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March 2011 Your Jeep

Posted in How To on March 1, 2011
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Tow Rig Tech
I have a Ford Super Duty F-250 that came with 3.73 gears about 31-inch tires (265/75R16). I put 33s on it, and of course, it cut the power. I am now putting 35s on it and since I tow my Jeep and carry a cabover camper, I know it will really cut the horses. I know that I need to either put 4.10 or 4.30 gears in it to keep it in the same RPM range in which the diesel was designed to operate. I also know that somewhere out there is a formula to determine the change in RPM with the change in tire size. Do you know where I can find that? Am I on the right track with the 4.10 or 4.30 solution?
Ron Dow
Via email

The formula you're thinking of is: 336 x (axle ratio) x (overdrive ratio) x (mph)/(actual tire diameter). Don't forget that when determining your tire diameter, you're only dealing with the static load radius of the tire, not the overall tire height. To determine the static load radius, take a measurement from the middle of the vehicle hub to the ground. That's your static load radius. Then double that number. That'll be the tire diameter measurement that you use when computing your formulas.

In your case right now with the 33s, I think the Ford four- and five-speed auto transmissions have a 29-percent Overdrive, so your formula would work out like this: 336 x 3.73 x 0.71 x 75mph/31.5 inches (actual static load radius of your tires x2) = 2,118rpm.

With your stock tires (roughly 30 inches), you were at about 2,224rpm at 75mph.

Upping the tire size to 35s (roughly 33 inches true diameter) would drop that down to 2,022rpm.

Going to 4.10s with the 35s would put you at 2,222rpm, and 4.30s would have you spinning at 2,331rpm.

If you're looking to replicate the performance of the factory tire size, then the 4.30s would be your best bet.

I will say this, however. It's been my experience that most diesel engines want to lug and tug against the gear. I wouldn't normally recommend skipping the gearing and going to power-adders on a regular Jeep, but in your case it may prove beneficial to take the money you would normally spend on regearing and put it towards some air, fueling, and programming modifications to your Super Duty to offset your power loss. I run a Dodge Megacab with the 6.7L Cummins. I've got 285/75R17 (roughly 34-inch diameter) tires and 3.73 gears and it loves to hunker down in the gear on long grades and build boost. It seems to be much happier than some of the 4.10 or 4.30-geared dualies I've trailered Jeeps with. I run a cold-air intake and no other power mods. I realize my truck is more powerful in stock trim (350hp/650lb-ft) than your Powerstroke, but with some very simple and safe modifications and perhaps some tranny cooling mods you could be equally happy with your factory 3.73 gears.

Stock Swapping
Hello, I have a quick question for you. I was reading a reader's response to an article concerning a '95 YJ without the Dana 30 CAD axle. I was curious about this. I have an '87 YJ and I would like to eventually swap out the stock Dana 30 for a non-CAD axle. I was wondering if this axle in question is a high-pinion or a low-pinion.
Diego Serna
Albuquerque, NM

All YJ axles are a high-pinion, CAD design. If you can weld, an excellent Dana 30 donor is the Jeep Cherokee XJ. The '95-up models will have the larger Dana 44-sized 760X U-joints as opposed to the smaller 260X-sized earlier U-joints. Pretty much all XJs had a high-pinion Dana 30 with no CAD. Some very early XJs had CAD and most '01 models used low-pinion TJ axles when Chrysler's supply of high-pinion XJ axles ran dry. Cut off the coil buckets and control arm mounts and weld on the YJ spring perches. The XJ axle is the same width as the YJ.

Or, you can more easily buy a one-piece passenger-side axleshaft and the correct seal from Superior Axle & Gear ( You'll still have the CAD housing, but without the sometimes troublesome CAD 2-piece long shaft.

Rambler Five-Speed
Will an AX-15 from a Wrangler bolt to an AMC 232? Are there issues with the clutch?
Rob Nichols
Via email

Yes and no. The early 232 engines from about mid-'68 and earlier have a bellhousing bolt pattern that differs from the standard AMC Jeep V-8/232/258/4.0L. If it's a later engine with the same bolt pattern as the modern I-6 engines (mid-'68 or so and up) then it should accept the AX-15 with no trouble. Use the 232 flywheel and pressure plate with an AX-15 clutch disc. I'd recommend running the '94-up external slave cylinder bellhousing rather than the older internal release throwout bearing model. Contact Advance Adapters ( for the correct pilot bushing for your transmission.

Sand Crawler
I own an '06 Wrangler Unlimited 4.0L with an automatic transmission and 32x12.50R15 tires. We spend a lot of time driving and fishing on the beach at Fort Fisher, North Carolina. The sand is sometimes heavy and sometimes like powder. My problem is my dealer tells me I should be running the T-case in High and my local mechanic tells me it should be in Low. I've had two Wranglers before-a '90 and a '93. Both were five-speed manual transmissions and I ran both with the T-case in High and they did fine. My current Jeep does not do as well in High, but it gets the job done. Although I have noticed that my engine temp climbs to 230 degrees on really hot days. I air the tires down to 20-22 psi. I just had the transmission fluid and filter changed and it was really dark, but didn't have any metal in it. If you would give me your opinion and any advice that you think might help, it would be greatly appreciated.
Donald Dawson
Wilmington, NC

If you don't have a long pavement drive after the sand to get to air, I'd begin by lowering the tire pressure to 11-14psi. That'll increase the contact patch and improve traction, which can reduce heat buildup in the auto tranny.

