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Top 10 4x4 Questions Answered

Posted in How To on February 1, 1999
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Photographers: Petersen Publishing Archives

Q Why must the axle gear ratios be changed after installing larger-than-stock tires?

A When the tire height of a vehicle is changed from stock, it changes the effective gear ratio of the vehicle. Axle ratios are carefully chosen by OEM to work with the engine and the stock tire height to create a good balance between acceleration performance and fuel economy and so that the engine will be within its optimum powerband when the truck is cruising down the highway. When larger tires are bolted on the truck but the axle gears are left stock, the effective gear ratio is reduced and performance and fuel economy suffer because the engine is forced to operate below its powerband. The only way to bring the engine back into its powerband is to change the axle gears to a lower (numerically higher) ratio. To figure out what ratio fits the bill for the larger tires on your truck, check out the next question.

Q How do I figure out which axle gear ratio is needed for larger-than-stock tires?

A We've included a chart that shows the standard gear recommendations according to tire height, but every truck has different needs and it's beneficial to do the math and find which ratio best suits your truck. There are several methods for determining what ratio is optimum for a truck and tire combination, but we've found the easiest and most accurate way is to find out what the stock cruising rpm of the truck was with the original tires, then match this rpm with the larger tires and gear ratio. To accomplish this, you need to know what tire diameter was original equipment and the actual tire diameter of the larger tires (actual tire height almost always varies from the sidewall spec), then plug the numbers into the following formula:

mph x gear ratio x 336 = rpm
tire diameter

For example, let's say the owner of a '77 Chevy truck wants to bolt on a set of tires that measure 36 inches in diameter. The truck is equipped with 3.73 gears and doesn't have Overdrive, and the stock tires are 31 inches tall. To find what the engine rpm is at 65 mph with the transmission in a gear with a 1:1 ratio (Third with an automatic, Third with a three-speed manual, or Fourth with a four- or five-speed), the formula would look like this:

65 x 3.73 x 336 = 2,628

Now that we know the stock cruising rpm is 2,628, with a little experimentation we can find what the gear ratio should be with the new tires (remember, lower gears are numerically higher):

65 x 4.56 x 336 = 2,766

A set of 4.56 gears will put the engine at 2,766 rpm when the truck is traveling 65 mph, which is about 100 rpm higher than original. By plugging a 4.10:1 ratio into the same formula, engine rpm would drop to 2,487, which might be a good choice if the owner is looking to improve fuel economy but not if there's a desire for more acceleration, towing, or off-road performance.

Aside from the tire diameter, other factors should be taken into consideration when choosing a gear ratio. An automatic tranny equipped with a non-lockup torque converter will slip more, while a manual tranny will not, and the actual cruising rpm will be higher with an automatic than a manual. Also, it's possible to choose a lower gear ratio on a truck equipped with Overdrive (a gear that is lower than 1:1 such as 0.73:1) for better crawling and acceleration performance without completely sacrificing highway fuel economy because the Overdrive will provide the necessary drop in engine rpm at speed. To add the Overdrive into the equation above, simply multiply the Overdrive ratio with the axle ratio.

Gear Ratios
This chart was compiled with information from 4 Wheel Parts Wholesalers and is based on a 65 mph cruising speed with a manual transmission in a gear with a 1:1 ratio. If the vehicle is equipped with an automatic transmission without a lockup torque converter, the true rpm will be higher due to slippage in the torque converter. Also, if the vehicle is equipped with an Overdrive transmission, the final cruising rpm in Overdrive will be 15-20 percent lower than indicated on the chart. For best all-around daily-driver performance, choose a gear ratio in the black area. For better acceleration, towing, and crawling performance but reduced fuel economy, choose a ratio in the light gray shaded area to the right of the black. For better fuel economy and highway driveability, choose a ratio in the dark gray shaded area to the left of the black.

