The great Henry Ford once said when introducing the Model T line of cars, “You can have any color you want as long as it is black.” Times have definitely changed and the wide variety of colors and paint schemes with automobiles is now endless. It is not only the paint schemes which are endless but also many things, such as wheel and tire options. This tech article will focus on the history of wheels and tires offered for diesel pickup trucks and possibly the best choice of wheels and tires for your truck.
Factory Options
It is important to look at factory wheel or rim options for eight lug pickups over the years. Light-duty truck manufacturers have had several different factory options for wheels in their light-duty trucks such as half-ton and quarter-ton trucks over the years. In contrast this has not been the case for three quarter-ton and one-ton trucks until recently.
Most eight-lug pickups over the years came from the factory with stamped steel wheels and hub caps. Almost all steel wheels were 16 or 16.5 inches in diameter. There were not any other options. It wasn’t until the late 80s and early 90s that we began to see factory aluminum wheel options for diesel pickups. The question could be asked as to why this is. One might conclude that this has to do with cost of aluminum versus steel, but the answer actually has a lot to do with weight, payload capacity and aluminum alloy.
Materials
Aluminum alloy strong enough for large payloads and gross vehicle weight consistent with that of diesel pickups were around 20 years ago, but aluminum wheel designs were few by aftermarket manufacturers for eight-lug trucks. There were also trust and liability issues with auto manufacturers and consumers in utilizing aluminum eight-lug wheels on diesel pickups early on, especially with dual rear wheels. There was more than one instance of aluminum wheel failure in three quarter-ton and one-ton pickups in the 80s and early 90s. A few of the failures included cracks, hole elongation, leaking beads and corrosion.
A stigma was soon formed about them and their integrity versus a steel wheel. The 90s brought about much change in the heavy-duty pickup industry and the realization of aluminum wheels on eight-lug pickups was made a reality by all three major diesel pickup manufacturers: Dodge, Chevy and Ford.
Going Lighter
Several manufacturers of wheels contributed to the growth, the largest of such being Alcoa Wheels. The aluminum wheel offered not only lighter characteristics of opposing steel wheels, but also offered similar strength to its steel counterpart. And last but not least, unsurpassed looks! Consequently several companies have merged on to the aftermarket wheel scene in the last 20 years and offer various aluminum wheel sizes, styles, and configurations for eight-lug diesel pickups.
Selecting A Wheel
Due to the large variety of both wheels and tires offered currently by auto and aftermarket manufacturers, many questions can be pondered when choosing the best for your truck. One question to ask is what size of wheel do I want? Some things need to be taken into consideration when pondering wheel size. First consideration is what size of tire will I run on my wheels and what type of ride do I want from my truck? This may seem irrelevant when considering wheel size, but it is important. If a 20-inch rim is selected, but will only be running a 33-inch diameter tire, the ride will significantly diminish in comparison to a smaller sized wheel with similar diameter tire installed.
Let’s do some math. The overall rim diameter is 20 inches and the tire diameter is 33 inches. This is only 13 inches of diameter difference, meaning there is only roughly 6.5 inches of side wall. A truck owner must understand that tires not only transmit horsepower and torque to the road, but also help the springs and shock absorbers soak up road conditions. A 6.5-inch sidewall will have a much poorer ability to varying terrain conditions versus a larger 8.5-inch sidewall. An 8.5-inch sidewall could be found on an 18-inch wheel wrapped in 35-inch rubber. This would provide a considerably smoother ride in comparison to the 20-inch wheel with 33-inch rubber.
Other Considerations
Several other factors can be taken into consideration when it comes to ride quality with tires. These considerations may include ply rating and rubber compound. If this is true, why are pickup truck manufacturers offering 18- and 20-inch diameter wheels as stock equipment on their pickup truck lines for three quarter-ton and one-ton pickups? Two reasons. One does have a lot to do with styling, but even more importantly, another is the simple fact that payloads have increased over the years for pickups and so have the brake sizes. It is impossible to fit a standard 16-inch diameter wheel over an 18-inch caliper and rotor. The pickup truck manufacturers not only like the styling appeal of larger wheels, but are forced to increase wheel size due to larger disc brakes and brake components.
Older pickups seem to have their ride affected most by diminished tire sidewall. The newer pickup trucks most likely have coil-over suspension or advanced torsion bars on the front and progressively rated springs in the rear and can easily compensate, but still a factor to take into consideration.
Diameter
Some other data to think about when choosing aftermarket wheels and tires is actual tire diameter. When tire diameter is chosen several points should be evaluated. The first point should be how much room is there vertically in my fender wells before tire rubbing occurs when the suspension is fully compressed? The second point could be tire and wheel width and body and suspension rubbing points when turning. Wheel offset can also greatly affect this second point. Wheel bearings suffer greatly from wider wheels, which have increased offset—especially since the introduction of the unitized wheel hub.
The third point to evaluate when choosing tires is the simple fact of the tire diameter changing final gear ratio and speedometer correction. Again let’s do some math. A 33-inch tire has a footprint of roughly 103.67 inches using the formula for circumference equals π X diameter = 2 × π × radius.
Each time the tire rotates one revolution it travels 103.67 inches. In contrast, each time a 31-inch tire rotates one revolution it travels 97.39 inches. If the tire is traveling 1000 rpms (rotations per minute), the 33-inch tire has traveled 8,639 feet in one minute. The 31-inch tire only traveled 8,115 feet. This explains why the speedometer reads lower than actual speed when a higher diameter tire is installed on a vehicle. Most current pickups offer an electronic speedometer reprogram correction to compensate for different-sized tires, but older pickups do not.
What’s Too Much?
Larger tires have several benefits when tackling terrain, but some other bits of data to digest when choosing a tire size are as follows.
Tire diameter increase will lower engine rpms for a given speed, but can come with a price if increased too much. When engine rpm is lowered at a given speed it steals overall torque to the road, resulting in a sluggish feeling and higher fuel consumption. Very large diameter tires can also be much heavier than a stock diameter tire depending on wheel size, further contributing to parasitic drag and fuel consumption. It is not just fuel mileage and power that is affected by very large-sized tires but also brake component wear. Isaac Newton’s first law of motion states that a body in motion (the tire) tends to stay in motion unless acted upon by an outside force (the brakes). Simply stated, the larger the tires the more inertia. The more inertia the more braking force it takes to stop them.
There is so much to consider when contemplating what wheel and tire combo is best for your truck. The possibilities are truly endless. Only a few of the main points have been touched upon. Maybe next time you ponder on which wheels and tires you want for your truck, you will think a little bit more about the far-reaching effects of wheel/tire combinations.
About The Author
Looking to learn more about diesel systems? Want to make a career out of building, repairing and installing performance components? Levi Perkins is the head instructor for the diesel program at the College of Southern Idaho located in Twin Falls, Idaho. As a student of the diesel program at CSI, you will also participate as a member of the Diesel Club, in which you'll be a member of various fund raisers, participate in the World Agriculture Expo, tractor pulling events and many other educational activities.
Classes are held from 8:00 a.m. to 1:00 p.m. Monday through Friday, and will give students the required knowledge and practical application experience for diesel systems, which include: engines, transmissions, axles, brakes, electrical and hydraulic systems and much more. The diesel program at CSI is proud to announce they've had a 100 percent placement in the last three years and look forward to giving future diesel mechanics and engineers a bright future in the diesel industry of tomorrow. For more information about the diesel program at the College of Southern Idaho, go to www.csi.edu or call 208-733-9554.