Tire Size Lawsby Richard Rowe
There is a common misconception about what type of guidelines are used to regulate tire size on modified off-road trucks. Most people mistakenly believe that the old federal system of maximum tire diameter is still universally enforced, but the newest system of enforcement has more to do with overall vehicle measurements and ground clearances, and varies from state to state.
The most universal laws apply to bumper height, which is of course affected by tire size, but varies by the amount of lift engineered into the suspension. States regulate bumper height to ensure that lifted off-road trucks do not have bumpers so high that they hit a car's windows instead of side panels or trunk lid in the event of a crash. States have many different ways of establishing appropriate bumper height.
Equally as important as bumper height is headlight height. After all, you can have the brightest headlights in the world, but if no one can see you in their rear view mirrors because your lights are 9 feet off the ground, then your headlights do little good. Most states limit headlight height to around 52 inches, but other states like Montana will allow as much as 72 inches.
One of the few universal tire restrictions on the books relates to tire width. The tires themselves can be as wide as you like, but almost no state allows the tires to stick out further than the vehicle's fenders. One state that allows this is Kentucky, where you could practically slap a license plate on Big Foot and drive it. This law can be fairly easily subverted in most cases by purchasing and installing aftermarket fender flares wide enough to cover the tires.
Like tractor trailers, pick-up trucks must also be able to fit under bridges on the freeway. Most states will give you 13 feet, 5 inches without a permit, which will allow you to clear any bridge on the interstate. Bear in mind, though, that roads designated as parkways (such as those found in New Jersey) often have bridge heights as low as 12 feet, and older cities like Chicago and Philadelphia have a great number of 10-foot bridges within city limits.
Increased tire size most often requires an increase in body height to accommodate the tires. Many states like New Jersey, Mississippi and Georgia have laws in place stating that a vehicle cannot be modified to go above or below 4 to 8 inches from the stock ride height, regardless of tire size.
Many states will allow larger vehicles to have larger tires and taller ride heights, which is usually dictated by the truck's gross vehicle weight. For instance, Ohio allows only a 24-inch front bumper height for those under 4,500 lbs, 26 inches for 4,501 to 7,500 lbs and 31 inches for vehicles over 10,000 lbs.
Richard Rowe has been writing professionally since 2007, specializing in automotive topics. He has worked as a tractor-trailer driver and mechanic, a rigger at a fire engine factory and as a race-car driver and builder. Rowe studied engineering, philosophy and American literature at Central Florida Community College.