Tire Gear Ratio
Dia. 3.07:1 3.21:1 3.31:1 3.42:1 3.55:1 3.73:1 3.91:1 4.11:1 4.27:1 4.56:1 4.88:1 5.13:1 5.29:1 5.38:1 5.71:1
26 2,579 2,696 2,780 2,873 2,982 3,133 3,284 3,452 3,587 3,830 4,099 4,309 4,444 4,519 4,796
27 2,483 2,597 2,677 2,766 2,872 3,017 3,163 3,325 3,454 3,689 3,947 4,150 4,279 4,352 4,619
28 2,395 2,504 2,582 2,668 2,769 2,909 3,050 3,206 3,331 3,557 3,806 4,001 4,126 4,196 4,454
29 2,312 2,417 2,493 2,576 2,674 2,809 2,945 3,095 3,216 3,434 3,675 3,863 3,984 4,052 4,300
30 2,235 2,337 2,410 2,490 2,584 2,715 2,846 2,992 3,109 3,320 3,553 3,735 3,851 3,917 4,157
31 2,163 2,261 2,332 2,409 2,501 2,628 2,755 2,896 3,008 3,213 3,438 3,614 3,727 3,790 4,023
32 2,095 2,191 2,259 2,334 2,423 2,546 2,669 2,805 2,914 3,112 3,331 3,501 3,610 3,672 3,897
33 2,032 2,124 2,191 2,263 2,349 2,469 2,588 2,720 2,826 3,018 3,230 3,395 3,501 3,561 3,779
34 1,972 2,062 2,126 2,197 2,280 2,396 2,512 2,640 2,743 2,929 3,135 3,295 3,398 3,456 3,668
35 1,916 2,003 2,065 2,134 2,215 2,328 2,440 2,565 2,664 2,845 3,045 3,201 3,301 3,357 3,563

36 1,862 1,947 2,008 2,075 2,154 2,263 2,372 2,493 2,590 2,766 2,961 3,112 3,209 3,264 3,464
37 1,812 1,895 1,954 2,019 2,095 2,202 2,308 2,426 2,520 2,692 2,881 3,028 3,123 3,176 3,370
38 1,764 1,845 1,902 1,966 2,040 2,144 2,247 2,362 2,454 2,621 2,805 2,948 3,040 3,092 3,282
39 1,719 1,798 1,854 1,915 1,988 2,089 2,190 2,302 2,391 2,554 2,733 2,873 2,962 3,013 3,198
40 1,676 1,753 1,807 1,867 1,938 2,037 2,135 2,244 2,331 2,490 2,664 2,801 2,888 2,937 3,118
41 1,635 1,710 1,763 1,822 1,891 1,987 2,083 2,189 2,275 2,429 2,599 2,733 2,818 2,866 3,042
42 1,596 1,669 1,721 1,778 1,846 1,940 2,033 2,137 2,220 2,371 2,538 2,668 2,751 2,798 2,969
43 1,559 1,630 1,681 1,737 1,803 1,894 1,986 2,087 2,169 2,316 2,479 2,606 2,687 2,733 2,900
44 1,524 1,593 1,643 1,698 1,762 1,851 1,941 2,040 2,119 2,263 2,422 2,546 2,626 2,670 2,834

Q What's the largest tire that will fit under a truck stock or with a lift kit?

A There's no easy or exact answer to this question for any vehicle because there are so many variables to consider. First, there are the differences between makes, model years, and types of trucks. While a '79 Bronco might be able to fit 33s without a lift, it may take a 6-inch lift to fit the same tires on a Chevy S-10. Stock and aftermarket suspension systems also vary in actual height and longevity, especially when the age of the truck and its usage are taken into consideration. A 4-inch kit from one manufacturer may start out as a 6-inch kit and settle to 4, while a kit from another manufacturer may not settle at all.

