We"d like to think you want more than a truck that just looks good in the driveway but if that"s all you need, you can turn these pages to the next story. If you want to have as much show as go along with something you can actually drive day to day, read on.
Step One: Tread Rightly!
The tread design you choose dictates performance on the street and on the trail. If you pick a tire biased for the street, you"ll pay a price on the trail. Pick a trail tire and you"ll pay a price on the street. For an "easy "wheeler," the choice is fairly easy and the compromises few. For a more hardcore trail traveler, still using his rig as a daily driver, the choice is harder.
Mud & Rock Tires: Visually, there is little to distinguish between a mud and a rock tire. Both do reasonably well in either venue and have deep, flexible tread designs with lots of void area. That void area allows the tire to clean quickly in mud and the tread segments to act as "paddles." On rocks, the tread blocks have higher contact pressure for better grip and are flexible to conform and adhere to irregular surfaces. A rock tire may have a softer rubber compound (which reduces tread life substantially) and more or stronger plies in the carcass (sidewalls, especially) to better resist rock damage. The tread designs may vary also, and there are nuances that offer better performance in either venue. The biggest downside of aggressive tires is that those deep, flexible tread blocks tend to be squirmy on the street. Compared to a street tire, or even an all-terrain, braking and handling is more imprecise. There is usually less siping (small cuts in the tread) to carry away water and ice, so the part of the tire hitting the pavement is essentially a slick. A very few MTs are studable, which can help on ice. MTs also have more rolling resistance and will usually decrease fuel economy.
Aggressive All Terrains: These are the "missing link" between a true mud/rock tire and a mild all terrain. They offer improved street performance, treadwear and fuel mileage over a mudder but also a nice bump in trail performance over the milder all terrains. They have more void areas than a "regular" AT and some have as much as an aggressive mudder. Usually the tread depth is less so the tire is less squirmy. You can even find a few aggressive ATs with soft compound rubber, which makes them even better in the rocks.
All Terrains: An AT will deliver better mpg and treadwear than a mudder or an aggressive AT, while offering better street handling. Pavement ice and snow performance is better as well. ATs will generally not offer great rock performance because the tread is less flexible but that quality adds greatly to street and inclement weather performance. That"s a good segue to mention that there are two snow tire ratings. First comes the old "Mud and Snow" rating (M&S), which allowed the manufacturer to rate the tire. That left a lot open to interpretation, so as of "99 some tires use the new Winter Tire Traction rating. Tires so rated have to meet a winter performance standard and wear a "snowflake-on-the-mountain" symbol. Many all-season and some all-terrains carry this rating. Many MTs carry the M&S rating, but the "S" part doesn"t mean much except in deep trail stuff.
All Season: There"s a place for the all season tire. Many FWD and AWD owners use their rigs as inclement weather transport. In this venue, they are generally better than any AT or MT. In this category would also fit dedicated snow tires which are your best bet for ice and snow country.
Step Two: Load Rating
Everyone should pay attention here, not just one-ton truck people. If you drive a light rig with anything above a Load Range C tire, you are sacrificing a lot of tire flexibility out on the trail. If you drive a truck, and use it for more than grocery hauling, you need a tire matched to its cargo capacity. Some half-tons can do well with a Load Range C tire, but some should have a Load Range D. Most 3/4 and 1-tons need a load range D and a few need a Load Range E. More to the point, look on the tire, or read on the spec sheet, for the actual load capacity and compare that to your truck"s payload, or GVW. The tires should be able to carry that payload, plus the weight of the truck, with some extra.
Step Three: Size Does Matter!
On the trail, bigger is better. A taller tire offers more clearance and a higher profile offers more flexibility. More width puts more tread on the ground, for better grip. For the trail, especially rocks, the high profile tire is the way to go. It has lots of air volume so it can be aired down more so the tire has the flexibility to conform to terrain irregularities. These attributes work against you on the street. Taller tires raise your center of gravity, but the high profile tire squirms and distorts in turning and stopping situations, reducing the performance in comparison to a lower profile tire.
The advent of larger diameter aftermarket rims and tires has made it possible to combine diameter for clearance with a lower profile off-road tire for better street handling. We"re not talking the "bling" look with the huge rims and tires that look like three layers of electrical tape. A larger rim and a moderate profile tire reduces the tire"s flexibility on the trail but many find it a workable loss. You can go too far in this direction but for most people, maintaining a minimum of about 8-9 inches of tire profile, regardless of rim diameter, is a good compromise.
Step Four: Wheel Rightly!
The tire needs to be matched to the correct rim. That starts with rim diameter but we think you know that 16 inch tires gotta go on a 16 inch rim. Width is less clear and a tire will mount and hold air, even if mounted on the incorrect width rim. The manufacturer of a typical 35x12.50-15 might list a width range of 8 to 11 inches, meaning you will get nominal performance from the tire if mounted on a rim in that width range. From there, look for a spec that lists "Measuring Rim Width" (MRW). That"s the rim width the tire manufacturer used to calculate the section width of the tire. It"s only a "12.50" when on the MRW.
