonne package. If your stock tires were 225, that still only makes a 4% difference. 2nd edit: to add that while 5.38 is available for a low pinion D30, 4.88 is as high as you can go on a high pinion. 4.88 is also the highest available for an AMC 20. Changed original post to reflect this. Link to comment Share on other sites More sharing options...
Philistine Posted December 20, 2009 Author Share Posted December 20, 2009 all depended on the package, but they were small. 225s were the biggest available and they went all the way down to 195s. Its the X package if that helps. I'm trying to figure out the original powerband. You also might want to upgrade some other things while you're at it, like front axle U-joints and that spindly front driveshaft if it's still there. Those are on the list too. While we are on the subject I recall a bit about better axle ujoints but I'm unsure which vehicle years they were found on. Link to comment Share on other sites More sharing options...
mvusse Posted December 20, 2009 Share Posted December 20, 2009 The better u joints were found on 89 or newer Cherokees with ABS, or any Cherokee 95 or newer I believe. Tape measure will tell you. The caps on the weak ones measure 1 1/16" diameter, while the stronger ones measure 1 3/16". Better front drive shaft on anything 87 or newer. Link to comment Share on other sites More sharing options...
Philistine Posted December 20, 2009 Author Share Posted December 20, 2009 I believe you have 4.10 axles already Edit: to add the AMC20 might mean you have the metric tonne package. If your stock tires were 225, that still only makes a 4% difference. While I haven't opened the diffs yet I've taken my RPMs at X speed, tire size and transmission and transfer case specs and used the online calculators to figure out I have 3.07 gears. Also even though its has an AMC20 it is not the metric ton package according to the VIN. As for 4.56 gears, using this calculator (http://www.grimmjeeper.com/gears.html) would put me at about 2400 RPMs at 65 MPH with a low range of 46:1. 4.88s would be 2600 RPMs at 65 MPH with a low range of 50:1. I'm just not sure how far I would be out of stock at 2400-2500 RPMs at 65 MPH. If the stock tire size was 27" then stock RPMs at 65 MPH would be just over 2000. Link to comment Share on other sites More sharing options...
Philistine Posted December 20, 2009 Author Share Posted December 20, 2009 The better u joints were found on 89 or newer Cherokees with ABS, or any Cherokee 95 or newer I believe. Tape measure will tell you. The caps on the weak ones measure 1 1/16" diameter, while the stronger ones measure 1 3/16". Better front drive shaft on anything 87 or newer. Thanks! Link to comment Share on other sites More sharing options...
Pete M Posted December 20, 2009 Share Posted December 20, 2009 Go 4.88s and don't look back. :D Also, you may want to buy a junkyard late model Dana 30 and use that for the new gears. Then you can sell off your current axle. Link to comment Share on other sites More sharing options...
mvusse Posted December 20, 2009 Share Posted December 20, 2009 I have never heard of 3.07 gears behind a 2.5 that I can remember. 3.07 comes with a 5 speed 4.0, but the 4.0 was not available in 86, and I believe the AMC 20 disappeared in 87. Either way, you should have 4.10. If you have 3.07 something weird is going on. Link to comment Share on other sites More sharing options...
Philistine Posted December 20, 2009 Author Share Posted December 20, 2009 Go 4.88s and don't look back. :D Also, you may want to buy a junkyard late model Dana 30 and use that for the new gears. Then you can sell off your current axle. I know someone who is replacing their HP D30 with 4.88 gears in the near future and he is willing to sell me it when he does the swap. I am leaning more towards the 4.88s for the better crawl ratio as the only highway driving it will see is to and from the trails around here. Link to comment Share on other sites More sharing options...
Philistine Posted December 20, 2009 Author Share Posted December 20, 2009 If you have 3.07 something weird is going on. Well it is a Jeep. Like I said I haven't opened the diffs yet but taking the RPM at X speed, tire size and transmission and transfer specs the online calculators gave me a gear ratio of 3.07. I actually got 3.11 but given I couldn't get exact RPMs and the tire wear, 3.07 makes sense. Link to comment Share on other sites More sharing options...
Eagle Posted December 20, 2009 Share Posted December 20, 2009 While I haven't opened the diffs yet I've taken my RPMs at X speed, tire size and transmission and transfer case specs and used the online calculators to figure out I have 3.07 gears. I VERY much doubt that you have 3.07 gears in a 1986 4-cylinder. They should be 4.10s. You have asked multiple times about the stock "power band." "Power band" means the range in which the engine produces the best usable power. The 1986 throttle body injected version produced 117 horsepower at 5,000 RPM and 135 foot-pounds of torque at 3,500 RPM. That means that your power band is from roughly 3,000 to 5,000 RPM. Is that what you were asking? Link to comment Share on other sites More sharing options...
Pete M Posted December 20, 2009 Share Posted December 20, 2009 quickest way to determine your gears is to jack up one of the rear tires, put the trans in neutral, and spin one tire around twice while counting the number of time the driveshaft rotates. If you have a posi rearend, then you'll need to jack up nboth rear tires and rootate them around only once. Link to comment Share on other sites More sharing options...
Philistine Posted December 20, 2009 Author Share Posted December 20, 2009 You have asked multiple times about the stock "power band." "Power band" means the range in which the engine produces the best usable power. The 1986 throttle body injected version produced 117 horsepower at 5,000 RPM and 135 foot-pounds of torque at 3,500 RPM. That means that your power band is from roughly 3,000 to 5,000 RPM. Is that what you were asking? Hmm... The engine is struggling at 3000 RPM (starts shaking something awful). The PO did swap the instrument cluster. I've read before on this forum that swapping an instrument cluster from a 4.0 into a 2.5 involves making a change to the tachometer. Could my RPM be reading lower if the instrument cluster in it now is from a 4.0? I've never pushed it above 3000 RPM as that seriously feels like the limit of the engine. Link to comment Share on other sites More sharing options...
Philistine Posted December 20, 2009 Author Share Posted December 20, 2009 quickest way to determine your gears is to jack up one of the rear tires, put the trans in neutral, and spin one tire around twice while counting the number of time the driveshaft rotates. If you have a posi rearend, then you'll need to jack up nboth rear tires and rootate them around only once. I'll have to try that. How would I know if its a posi rear end? Link to comment Share on other sites More sharing options...
1986Comanche Posted December 20, 2009 Share Posted December 20, 2009 I'll have to try that. How would I know if its a posi rear end? Jack up the rear of the truck and turn one of the tires, if the tire on the other side turns in the same direction then you have a posi rear end. If it turns the opposite direction you have an open differential. I hope this info helps. Link to comment Share on other sites More sharing options...
1986Comanche Posted December 20, 2009 Share Posted December 20, 2009 What was the stock tire size for an '86 MJ There were 4 stock tire sizes available on a 1986 Comanche and they were P195/75R15, P205/75R15, P215/75R15 or P225/75R15. I hope this info helps. Link to comment Share on other sites More sharing options...
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