While the following news article from Off-Road Magazine (August, 1999 Issue) uses truck tires for analogies, many of the underlying principles apply to all tires in general:
The Pros and Cons of Big Tires
Big, bigger, best-that's the American way
That philosophy certainly holds true when it comes to tires on 4x4s. Maybe it has something to do with the old shoe size myth, but we think guys would run earth-mover tires if they could figure out how to make them fit. Who cares if the law hassles you? You'd be the biggest, baddest dog on the block.
Of course, your rationale for the big tires is performance and ground clearance. Those are valid points, but we've seen rigs conquer some mighty tough terrain on skinny, old Army surplus ribbed tires that didn't have any catchy names on the sidewalls. If you call have something that looks cool and also works well, that's a tough combination to beat. By choosing wisely, you can have the best of both worlds.
Deviating from the factory recommended tire sizes involves a number of compromises. Probably the two biggest issues are clearance problems and gear ratio changes. Installing a bigger set of tires is seldom a one-step process. More often, It's one of those "in order to run these, you have to modify this other thing, which means a couple more items need to be changed," chain reaction events.
Fit is usually the first hurdle if you don't count coming up with the money to buy the big tires and wheels. If a tire and wheel combination won't fit, that's all she wrote. Of course, there are members of the "if at first you don't succeed, get a bigger hammer" school of truck modifying who will make any tire fit, regardless of the consequences. We're talking about fitting big tires with a reasonable amount of additional changes to the truck.
Two ways to gain clearance are to lift the body and enlarge the wheelwells. Lift kits are widely available, but depending on the amount of lift and the particulars of your truck, you may need to change some suspension and steering components.
Body clearance problems diminish when a lift kit is installed, but there still needs to be adequate distance between the rubber and the body. This clearance needs to allow room for suspension travel. It's one thing to drive in a straight line on a paved road with tight body clearances, but if you actually use the tires in the type of terrain they were designed for, the body will cause interference. The easiest way to gain wheelwell clearance is to enlarge the openings and install aftermarket fender flares. Sometimes the front bumper will need a little trimming too.
Fitting bigger tires is more of an art than a science. Tire stores and off-road equipment shops have charts that will help you get close, but charts can't account for the variations on your truck. The charts tend to be a little conservative, which is good. Shops don't want you coming back unhappy with damaged tires. Also, you can count on a little extra room when tires are mounted on the truck. The weight of the vehicle tends to flatten the tire a little and push out the sidewalls. It's not uncommon for a mounted tire to lose an inch from its off-the-truck height. This phenomenon is known as deflection and loaded radius.
Deflection is a good reason for not measuring your tires while they are supporting the truck. One way to prove that is to measure the distance from the center of the hub to the ground and to the top of the tread. The top radius will be larger. If you are comparing tires on another truck, you need to account for the vehicle's weight. The heavier the truck, the greater the deflection.
Clearances should be measured on flat pavement with the tires straight ahead. Measurements should be taken up, forward, and back of the tire. If you think of the tire as a clock, measure at 9, 12, and 3 o'clock. Besides measuring with the wheels straight, turn them all the way to the right and left. Due to the nature of Ackermann steering, clearances can be different depending on which way the tires are turned. Not all bodies are mounted perfectly square on frames either. If your truck has ever taken a super-hard off-road pounding or been in an accident, that could affect side-to-side measurements.
You can measure tire diameter with the tires on the truck if you jack up the tire to the point where it clears the ground. You can also run a cloth (sewing type) measuring tape or a piece of string around the circumference of the unloaded tire. That number divided by Pi (3.14) will give you the exact diameter.
Suspension travel is an important consideration when contemplating bigger tires, A way to approximate the necessary clearance between the tires and the body is to measure the distance between the bumpstop and the suspension component it is meant to protect. Allow up to an extra inch for the compression of the bumpstop. Add those dimensions to determine the clearance.
An example is a truck currently running 29-inch-tall tires. At rest, there are 5-1/2 inches between the top of the tire and the closest body point. There are 3 inches between the bumpstop and the frame. Add the inch for snubber compression. That's 4 inches of travel, which is subtracted from the 5-1/2-inch body clearance. Theoretically, there's 1-1/2 inches of body clearance at full jounce.
Going to a 31-inch-tall tire adds 1 inch to the top of the tire (no one ever had a clearance problem at the bottom of the tire). Subtract the 4 inches of travel plus the 1-inch diameter increase from the 5-1/2-inch body clearance and you should still have 1/2 inch of clearance at the top of the tire. That doesn't mean that there might not be some interference at the 9 o'clock and 3 o'clock positions. These types of calculations are good starting points.
Something we like to do is talk to owners of similar trucks and check out the clearances they have with tires we are considering. If you have a cooperative friend with big tires, try them on your truck, For static suspension clearance checks, we like to put a bunch of heavy friends in the bed.
Once you have your tall tires and everything fits, there is still more to be done. Going up a couple of inches in tire diameter makes a big change to your final drive ratio. Installing tall tires without changing the rear axle ratio will make your truck seem like it can't get out of its own way. The added weight of the big tires will also hamper performance.