For every half-inch wider or narrower than the MRW, the section width changes a quarter-inch in the same direction. Say the MRW was nine inches, the section with of the tire is 12.50 and the rim width range was 8-11 inches. If you mounted the tire on an 8 inch rim, you"d reduce the section width by a half inch. That might be just enough to clear that last little bit of rubbing. Bear in mind that when you go towards the narrow end of the range, it tends to pull the edges of the tread up. When you go wide, it tends to pull the center up. Those effects are minimal within the width range, but wear and performance can still be effected negatively. The MRW is usually the ideal.
Two wheel-related terms you need to understand are backspace and offset. Backspace is the distance from the inside wheel mounting flange to the inside bead. Offset is the distance the mounting pad is offset from the wheel centerline (see nearby illustration). Negative offset means the wheel rim is moved outboard on the wheel flange. If you keep the same wheel rim width, adding more negative offset will reduce backspace. You can determine offset by measuring backspace, subtracting it from the rim width and then dividing by two. An 8 inch rim with 5.5-inch backspacing has a 1.25-inch offset (8.0-5.5=2.5/2=1.25-inches). Remember that rim width is technically from bead to bead, not between the outer edges of the rim.
The extra width needed for a bigger tire is best added equally to the inside and outside to keep the offset roughly the same as stock and keep load on the wheel bearings where the factory intended. A stock 7 inch rim with 3.5-inch backspace might be replaced by an 8-inch rim with 4-inch backspace. One-half an inch is added to the inside and the other half-inch to the outside.
How much you can add to the inside may become a problem, especially on the front, so you may have to add more of the extra width to the outside edge. Using the example above, the 8-inch rim with a 3.75-inch backspace adds a quarter-inch to the inside and three-quarters to the outside. That changes the backspace minimally but adds more negative offset and loads the bearings a little more. Three-quarters of an inch isn"t a biggie but changing the offset a great deal in either direction can cause early wheel bearing failure and tire wear issues.
Your wheel should be rated for the load it will carry. This spec is available from the wheel manufacturer"s specs and should be cast into the wheel also. This usually works itself out, as an 8-lug wheel is usually rated for the load an 8-lug truck might carry. Trouble is more likely in the half-ton truck, five-lug realm where the bolt pattern is shared with cars. You can lessen the chance of a problem by using high quality cast or forged wheels, avoiding composite wheels that have a cast center with a spun aluminum rim.
Step Five: Let"s Be Practical Here
Bigger tires equals bigger expense. That"s true for the tires and wheels themselves and for all the other stuff that goes with them. Bigger tires need taller and more expensive lifts, gearing changes and often need axle strength... or a complete swap. If you don"t address the whole picture, you leave a weak link to bite you in the butt when it will hurt the most. Every rig has a range of lift and tire combinations that are practical. If you don"t see the lift kits and other accoutrements needed to get a specific sized tire on your rig... take the hint... or maybe get a second mortgage.
Tire Manufacturer Sources
BF Goodrich Tires
www.bfgoodrichtires.com
Dick Cepek Tires
www.dickcepek.com
Interco Tire
www.intercotire.com
Mickey Thompson Tires
www.mickeythompsontires.com
Pro Comp Tires
www.procomptires.com
Sidebar One:
The Spare Back There
On the trail, replacing a damaged tire with a bald spare reduces your traction potential by as much as 25 percent and you struggle on the terrain you would normally ace. If the diameter is substantially different from the other tires, your automatic locker will go nuts... and may go bye-bye. The best idea is to have five identical tires and rims. If you include the spare into the rotation process, you will be able to significantly extend the life of any set of tires.
Sidebar Two:
Gear Ratios
A substantial tire diameter increase requires a change in the axle gear ratios to maintain performance. That"s not cheap or easy and many people try to avoid the issue. The resulting performance issues are why gearing is, and always has been, the number one topic in our "Gearhead" tech column. There are two ways to figure the changes needed, a simple "Equivalent Ratio" formula or by using the "Shotgun" chart below.
Equivalent Ratio Formula:
new tire diameter
______________ x original gear ratio = new ratio
old tire diameter
Note 1: If your current gears are taller than 3.55:1 (3.21:1, 3.7:1, 2.73:1, etc.), you get better performance results with the formula by using 3.5:1 as your current ratio.
Note 2: You will get more accurate, real world results, measure the loaded radius of the wheel and tire (from the ground to the exact center of the wheel) aired up to normal street pressure, times 2 and use that as you diameter.
Shotgun Ratios:
These are rough rules of thumb that seem to work more or less universally on rigs with average power to weight ratios. On underpowered rigs, bump to the next lowest ratio.
Tire Diameter Gear Ratio
31 3.73-4.10
33 4.10-4.56
35 4.56-4.88
36-37 4.88-5.13
38+ 5.13- 7.0 (as needed)
