Odo Adj
Q I recently installed bigger tires/different gears and now my speedometer isn't accurate.
A It's simple math, but if you increase your tire size by 10 percent, your speedometer will read slower by 10 percent. If you increase your gear ratio by 10 percent, your speedometer will read 10 percent faster. Whichever modification you're working with, there are a few methods of correcting your speedometer no matter what year Jeep you have.
Older Jeeps with mechanical speedometers will need to run a drive joint with a specific ratio to either slow down or speed up the speedometer. The drive joint mounts in between the transfer case speedometer gear drive and the speedometer cable and is currently offered by Stewart Warner (stewartwarner.com).
Jeeps from the '70- to mid '90s that run fully mechanical or mechanically-driven electronic speedometers can have new speedometer drive gears installed into the transfer case tail housing. Replacement gears with 26-38 teeth are available to correct for several ratios and tire size combinations. The gears are available through numerous distributors including 4 Wheel Drive Hardware (4wd.com) and Quadratec (quadratec.com).
Finally, vehicles with fully-electronic speedometers that use a Hall-effect pickup, and even mechanically-driven electronic speedometers, can use an electronic conversion box, such as the Superlift True Speed or the Jet Performance Speedometer Calibrator. Or there are a number of performance controllers and hand-held electronic programmer boxes that can adjust the speedometer readings by accessing the factory computer software through the OBD II port.
Power Play
Q I want to swap a V-8 in my Jeep. Will I need to modify the firewall?
A This one always cracks us up because a V-8 swap involves so much work that modifying the firewall seems simple in comparison. Regardless, swapping a V-8 can generally be done in any Jeep without firewall modifications.
About the tightest and most common swap is sticking a small-block Chevy into a flatfender. The two biggest factors in fitting everything in the stock location is the large distributor cap on the factory Chevy HEI distributor and the more common long water pump assembly. Swapping to a short water pump and accessory package and swapping out to an aftermarket small cap electronic ignition distributor will allow you to just squeak a SBC under the factory flattie hood.
For an even cleaner swap, we'd consider a small-block Ford, a Buick V-6 or V-8, or even an AMC engine. All of these designs have front-mounted distributors that will allow the engine to be positioned a little farther back in the engine compartment without firewall interference.
Tread Troubles
Q I use my Jeep mostly for commuting, but hit the trails about one weekend every month. Should I buy
an all terrain or mud terrain tire?
A There's really too much personal preference involved in a loaded question like this. However, although you'll almost always get more mileage out of a set of all terrain tires, manufacturers nowadays are employing some pretty advanced computer designs to create moderately aggressive mud tires that are quiet and that roll smoothly on the street. While you won't get the animal traction of a gnarly Interco Swamper or BFG Krawler, a mild mud tire such as the Toyo Open Country MT or the Pro Comp Xtreme Mud Terrain will get the job done off-road without wiping out your ear drums on the street.
Ho-Hum Drum
Q What do I need to convert my early Jeep from drum brakes to disc?
A No matter how much we cover this question in the magazine and on our website, we always seem to get asked about converting drum brake axles to disc. For starters, visit jpmagazine.com and check out the story, "Early Jeep Disc Brake Conversion-Timeless Tech Drum to Disc" by former Jp staffer Verne Simons.
Early Jeep Dana 25, Dana 27, Dana 30, and Dana 44 axles originally equipped with drum brakes share the same round six-bolt spindle and backing plate bolt pattern as later GM, Ford, and Dodge Dana 44 disc brake axles. So, it stands to reason that you can use some combination of these parts to convert to disc brakes. For the story, Simons used the following: calipers from a '71-'78 Chevy 1/2-ton or light-duty 3/4-ton 4x4 with a Dana 44, caliper mounting brackets from a '73-'91 1/2-ton Chevy 4x4, Rotors from a '77-'78 CJ (These rotors are 11/8-inch thick), ten longer wheel studs (he used Dorman PN 610106), Chevy 1/2-ton brake lines (Pro Comp PN 7220), a grinder, standard hand tools, brake fluid, new hard brake line, banjo bolts, and a bleeder kit.
You may be able to do the same swap on the rear Dana axle providing it has the same round six-bolt backing plate pattern as the front.