As for Low or High range, it's a tricky decision. On the one hand, hitting the sand in High range will keep the transmission in the taller (First or Second) gears, while Low range may allow the transmission to operate in the sand in Second or Third gear. You just don't want to hit Overdrive. I'd suggest trying Low range and locking out Overdrive. This will prevent excessive heat buildup. It will also reduce the strain on the automatic transmission thanks to the mechanical advantage of the 2.72:1 Low range planetary in the T-case.

Give the lower tire pressure and Low range a try. Worst thing is it won't work as well for you and you just go back to how you've been driving it.

Rover on Over
Am I wasting my time or do you think I can repair my T90 and make it last? It's in a flattie that is now running a healthy 5.0L alloy Rover V-8 making 250-300-ish horsepower. I'm running 36-inch IROKs, a Ford 9-inch rear, Dana 44 front with 5.38 gears, and the factory Spicer 18 T-case. A bigger gearbox will mean more weight, which is what I'm trying to avoid. I thrash the Jeep more often than not, and as you know, breaking down on the trail sucks balls. So any advice would be awesome.
Craig McGinnity
New Zealand

If you were running that engine with smaller 31-32-inch tires or running the larger 36s with less engine, I'd say rebuild the tranny and be careful. But with 36s and a healthy V-8, I just don't see it lasting unless all you're doing is driving it down the road going easy on the throttle. And you've already admitted that's not your driving style.

The Rover V-8 is based off the older Olds/Buick 215 aluminum block and should accept a Buick bellhousing. They're not super-common, but you could use the GM Buick bellhousing to mate an SM420 behind your Rover engine. Then you could buy a Novak SM420 to Spicer 18 adapter. I think the Spicer will survive just fine behind the Rover V-8 (they're not super-torquey) as long as it's rebuilt or in good shape. It's once the bearings and/or intermediate shaft start wearing that the Spicer 18 suffers tooth damage. If the gears cock at all they'll chip or break. Worn bearings allow 'em to cock. Or, old or inferior intermediate shafts will allow accelerated wear because all the power output is sent through the intermediate gear instead of straight through as on a Dana 20 or Dana 300. Novak makes one of the best rebuild kits for a Spicer 18 out there (, and the company has by far the best intermediate shaft.

FSJ Fooling
Hey, all-knowing Jeep dude, I just inherited a '70 J3000 1-ton truck with the 350 Dauntless engine, Dana 60s, and so on. It has a beautiful Pioneer town, never-garaged patina paint job, but mechanically it is top notch. My brother-in-law was an auto mechanic and took care of it. Any guesses as to the value or rarity of such a money pit? This is my second Jeep, so as you can see, my addiction is getting worse, not better. If this truck is as hip as some have said, I will bring it to a local event this year. Any help with value or advice about weak links and so on would be great. Also,in Jp's Hot Dog project, the '73 truck from a few years back was running an aftermarket front bumper. Who made that?
Via email

It's hard to assess a value without a photo. I bought a '68 J2000 for $800 and it had very little rust or rot and was in pretty good mechanical condition.

My ballpark guess: maybe $1,000 minimum? Maybe a bit more without rust or rot. Even more if it has the "Camper Special" package with the T-18 four-speed in place of the more-common TH400 three-speed auto. More if it's got A/C, and so on. Price pretty much goes up from there. The Buick 350 is a good engine and is sought after in some J-truck circles. The downside is the axles. I'd also check the axles a bit more closely. Chances are that the front is a closed-knuckle, drum brake Dana 44. Depending on the GVW, the rear could be a Spicer 53 or Dana 60. Only the dual-rear-wheel models got the Dana 70 rear.

The Hot Dog '73 J2000 project Jeep ran a BJ's Off Road bumper (

Locker Loving
I read about the ECTED that you put in your FSJ project and was wondering how it is performing for you. I am thinking about putting one in my CJ. Would you still recommend this locker?
Levi McLsaac
Via email

John Cappa replies:
Yep, I do. If you are looking for a selectable locker, I think it's a really good option. In my opinion, pretty much every selectable locker has some sort of disadvantage. The ARBs require a pump, the whole system is expensive, and it can leak air, but the mechanicals are nearly bombproof. The OX has a heavy cable and bulky mechanical shifter that needs to be routed and mounted, but it too is really strong. The ECTED has wearable parts that cannot be replaced at home and I think it's likely on par with the OX in terms of strength.

If you want a selectable locker, you have to simply take the good with the bad. I really love the limited-slip feature of the ECTED and the ability to lock it on demand. There is no waiting, no flashing lights, or anything. It locks and unlocks quickly with the flip of a switch. The limited-slip is the most aggressive that I have ever used. It works really well and in some cases I didn't even need the locker. However, recently I broke several teeth off of the ring gear of my Dana 44 and the bits got wedged inside the ECTED, causing it to not unlock anymore. It's really no fault of the locker that the ring gear failed before the locker did, so I'd call it a success story.

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