Beyond the truck and suspension variations, wheel width and offset, as well as actual tire height and width, also come into play. Aftermarket wheels can be ordered with different offsets and widths from stock to accommodate larger tires, but a wider tire may need a more negative offset wheel to fit, while a skinny tire of the same diameter might need a positive offset. Actual tire heights and widths are almost always different from one manufacturer to another and rarely match the size printed on the sidewall. Therefore, a 35x12.50 from one manufacturer might fit but one from another manufacturer might rub. And as though all this weren't enough, actual tire heights will vary depending on the width of the rim that the tire is mounted on.

The bottom line is that without knowing every variable, there's no way to accurately state whether a certain tire height will fit a certain truck, and even when all the variables are known, you can only make an educated guess without actually seeing all the components installed on the truck. Virtually all suspension manufacturers have tire-height recommendations, but it has been our experience that some are too conservative and others are a little optimistic, so it's best to use their recommendations as a guideline and discuss your proposed lift, tire, and wheel combo with your local off-road shop or someone with a similar truck and setup as the one you want. For further guidance, check out "Tire Fitment Guide for Lifted Trucks" and "Size Counts" in the July '98 issue. In the end, if you're still not sure whether the tires you want will fit, it's always best to err on the conservative side.

Q What relatively simple bolt-on parts are recommended for increasing acceleration and enhancing power?

A Bolt-on power is all the rage, and lots of companies manufacture simple-to-install parts designed to squeeze more ponies out of stock engines.

The biggest gains for any engine can be found by eliminating potential restrictions. Replacing a paper filter element with a high-flow air filter (one of the first mods people perform) is a good start. The exhaust manifolds on stock engines are notoriously restrictive, so swapping these for a set of quality headers can make surprising gains. The same can be said of OE intakes, so an aftermarket dual-plane can increase power as well as reduce some weight. High-output aftermarket ignition can also free up a few ponies since many factory ignition systems aren't as powerful as they could be. On late-model trucks, combining some of these mods with a set of underdrive pulleys and a performance computer chip can add significant power.

Before adding any aftermarket performance parts, check the local smog laws. Many states have laws for altering emissions equipment, so make sure additions conform with the laws of your area or have an E.O. (Executive Order) number that permits the parts to be used on emissions-controlled vehicles.

Q What does it take to perform an engine swap?

A Virtually every engine-swap question we receive is based on a desire to put more power under the hood of a truck. Although we receive questions about putting larger displacement engines of the same configuration and make into a truck (such as a Ford Explorer V-6 into a Bronco II or installing a Suzuki Sidekick four-cylinder engine in a Samurai, both of which are possible and relatively simple), most questions are centered around V-8 swaps. The information for most swaps could fill several magazines, so it's impossible to provide all the info for every swap in the limited space here. However, several companies, such as Advance Adapters (Dept. 4WOR, P.O. Box 247, Paso Robles, CA 93447, 800/350-2223, 805/238-7000), JB Conversions (Dept. 4WOR, P.O. Box 2683, Sulphur, LA 70664, 318/625-2379), JTR (Dept. 4WOR, P.O. Box 66, Livermore, CA 94551, 925/462-3619), and Kaufman Products (Dept. 4WOR, 12400 Benedict Ave., Downey, CA 90242, 562/803-5531), offer essential swap components and valuable installation manuals that can take much of the headache and guesswork out of the more popular swaps. Even with all of this help, an engine swap is still a major undertaking, so before getting tangled up in a conversion, it's important to do as much research as possible and then decide whether you're comfortable handling the proposed swap on your own.

As a general rule, swapping engines within the same make (such as replacing a Jeep CJ inline-six with an AMC V-8 or putting a big-block in place of a small-block) is easier than trying to mate the engine from one manufacturer with the drivetrain from another. In fact, most Chevy big-block swaps are fairly straightforward and require minimal modifications. In the case of replacing a six-cylinder with a V-8, a couple of frame brackets and other accessories are needed, but these items are attainable at salvage yards and take little, if any, modification. Be aware, however, that most engine swaps will require different bellhousings and transmissions or an adapter.

