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Aftermarket gearing and highway speeds?


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Hello everyone, I just purchased my mj not very long ago. Got it as a "running project" from a gentleman who seemed to really want/need it gone. My question is when I try to drive on a road with a higher speed (55+ mph) the jeep seems to run out of gearing in 5th at 50mph. I can hear it really whining out, plus the idiot shift light comes on at about 44-45 mph. Also, I bought the jeep with the front axle out of it, laying in the bed because the yoke has a ear snapped off. So I've been driving it for the last few months in 2wd, and haven't tried putting the axle in, out of fear the the front axles gearing is different then the rear.

What do you guys think? What would be your next step if you were me?Any input would be wonderful.

1988 jeep mj

5speed

4.0

31inch all terrains

190xxx miles

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I would guess that if your trucks current running gear is stock, your probably running 3.07 gearing. Have you looked for a tag on either differential pumpkin? If its there, it has the ratio on it. If it is 3.07's then your rpm is probably around 3k at 55=60. As far as what I would do? I would source a cluster with a tachometer in it and full gauges from an 87 or 88-90 Comanche or Cherokee. Then you would know what rpm your actually running. Also, what I've found is that shift light is useless if you have over size tires, or your going up a hill, or anything really. And finally, if you can't find the tag on any of the differentials, then pull off the cover and do some googling to find the ratio. 

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To put it into perspective, I have an '87 with factory 3.07 and I must be going at least 60 before I can put it in 5th gear.  Factory gearing was way too high.  In my youth, I always thought that poor 4.0 was a dog until I put a tach in.  Then I saw what was really going on.   Also, I Ignore that shift light.  It was never accurate, even when new. I treat it as a piece of nostalgia.  

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A third on the tach. I pulled the bulb on my shift light out. The other thing to check is if you have the right speedo gear in your truck. I've got a stock 88 4.0 5 speed 2wd with 3.07s and 235/75/15s and at 70mph I run about 2k. With 31s you should run slightly less. Somewhere around here there is a spreadsheet with what speedogear you need based on tire size and gear ratio

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Sounds to me like everyone's looking at this the wrong way... "run out of gearing" and "really whining out" seem to me like he's got some fairly low gears installed. You're saying you hit absolute top speed at ~50mph?

 

 

Wouldn't surprise me if it had a lift and oversize tires on the stock axles, the guy took it offroad and the yoke broke due to added stress, the guy gave up on project, pulled the lift and tires off and left the gears in there. In that case, likely the only way to know what gears are in there is to pop the covers off and see. If you had a tach, you could compare your speed/rpm/tire size against this chart: http://comancheclub.com/topic/27178-tire-size-gear-ratio-rpm/

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Thanks for the great ideas guys. As an update, I got paid this morning so I ran to autozone and got a small aftermarket tach (and a bucket of bondo :)) also, I compared my speed to a good gps and one of those useless police mph signs, and found that my speedo is spot on + or - 2 mph. Also, I crawled up under her and never found a tag. I did find a rivet where it seems one should be. And yes, my top speed is around 55-60. I can shift easily into 5th at 30mph.

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a quick and dirty way to estimate the ratio is to jack up one tire, put the trans or tcase in neutral, and then turn the lifted tire around 2 full revolutions while counting the number of times the driveshaft spins.  if the shaft spins around 3 and a half times, that would indicate 3.55 gears.  a bit over 4 times would be 4.10 gears, etc.  if the axle has a posi or locker then you'd have to jack up both tires and rotate one of them around once (which assumes the other tire is also rotating around the same amount).

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What do you guys think? What would be your next step if you were me?Any input would be wonderful.

1988 jeep mj

5speed

4.0

31inch all terrains

190xxx miles

 

My next step would be to figure out what ratio your rear axle is. The most accurate way is to remove the cover and count gear teeth on the ring gear and the pinion. Divide the tooth count from the pinion into the ring gear count, and that's your ratio.

 

You don't have the new tachometer hooked up?

 

I also like the idea that it's in low range. With 3.07 gears and 31" tires, 55 MPH would be 1914 RPM in 4th gear and 1435 RPM in 5th gear. That's barely above idle.

 

BUT ... multiply that by 2.72 (the low range ratio) and now you get 5206 RPM in fourth (that really is maxed out -- we have a 5500 RPM redline), and 3903 RPM in 5th.

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2 simple tests......shift it into 2wd.......drive till you are doing 20 mph.....take a tach reading

 

Shift into 4WL....drive to 20 mph....take a tach reading......is it different?

 

If not your stuck in low.

 

Since it is a project....do some maintenance and answer your question at the same time......go get some diff lube...jack the rear up .....put it on stands....take off the diff cover ......rotate the diff until you can see the numbers stamped on the large gear.........post the numbers. (you divide the bigger by the smaller to get the ratio).....put the cover back on and lube up with the new gear lube.....now you know your actual ratio and that you have good lube in the rear end.  :thumbsup:

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Okay so an update, I installed the new tach and found that at 55mph I was at about 2500 to 2600 rpms. With my tire size, this should put me somewhere in the 4.27 gearing range if I'm not mistaken. So i suppose I was wrong when I said that I felt like it was whining out at 55-60.

I think that the only gear set ratios for your axles would be 4.10 & 4.56

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Okay so an update, I installed the new tach and found that at 55mph I was at about 2500 to 2600 rpms. With my tire size, this should put me somewhere in the 4.27 gearing range if I'm not mistaken. So i suppose I was wrong when I said that I felt like it was whining out at 55-60.

2500 RPMs is not burning up the engine, since the redline is about twice that ... but it's not right, and IMHO it's excessive for 5th gear with 31" tires. If that's in 5th gear, it does NOT work out to 4.27 gearing. In 5th with 31" tires, 4.88 gears only produce 2281 RPM ... and 4.88 is the deepest gearset available for an XJ or MJ with a Dana 30 front axle.

 

Was your speedometer recalibrated when you put on the 31" tires? If not, your actual road speed is between 10 and 13 percent faster than what the speedometer reads -- meaning that when you see 55 you are actually traveling at more than 60 MPH (maybe close to 65).

 

You really need to open up one of the differentials and read the numbers stamped on the edge of the ring gear.

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60 MPH in 5th gear with 31" tires with 3.07's in low range = 4310 RPM.

 

60 MPH in 5th gear with 31" tires with 4.88's in high range = 2520 RPM.

 

Actual RPM will be higher because I used 30.5 which is the nominal size for a 31" tire. With the weight of the vehicle reducing that a little would raise RPM.

 

I don't think you're in low range. I do think your gear ratio is close to 4.88.

Only way to know is to pop that rear cover off and see what the numbers on the ring gear say.

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