Eagle

Somewhere around 2250 to 2500 is a sweet spot for normal driving, but if you set up your gears to run those RPMs at 75 MPH then at anything under 60 MPH 5th gear will be useless, and 4th gear will be almost useless. Also don't forget that drag increases exponentially with speed, so to push the same "brick on wheels" down the road at 75 MPH requires a whole bunch more horsepower than pushing the same brick on wheels at 50 MPH. 2250 is the torque peak, but it isn't the horsepower peak. What you're shooting for would be almost exactly the same as stock 3.08 gears and stock tires. That setup runs a bit over 2100 RPM at 75 MPH, and 5th gear is totally useless under 60. Not only that, I have run the same route, one way using 5th gear and the other way using only 4th, and I got as good or better gas mileage NOT using 5th gear. The 3.08 ratio was an idiotic choice by AMC/Jeep, but they did it because the fuel mileage and emissions testing was run indoors on a dynomometer, so there was no aerodynamic drag to be concerned with. In other words, they weren't realistic numbers, and the gear ratio was chosen to beat the test, not because it was in any way a good choice for real world driving. So what you would be doing trying to run 2250 RPM at 75 MPH is to duplicate the stock 5-speed overall drive ration, while struggling to spin much larger and heavier tires. Keep in mind that the 4.0L engine (and the 2.5L) is derived from AMCs old I-6 that was first introduced all the way back in 1964. Back then it was carbureted, of course, but the basic block and engine was the same. And in those days we didn't have overdrive, a manual transmission was "three on the tree." Top gear was third, with a 1:1 ratio in the transmission and 3.08 gears in the axle. In my 1966 Rambler American, that worked out to exactly 2500 RPM at 60 MPH, and 3000 RPM at 72 MPH. They ran forever like that. Mine got 28 MPG highway and mid-20s around town. I was in the Army then and I traded mine in for a new Javelin with my combat pay. My brother had a 1970 Gremlin with the same drive train that went something like 350,000 miles. And he autocrossed it on the weekends. If you set yours up to cruise at 3000 RPM at 75 MPH, you won't be "burning up" the engine, and you'll have a much happier rig.

Sure looks like eaton owns detroit http://www.eaton.com/Eaton/ProductsServices/Automotive/AutomotiveAftermarket/Differentials/PCT_221432 I never knew that. I have a TrueTrac in my '88 MJ. I like it ... a lot.

The disadvantage to 4-door XJ seats is that they don't fold forward. MJ seats, and 2-door XJ seats, do fold forward. You can always swap the upholstery from the XJ seats onto the MJ frames -- you'll just have to cut a small slit for the lever that releases the backs to fold forward. Is that worth $150? I dunno -- that's a very subjective call, and while it might be a good deal for one person, it might not be a good deal for someone else.

In terms of revolutions per mile (which is the only important criterion when matching gears to tire size), the 265/75-16s are identical to 33x12.50-15s (637 revolutions per mile vs. 636 revolutions per mile). Using that as the base, my speed to RPM chart (which is corrected for actual revolutions per mile, not theoretical tire diameter that doesn't accound for sidewall squat), I get the following (in 5th gear): 4.10 gears ==> 2282 RPM at 70 MPH (my chart doesn't go up to 75 MPH) 4.56 gears ==> 2538 RPM at 70 MPH 4.88 gears ==> 2716 RPM at 70 MPH Crunching the numbers, to convert that to 75 MPH we get: 4.10 gears ==> 2445 RPM at 75 MPH 4.56 gears ==> 2719 RPM at 75 MPH 4.88 gears ==> 2910 RPM at 75 MPH You should be changing to 4.88 gears. 75 MPH cruise at 2900 RPM is NOT pushing the engine too fast. Conversely, 4.10 gears isn't nearly enough gear for those tires. Remember, a "31x10.50" tire is NOT 31 inches in diameter -- they are typically between 30.3 and 30.5" overall diameter. So if you're thinking that your chosen tires are close to 31s ... you're wrong. If you're not comfortable with 4.88s, go with at least 4.56 gears. Putting this is perspective, a stock XJ with automatic and 225/75-15s turns 2261 RPM at 70 MPH, so if you go with 4.10s you're getting exactly the same final drive ratio as a stock vehicle ... but you're trying to spin much larger and heavier tires.