When transplanting the engine from a different manufacturer into a truck, there are several factors that can add to the complexity of a swap. First, if the swap is uncommon (such as transplanting a Chevy engine into a Ford), there won't be much information available and you won't find many adapters to assist with mating the driveline components to the engine and sorting out the details. Many of these types of swaps will require custom fabrication that can be very expensive, so unless you're really after something unique or possess an above-average amount of mechanical skill, we recommend sticking with the more popular swaps.

When installing a large engine into an engine compartment designed for a small one (such as stuffing a V-8 under the hood of a Toyota), fitment and proper positioning become critical, and many components, such as the radiator, steering shaft, and accessories, may need to be relocated to make enough room. Larger displacement engines will generate more heat, so cooling upgrades such as a larger radiator and a custom fan shroud are almost always necessary. Also, even though it has great benefits for off-road use, swapping a fuel- injected engine is much more complex than a carbureted swap due to the maze of wires, sensors, hoses, and other components that will need to be transplanted and adapted to work with the vehicle.

Once general information and fitment issues are addressed, it's necessary to determine if the driveline (transmission, transfer case, driveshafts, and axles) can withstand the additional torque of a larger displacement engine. Often the total swap involves more than just the engine. Even if the original drivetrain will work with the new engine, it's necessary to figure out how to mate the engine with the rest of the driveline. As we mentioned before, several adapters are available for popular swaps (virtually every Jeep transmission and transfer case has an adapter available, for example), but they usually require complete disassembly of certain driveline components to install a custom input or output shaft.

Finally, there are hundreds of little but essential items to attend to, including addressing smog issues for registration. As you can see, an engine swap can breath new life into an old truck, but it takes a lot of patience and perseverance to make one come out right.

Q What are some of the best ways to increase a truck's off-road performance?

A Most stock trucks lack two things that are key to off-road performance: ground clearance and traction. Therefore, addressing these two shortcomings should top the list of modifications.

Installing a lift kit will both increase the clearance between the body and the ground and provide enough room in the wheelwells to mount larger-than-stock tires. A lift kit may also increase axle articulation, which is an added bonus. Traction problems can be solved by using tires with a more aggressive tread, having a suspension that provides more flex, and installing traction-aiding differentials.

Beyond ground clearance and traction, the rest of the recommended modifications depend on what type of off-roading the truck will encounter most. Mud monsters may need a big lift, tall tires, and extra horsepower, while a rockcrawler may need more gear reduction and suspension flex without as much height. For frequent fire-roading, items such as dual shock kits, axle trusses, and long-travel suspensions capable of soaking up jumps and bumps work best.

There are many different directions to take when building a truck, and not all possible modifications will work with every type of four-wheeling. Once you have the basic mods out of the way, the best plan is to use the truck as you intend to and talk to other people with similar interests and trucks, then make the modifications that best suit the truck, the terrain, and your budget.

Q What are limited slips and lockers, and which do you recommend more for off-road use?

A Most stock trucks are equipped with open differentials. With this type of diff, power takes the path of least resistance, so if one tire is in the air while the other tire on the same axle is on the ground, the tire in the air will always spin while the one with traction (the one you need to receive power most) stays still. A limited slip is similar to an open differential but with one key difference: A limited slip uses a torque-biasing device, such as clutches or a gear assembly, to resist slippage allowed between the two tires on the axle. Using the same scenario of one tire in the air and the other on the ground, the torque bias of a limited slip will redirect power from the tire in the air to the one on the ground and hopefully move the truck forward. However, the resistance provided by a limited slip eventually can be overcome, so in extreme situations, a limited slip will still permit the tire in the air to receive all the power while the tire on the ground won't move.

