Eagle

I was going from memory, which ain't what it used to was.

Based on the math of tire specs, a 205/75-15 turns 760 revolutions per mile and a 235/75-15 turns 722 revolutions per mile. 38 divided by 760 gives us a 5% difference. Theoretically, then, at 70 on your speedometer the actual speed should be 73.5. But ... stock speedometers usually read slightly on the fast side, so 70 indicated probably wasn't really quite 70 on the factory tires, and that could easily account for another 1.5 MPH. Anyway, we're in the ballpark, and these examples show how it all works. My '88 XJ came with 205/75-15s, and a long time ago I calculated that the difference from those to 30x9.50s was 6 percent, and the difference from the 205s to 31x10.50s was 11 percent.

The only maintenance you would do inside the differential would be to clean out the old gear lube, make sure you get the cover sealed on well when you reinstall it (there's no gasket, it uses RTV), then fill it up to the plug with new gear lube. The factory spec for an '86 was (IIRC) 80W90, but if you want to be nice to the old gears you could go with synthetic 75W140.

Obviously a rare, hybrid prototype. I think you should buy it and restore it. It'll be the only one in captivity, and worth big bucks. Then somebody will convert it to the open cooling system and it'll instantly be worth $75 as scrap.

Since just removing the diff cover doesn't touch any moving parts, it's unlikely you could break anything -- and with an old truck, it's probably long overdue to change out the gear lube anyway. But, if you really don't want to remove the cover, you are left with the turn a wheel and watch the driveshaft method. First off -- do you have an open diff or do you have limited slip? This is important because the test is done differently with a limited slip. With a limited slip (if it's working), you have to jack up both rear wheels. When you turn one wheel, the opposite wheel should turn in the SAME direction. If so, mark the driveshaft with a white or yellow chalk or crayon on the side, where you can see the mark when you're sitting or kneeling at the tire. Get the mark where you can see it, then mark a spot on the tire either at the very top or the very bottom. Now, slowly rotate the tire exactly one revolution, and count how many turns the driveshaft makes. That'll be your ratio. If you have an open differential (no limited slip or locker), you only jack up ONE rear wheel. Keep the other wheel on the ground so it can't turn. Mark the tire and the driveshaft. With an open diff, you rotate the tire through TWO revolutions and count the number of driveshaft revolutions. Either way, be sure to mark the tire so you know you are turning it exactly one or two revolutions. If that's not exact, your count on the driveshaft (which is going to be an approximation at best) will have to be off.

You don't have to count anything. Remove the rear diff cover. The large gear with its edge facing you is the ring gear. Rotate it slowly -- there will be a pair of numbers stamped into the flat surface of the edge. One number is the number of teeth on the ring gear, the small number is the number of teeth on the pinion gear. Divide the larger number by the smaller number and that IS your gear ratio. Here's an example: Looks like the numbers in this photo are 46 and 13, so 46 divided by 13 would be 3.5385 ... which rounds off to 3.54. And, just for the record, you very clearly DID say you don't believe us:

No, there are not many possible options it could be. If it's a 5-speed, it can ONLY be an AX-5 or a T-5, and the likelihood of a 1986 XJ or MJ having a T-5 transmission is very close to 0.00000000001. The T-5 was an alternate standard transmission when XJ production was first ramping up -- which was in late 1983 for the introductory 1984 model. By the time the 1986 model year came around, they had the supply stream pretty well normalized. For rear axle, again there are only two possible choices (unless a previous owner swapped it): A dana 35, or an AMC Model 20. The Model 20 is immediately obvious, because the cover is perfectly round, and it's HUGE. I posted a photo of one above. If your differential doesn't have a perfectly round cover ... it's a Dana 35. You can't have a Dana 30 in the rear, because it's only used for the front axle. Seriously -- look at post #51 -- you can't tell the difference between an oval and shape that's nearly square? It's about at the end of the day ... where are those pictures you were going to take today?

They did. Are you certain it's a 1991 model? Is there a computer mounted to the left inner fender, next to the air intake box?

The two times I used my wife's Tom Tom, both times there were errors. On a trip to New Jersey, it told us to turn left into the motel when we were still five miles away, and not even in the right town yet. On a trip to Massachusetts to pick up some MJ parts, the turn instructions didn't match the street names. I finally got where I was going by turning left where it said to turn right, driving a couple of blocks, and then letting it recompute the route. Not ideal. Any Magellan users out there? Any other feedback on any of the three brands?

Tom Tom, Magellan, or Garmin? My daughter gave my wife a Tom Tom for Christmas a year and a half ago. I've used it twice, so I guess it could be said I'm "familiar" with it (sort of). No experience with either of the other two. I would use it occasionally, so I don't need top-of-the-line. I'm looking for more of an entry-level or lower mid-tier unit. If they're all about the same, I'll either go with another Tom Tom since I (sort of) know it already, or buy strictly based on price. Any recommendations to steer me off that course? What you y'all use and like -- and why?

Then how can you be so certain that all of us, who DO know something about these vehicles, are wrong when we tell you that your rear axle is almost certainly a Dana 35 and that your transmission is almost certainly an AX-5?

If your police officer friend will agree to check you with radar, be sure to get at least two readings, like at 25 and 50 or 30 and 60. You need to find out if the difference is a constant (like 3 MPH at all speeds) or proportional (like 3 MPH at 30 but 6 MPH at 60).

