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Top 25 Most Asked Jeep Tech Questions

Christian Hazel
| Brand Manager, Four Wheeler
Posted April 1, 2010

The Questions That Haunt Us Monthly

Sometimes it gets a bit comical. We get dozens of emails every day asking nearly the same thing: "What gears should I run?"; " Why is my speedometer off?"; "What size tires can I fit?" You get the idea. Sometime we'll get two or three identical question back-to-back. It's almost like somebody out there is messing with us. So to up the sanity level just a tad, we compiled the top 25 tech questions we are answering over and over again.

Gear Grinding
QI just installed XYZ-sized tires and now my Jeep is a dog. What gear ratio should I run?

A There are a lot of variables, such as your engine type and size, manual or automatic transmission, overdrive or not, driving style, vehicle's primary use, and so on. For starters, you've got to keep in mind that a smaller four-cylinder engine will require more gearing to regain the performance and loss of drivability incurred with larger tires than a larger engine making more power. Also, an automatic transmission without a lockup torque converter will allow the engine to spin roughly 100-150 rpm higher than the same setup with a manual transmission or an automatic with a lockup converter due to slippage in the torque converter. With those caveats, take a look at the chart below for a few general gearing guidelines that have worked well for us in the past.

Fragged Five-Speed
Q My AX-5 is grinding and needs to be rebuilt or replaced. What do you suggest?

A The AX-5 found behind 2.5L four-cylinder engines is okay at best. However, we can't see ourselves ever rebuilding one, so it's hard to recommend our readers do the same. Instead, we'd look into replacing the little five-speed with an AX-15 from a 4.0L YJ, TJ, or XJ with an Advance Adapters conversion kit. You'll also need to replace the input gear on the NP231 T-case to convert from the AX-5's 21-spline output gear to the AX-15's 23-spline gear.

Another option if most of your driving involves hardcore wheeling is to swap in an SM420 using a 60-degree V-6 bellhousing from a GM Camaro, S-10, or Blazer. The GM bellhousing will allow the SM420 to bolt up to the 2.5L. I'd use the factory GM slave cylinder, clutch fork, and throwout bearing. You'll need a conversion pilot bushing for the SM420's 0.590-inch pilot tip and a 10-spline clutch disc. Or, if you don't want to scrounge junkyard parts, you can use a Novak Adapters PN B150GM to bolt the tranny to the engine. There are several different types of adapters available to convert the SM420 to the Dana 300/NP231 pattern T-case. Although you'll loose the overdrive, if you do a lot of off-roading, the 7.05:1 First gear of the SM420 offers a huge increase in slow-speed gearing compared with the AX-5's 3.93:1 First gear.

Engine Tire Size Suggested Axle Ratios for Specific Transmissions
(diameter, in) 3-spd. Auto 4-spd Manual Overdrive
Four-cylinder 33 4.56 4.56 4.88-5.13
35 4.88 4.88 5.13-5.38
37-40 5.13 5.13 5.38-up
Six-cylinder 33 3.73-4.10 3.73-4.10 4.10
35 4.10 4.10 4.56
37-40 4.56-4.88 4.56-4.88 4.88-5.38
Eight-cylinder 33 3.55-3.73 3.73 4.10
35 3.73-4.10 4.10 4.56
37-40 4.88 4.88 4.88-5.38

Steer Clear Rear
Q I've got a Dana 20 rear axle and want to build it for off-roading with big tires. What do you suggest?

A For starters, a Dana 20 is a transfer case used in late '60s to late '70s Jeep vehicles. A Model 20 is a rear axle put in full-size Jeeps and CJs from the mid-'70s until the early '90s.

There's really nothing wrong with the Model 20 that came in full-size Jeeps. These axles used heavier axletubes and good one-piece axleshafts.

While the CJ Model 20 used the same 8.875-inch ring gear and 29-spline carrier as the full-size Jeep, these axles have relatively weak axle tubes that are prone to flexing and bending with heavy use. Furthermore, the casting where the tube enters the centersection is comparably weak and has been known to crack and fail and/or the tubes can spin in the centersection bores, allowing the pinion to shoot straight up at the floorboards. Finally, the CJs were given a horrendous two-piece axleshaft setup that used a nut and Woodruff key to attach the hub to the shaft. In the field, the nut could back off and/or the Woodruff key could shear, effectively ruining your day.

We'd recommend adding some one-piece conversion shafts and hitting mild trails with 31- or 32-inch tires and a limited slip. But if you're thinking of punishing your CJ Model 20 off-road with 33s or larger tires, we'd put our money into a Dana 44, Ford 9-inch, Dana 60, or some other beefy axle swap.

Although there are a number of one-piece conversion shaft kits on the market, it's really a waste of money if heavy wheeling is in the axle's future. Because of the tube flex, axle seal life is shortened, leading to leaks. And centersection cracks and bent tubes are likely with big tires and heavy off-road use.

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