Unlike a limited slip, when torque is received from the engine, a locking differential mechanically locks the two axleshafts together and forces them to rotate at the same speed regardless of traction. So if one tire is in the air and the other is on the ground, both tires will still rotate. With an automatic mechanical locker, both axleshafts are normally locked together until the traction from the road overcomes the torque received from the engine, such as during deceleration through a corner. When this happens, the differential unlocks and allows one tire to spin faster than the other. But once torque is received from the engine, it locks the two back together. Another type of locking differential is driver-controlled, such as the ARB Air Locker. The unit acts just like a standard open differential until it's engaged by compressed air from a remote compressor, and then it locks. With this arrangement, the driver can turn the locker on and off as traction demands.

Both lockers and limited slips have advantages and drawbacks. Off road, a locker is hard to beat because of its superior traction capabilities. However, many people feel a locker's on-road manners are a major disadvantage because it makes the truck handle differently from stock, and on some models a ratcheting noise can be heard while going around corners along with occasional pops and bangs. Lockers also tend to accelerate tire wear, and running one in the front can make turning while in four-wheel drive a challenge. A limited slip has better manners on the street because it will allow the two tires on an axle to rotate at different speeds easier than a locker, but a locker can outperform one off road. Also, a clutch-type limited slip will eventually wear out.

As for what combination works best for off-road use, the correct answer depends on how often and how hardcore you consider your four-wheeling habits. Generally speaking, a limited slip in the rear or one in both axles works well for people who do occasional mild-to-moderate four-wheeling. Many moderate-to-serious four-wheelers run a locker in the back and a limited slip up front to retain turning ability, while most gonzo four-wheelers run lockers front and rear. And when you consider using a driver-controlled locker such as an ARB, there are even greater possibilities for combining on-road manners with off-road capability. Our best advise is to testdrive both and then base your decisions on your driving impressions and traction needs.

Q I'm having difficulty finding after- market parts for my truck. Where are the best places to look?

A The answer to this question is much easier for popular trucks-such as fullsize Chevys, Dodges, and Fords, Jeep CJs, YJs, and TJs, and Toyota trucks and Land Cruisers-which all have a wealth of aftermarket parts available. One of the best places to start searching for parts and ideas is right in front of you: Every month, this magazine has around 175 pages of editorial and advertisements devoted solely to upgrading and modifying 4x4s. Catalogs for large mail-order houses such as 4 Wheel Parts Wholesalers (Dept. 4WOR, 801 W. Artesia Blvd., Compton, CA 90220, 800/421-1050), Dick Cepek (Dept. 4WOR, 17000 Kingsview Ave., Carson, CA 90746, 800/992-3735, 310/217-1805,, and Summit (Dept. 4WOR, P.O. Box 909, Akron, OH 44309-0909, 800/230-3030, 330/630-0200, contain thousands of products for everything from engine performance to bed accessories. These companies also have well-staffed technical departments to answer your questions before you buy. If you're looking for more face-to-face service and more detailed information, try local off-road shops.

For owners of less common 4x4s, specifically vintage iron and many of the import brands, there's more of a challenge. Parts for trucks such as a Nissan Patrol or an Isuzu Trooper are in less demand than parts for an application such as a mid-'70s Chevy truck, and, therefore, aftermarket companies have less motivation to manufacture or stock the parts. In these instances, it may be best to search out a club (check our "Giant 4x4 Club Directory," Nov. '97) or the internet for information. Also, there are a few companies that specialize in hard-to-find performance parts such as lift kits and traction differentials, with Calmini Products (Dept. 4WOR, 6600-B McDivitt Dr., Bakersfield, CA 93313, 800/345-3305) topping the list for import trucks.

If none of these places net you any leads, then your only option is fabricating or swapping in parts from more common vehicles. A shop that specializes in custom fabrication can do just about anything you want, provided you have the resources and the patience to see it through. In the end, owning a unique, modified 4x4 is cool, but if you're looking to modify a 4x4 on a budget and with minimal hassle, it's often easier to invest in a more popular application and modify it rather than attempt to modify a less common one.

Q What is involved with an axle swap?