??? What are you talking about? I dunno what you have, but my wife's Tom Tom GPS gives a constant true speed read-out. My friend down the road didn't have a working speedo in his MJ for a couple of years, and he used his GPS as a speedometer. It gives a constant read-out -- all you have to do is drive. Look at the speedo, look at the GPS, and see how far apart they read.

Why not? The Dana 44 was not an option in 1986. The heavy-duty axle in 1986 was the AMC Model 20. The diff cover on the AMC 20 is perfectly round, and it's HUGE. I seriously doubt that you have an AMC 20 axle. http://www.drivetrainshop.com/AMC_Model_20_s/115.htm Unless you have a very heavy foot, the Dana 35 isn't as bad as some people claim. My '88 XJ has 287,000 miles on the original Dana 35 rear axle. It has been wheeled at Paragon, on various trails in the Mohawk Trail region of western Massachusetts (including Old Florida Road), on power line trails in Rhode Island, and on various fire roads in northern New Mexico. My '88 MJ also has a Dana 35, with the TrueTrac limited slip. It has also been all over Paragon, the Pine Barrens in New Jersey, and the power line trail in Rhode Island. If you don't hammer it, it'll be nice to you if you're nice to it.

Probably. It's upside down because it's not right side up. Look closely -- each rib has a bend that, on that vehicle, is about 1/3 down from the top. Flip the grille over so it's 1/3 up from the bottom and the bend aligns with the line of the tops of the parking lights -- and the bend in the side marker lenses.

My Verizon contract finally rolled over so I used the renewal to upgrade from my four (or six?) year old flip phone to a Samsung Galaxy SIII. I figured my wife and daughter both have smart phones, it was inappropriate for the guy who pays the bill to be the only one still in the telecommunications dark ages. So now I have it (got it today) -- what do I do with it? What are some must-have apps for the 'Droid platform? Don't worry about music and entartainment apps -- I don't even listen to the radio in the Jeep when driving. I'm thinking more along the lines of what apps would be useful oriented toward driving, Jeeping, and shooting. Also, while in the Verizon shop, I notived they are selling a Delphi vehicle information doohickey that plugs into an OBD-II port and broadcasts vehicle data that can be read by a smart phone or tablet. Has anyone used that? Is it worth the $249 they want for it? Would it tell me anything my Harbor Freight Tools code scanner won't tell me?

According to my research, a 205/75-15 turns approximately 760 revolutions per mile. A 225/75-15 turns approximately 730 revolutions per mile. That's a difference of 4 percent. That means your speedometer would be off by 4 MPH at 100 MPH, or 2 MPH at 50 and 1 MPH at 25. Changing tire size NEVER makes a speedometer read 'X' number of MPH fast or slow at all speeds. It's a proportional difference. Have you checked your actual speed against the indicated speed using a GPS? Do it at different speeds, like 25, 50 and 75 MPH and see what the error is for each speed.

The numbers aren't on the carrier. They're stamped into the outer edge of the ring gear.

ok, what if ya don't know if ya have the right drive gear? Redwolf Then you take out your drive gear and count the teeth, then look it up in a FSM or an on-line chart. However, to look it up you need to know both the tire size and the gear ratio, and IIRC you aren't certain about your gear ratio. So basically the question would be: was your speedometer reasonably accurate before you changed to the 31" tires? If so, you can back into the speedo gear charts that way. The charts tell you which gear to use for an axle and tire combination. If you count the teeth on your speedo drive gear, then you know what and you know the tire size, so you go into the chart and find the block with that number of teeth with your stock tire size, and that should tell you what axle ratio you have. Then you follow down (or across, depending on how the chart is laid out) to the new tire size, and that tells you how many teeth you need on the new drive gear. As a rule of thumb, going from stock 225/75-15s up to 31x10.50s is a jump of 7 percent. If the original tires on your truck from the factory (which is what the speedo gear will be calibrated for) were 215/75s, the jump will be about 9 percent. All speedo gears cover a range of two or three tire sizes, which is why the speedometer typically isn't exactly accurate even from the factory. So you can get close just by counting the number of teeth on your drive gear and choosing a new gear that has +/- 8 percent fewer teeth. Here's a link to a chart. 225/75 tires are 28.3" in diameter, 215/75s are 27.7" in diameter, so call it 28" for your stock tires. 31x10.50s typically run a bit less than a true 31" but call it 31". http://www.quadratec.com/jeep_knowledgebase/article-46.htm So, for argument, lets say you have 3.55 gears. On the chart, 3.55 gears with 28" tires should have a 35-tooth drive gear. 35 less 8 percent is 32.2, so my backwards math method would say you need a 32-tooth gear for 31" tires. Looking at the chart, if we find 31" tires and 3.55 gears, it tells us you need ... 32 teeth.

The 2.8L automatics came with 3.73 gears. The 3.54s could easily have been from the factory behind a 5-speed. AMC typically went one step "taller" in gearing behind the 5-speeds than they did with the automatics.

I wouldn't trust the bumper hitch even for a "light" camper trailer.

There is no way to do it yourself other than swapping speedo gears. That usually gets you to within about a +/- 5% accuracy, which translates to being off by about 2-1/2 MPH at 50 MPH. To get it closer than that, speedometer calibration shops play with the return spring on the needle. But they would insist on having the correct speedo drive gear for the tire size before they even start tinkering with the speedo head.

3.55 (or 3.54) was the factory ratio for a 4-banger with the AX-4 transmission.

^^^^ Agreed. Also, the turbine wheels originally had center caps. The wheel in your photo is missing the center cap. The turbines are 15x7". The spare is 15x6".

I once had a professor who used to say that a lot: "It's exactly the same except it's different." ???