The TrueTrac is also not made by Eaton. It's made by the same folks who make the Detroit Locker.

Somebody did that. We had photos of it on here a couple or three years ago.

And reduced backpressure in an exhaust = loss of torque at low and mid-range RPMs. Since I never operate a 4.0 liter higher than 2500 RPM (and rarely even that high) I'll take the torque that I use every day over the 5000 RPM horsepower that I'll never use -- ever.

I believe factory spec is 750 RPM.

Nope.

No, I didn't. &%#&% zombie threads.

The AX-4 4-speed and the AX-5 5-speed are the same transmission. The 5-speed just has some extra internal parts to run the overdrive gearset. I'm not certain, but the long tailshaft on the 2WD 4-speed might be the same length as the transfer case on the 4WD, which would allow you to use the same rear driveshaft. Yes, you have to do something to make the ratio in the rear axle match the front. The 2.5 liter 5-speed had 4.10 gears. Your 2WD 4-speed has a 3.54 rear axle.

Those numbers don't even say what they represent. Is that supposed to be RPM at 60 MPH? If so, is that in top gear (overdrive) or 1:1 ratio (4th in the 5-speed, 3rd in the auto)? Assuming they represent RPM at 60 MPH, they are completely wrong. I compared them to the numbers I worked up in a spreadsheet, using actual tire manufacturers' revolutions-per-mile data, and the numbers in this chart simply don't match up with ANY RPMs in my spreadsheet. The above chart is somewhat useful as a general guide (bigger tires = less RPM, more gear = more RPM), but not helpful beyond generalities.

The tachometer was optional on the Pioneers in '88 and '89. Gauges rather than idiot lights were included with the Pioneer package, but the standard cluster had the large fuel gauge. I've owned two Pioneers (my '88 XJ and an '89 MJ) that had the tachometer. So don't skip over a vehicle just because it's a Pioneer. It could have the full cluster with tach.

Used AALs? Craigslist? There is NO way to know what they are or how much lift they'll produce unless you buy them and install them in your truck. Unless they are VERY cheap, I wouldn't bother.

Any taper flared not bubble flared) brake lines are double flared. DO NOT use a single flare for brakes, and DO NOT use compression couplings. Either do it right, and be safe ... or don't do it at all.

They aren't even satin -- on my red '88 they are matte black, along with the black accents on the hood. The font is something like Helvetica Condensed (or Helvetica Narrow), and slanted.

Have the alternator checked. I've had some brand new ones that were DOA.

There should also be a plaque pop riveted to the driver's door (the door, not the door post) that has the color codes. My original XJ is an '88 with the dark blue paint. I don't think it was called Baltic Blue, I actually think it was called Teal, and it isn't like a Navy Blue at all. I'll try to remember to check the paint codes when I'm out by the Jeeps this afternoon.

The XJ transfer case skid is the same one they used on the MJ.

You won't run 12-1/2" wide tires on stock rims without spacers. 31x10.50s on factory rims leave VERY little clearance between the sidewalls and the rear springs, and when the rear axle gets at all crossed up the inner sidewall at the top rubs the inner fender well. 12.50s will be an interference fit.

Back up and start at the beginning. First, how do you plan to use the truck? Second, what size tires do you plan to run? Third (and well after the other two questions have been answered), how much lift you you think you need? Ideally, you want the least amount of lift that will get you where you're going. You can run 31x10.50s in factory rims with NO lift, and that's probably enough to get you to most fishin' holes right there. A spring over conversion is going to lift the rear by around 5-1/2 inches, which means doing a lot to the front to lift it my a comparable amount.

Yes, he should. The rear height-sensing valve should not eliminate the rear brakes under normal operating conditions, but it does severely reduce the amount/pressure of brake fluid to the rear wheels when the truck isn't loaded. Opening a front bleeder to simulate a front circuit failure doesn't bleed the "normal" circuit, this is done to bleed the emergency bypass circuit to the rear (the second line -- which is NOT a "return" line, as implied in the old posts in this thread).

The mystery filter must be made by FRAM.

The TCU harness is separate. You have to get the TCU and the harness from the donor vehicle.

What clip are you talking about?