A The difficulty of an axle swap depends on whether the axle was offered as original equipment for the truck. If so, then it's normally a bolt-on operation. But since most swaps involve putting an axle from a 3/4- or 1-ton truck under a lighter-duty vehicle, this is rarely the case. Several companies such as Currie (Dept. 4WOR, 1480 N. Tustin Ave., Anaheim, CA 92807, 714/528-6957), Tri-County Gear (Dept. 4WOR, 1143 W. Second St., Pomona, CA 91766, 909/623-3373), and DynaTrac (Dept. 4WOR, 7236 Garden Grove Blvd., Westminster, CA 92683, 714/898-5228) specialize in building custom axles for nearly any application, but if you're planning to attempt the project yourself, here are some of the highlights:

Swapping heavier-duty axles under a fullsize truck is relatively easy, especially when the axles are from the same manufacturer as the truck (such as swapping a 14-bolt under a 1/2-ton Chevy). Most of the time, the front axle from a 1-ton will bolt right under a 1/2-ton, but do some research beforehand to make sure this is the case with your application. On rear axles, there may be some differences in the track width and spring-pad location. For example, not all 14-bolts will bolt directly under 1/2-ton Chevys; it depends on what application the axle came from. In many instances, relocating the spring pads is required (a relatively simple process), so measure the distance between the spring pads before buying if keeping hassle to a minimum is a concern. Since heavier-duty axles have a longer pinion and use a larger U-joint than their 1/2-ton equivalents, driveshaft modifications are usually necessary.

When swapping fullsize axles under a downsize vehicle, such as a Jeep, the axles will need to be cut to accommodate the vehicle's narrower track width if it's also to be driven on the street. Cutting down an axle, especially a front one, is a complex process that is best left to a professional. Also, since the shorter axle will no longer accept standard-length axleshafts, custom shafts will be required, and new spring pads that match the location of the vehicle's springs will need to be welded into place. After the axle has been hung under the truck, a new driveshaft will need to be fabbed. For front axles, the steering linkage will likely need to be fabricated or adapted to work with the vehicle's steering box.

Beyond bolting the axle into place, there are other concerns that need to be addressed. If one of the axles you're swapping in has a different wheel-bolt pattern then the other, one of the axles will need to be adapted unless you want to carry two spare tires. Also, different brake lines are usually necessary, and since heavier-duty axles often have larger brakes, proportioning valve and master cylinder changes may also be required. Check out "Fixing a 1/2-Ton's Worst Problem" in this issue for details on a 3/4-ton axle swap.

Q What is a spring-over conversion and how difficult is it to perform?

A On some vehicles, such as Jeeps and Scouts, the axles are positioned above the springpacks. In the last few years, it has become popular to remount the axles below the stock springs similar to the way axles are mounted on fullsize trucks. This conversion nets a 4- to 6-inch lift (depending on the springs used) and many claim it also increases trail performance. However, there's more to a spring-over conversion than meets the eye.

To the best of our knowledge, no company manufactures a spring-over conversion kit, so the whole process involves fabrication. Since the axles are moved below the springs, the stock spring pads must be cut off and rewelded above the axle tubes. During this process, driveline angles must be taken into account or vibration will result, and longer driveshafts are normally required. New U-bolts and clamps are also needed, as are extended-length brake lines. The steering must be corrected similar to a standard lift kit, but on some applications the new location of the front springs interferes with the steering linkage, so often the tie rod must be relocated above the steering knuckles. This requires redrilling and tapering the knuckles as well as adding new tie-rod ends.

Since most spring-over conversions retain the stock springs, the ride remains close to stock but the increased leverage on the springs often causes problems with axlewrap both front and rear. There are several ways to correct this; one of the most effective solutions we've seen is mounting adjustable heim-jointed bars between custom brackets on the frame and above the axles.

When done properly, a spring-over yields an impressive amount of flex and handling and ride characteristics that remain close to stock. However, unless you possess the skills and tools to fabricate and deal with the problems a spring-over causes, you may be better off with a standard 4-inch lift kit that leaves the springs in the stock location.

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