Taller tires also affect the speedometer calibration. The speedometer will read slower than your actual speed. In modern trucks, the electronic speedometers are part of the whole computerized infrastructure of the truck, The speedometer can control transmission shift points, which affect engine speeds. With old bare-bones trucks, you could time yourself at speedometer check areas on the freeway and approximate true speeds. What was an inconvenience on old trucks can be a big problem on new ones.
Another potential problem with taller tires and lift kits is driveshaft lengths. As angles increase due to additional height, the driveshafts need to be longer to properly reach the differentials. These increased angles can also affect U-joints, which is one more example of how one thing leads to another.
Bigger tires can reach a point of diminishing benefits. Tires that are too tall can compromise stability. That necessitates stiffer springs. If the tires are super wide, that limits your ability to traverse narrow trails. Realistically, 35-inch-tall tires are a reasonable limit for most street-driven trucks (33-inch tires are even more realistic). A 33- or 35-inch off-road radial tire gives ample ground clearance, shouldn't require extensive body modifications, won't cost a fortune, and will still look great.
The bottom line on choosing tire sizes for your truck is personal preference. You should understand the compromises and limitations inherent with different size tires. It also helps to separate your fantasies from your real-world needs, in the long run, you'll probably be happier with bigger rather than biggest.
Big Tire Mathematics
A little number crunching will help you determine the effective final drive ratio when you change tire sizes. Taller tires will hurt the acceleration and power of your truck. The taller the tire, the higher (lower numerically) the final gear ratio will be. Unless you want to run your truck on the Bonneville Salt Flats, you will need a gear change to regain the effective ratio of the original gears.
You can approach these calculations from two directions. You can determine the effective drive ratio, which is how your current gears will behave when you install the new tires. You can also determine the equivalent drive ratio. This tells you what gear ratio you need to get in order to maintain your original performance level.
EFFECTIVE FINAL DRIVE RATIO = CURRENT TIRE DIAMETER DIVIDED BY THE TALLER TIRE DIAMETER AND MULTIPLIED BY THE ORIGINAL GEAR RATIO.
EXAMPLE: Your current tires are 29 inches tall and you want to install 33-inch-tall tires on a truck with 3.50:1 gears.
29/33 = 0.8787
0.8787 x 3.50 = 3.0757
That means your current 3.50 gears will seem like 3.08 gears when you switch to 33-inch-tall tires.
EQUIVALENT FINAL DRIVE RATIO = NEW TALL TIRE DIAMETER DIVIDED BY CURRENT TIRE DIAMETER AND MULTIPLIED BY THE ORIGINAL GEAR RATIO.
EXAMPLE: Using the same 29-inch original tires and 33-inch new tires on a truck with 3.50 gears gives the following:
33/29 = 1.1379
1.1379 x 3.50 = 3.9827
That means you would need to install new gears with a 3.98-plus ratio to maintain your current performance level. You can't always get the exact ratio, so you need to use the closest one available. We recommend going to a lower gear (higher numerically), so in this example, we would suggest a 4.10 gear set. A slightly lower gear ratio will help move the added weight of the big wheels and tires.
Notice that the two equations are very similar. The difference is which diameter comes first.
The selection of aftermarket 4x4 tires is immense. Sometimes the number of choices can be overwhelming. Events geared to off-road enthusiasts often have mini tire stores right on the grounds. These stores often have great show specials.
Typical wheel flares are made out of plastic or fiberglass. That means they can be vulnerable to trees and other obstacles, but these flares are affordable and easy to replace.
Just because a wheel and tire combo will bolt on to your hubs doesn't mean there won't be body clearance problems. Extra lift or more fender clearance can help you avoid bashed bodywork like this.
Tire width is a big consideration when selecting larger tires, Wider tires have more floatation ability. These tires are mounted on wide wheels, which makes them protrude from the wheelwells. They're an invitation to a mud bath.
Running the original wheels and tires after a vehicle has been lifted for big tires looks pretty odd. If the gears were changed to compensate for the taller tires, returning to short tires is like installing very low (high numerically) gears.
Going to bigger tires involves much more that choosing your favorite tire. Body lifting and suspension upgrades are often part of the deal.
Multiterrain tires are one of the best bets for trucks that are primarily street driven. The BFGoodrich radial All-Terrain T/A is one of the best known examples. Multi-terrain radials are much quieter than aggressive tread patterns.
There just isn't adequate clearance here, witnessed by the damage to the back of the wheelwell opening. Besides harming the body, situations like this can damage the tires.
Automotive swap meets can be a good source for affordable used wheels and tires. Be cautious though, because these tires are sold "as is" without any warranty. Our first question is, why did the seller quit using these tires?
Most tire companies offer several variations of their 4x4 tires. The BFGoodrich radial Mud-Terrain T/A is a more aggressive version of the All-Terrain T/A. More tread goes around to the sidewalls far increased traction in mud.
A set of custom wheels and some tall, aggressive tires are the easiest way to make any truck stand out from the crowd of stockers. Tires like this let people know that you're a serious off-roader, not some status-seeking yuppie in a luxury SUV.
By Bruce Caldwell of Off-Road Magazine
- August, 1999 Issue
[ July 09, 2001: Message edited by: Turbo Steve ]