Wednesday, February 2, 2011

About Your Tires



Rub a piece of chalk over any markings that are difficult to see/read; the chalk makes the letters stand out and doesn’t hurt the tire. Sidewall reading “how-to” for passenger car and light truck tires:

  • LT” = “light truck”, but not all “light truck” tires are appropriate for RV use!

  • “225/75R/16” is the tire size on the Ford E-450 chassis. “225” is the tire’s width in millimeters. ”75” is the “aspect ratio”, i.e., the relationship of the sidewall height to the tire width; the sidewall height is 75% of the tire width. “R” indicates a “radial” tire. “16” is the diameter – inches – of the wheel (rim).

  • “Load range” relates to the maximum load an individual tire can carry; on LT tires, a letter indicates the load range, e.g., “D” or “E”.

  • The maximum load capacity for the individual tire (stamped on the sidewall of the tire) is indicated as “X” pounds when the tire is used as a single, and “X” pounds when used as a double; the Xs aren’t the same weights!

  • The tire’s maximum cold pressure is indicated near the maximum per-tire load ratings.

  • Tire markings vary according to the manufacturer, but the tire’s description/specs should include its intended use, e.g., commercial, “highway all-season”, etc.

  • The number and composition, i.e., polyester or steel, of belts under the tread and plies in the sidewall are (usually) indicated near the tire size marking. “E” load range tires should be “10-ply”.

  • The date of manufacture of the tire is located in one of the “DOT boxes”, small rectangles containing the tire’s identification, serial number, etc. Beginning with the year 2000, the manufacturing date is indicated by a four-digit number; the first two digits represent the week of manufacture, and the last two show the year of manufacture. For example, “3506” means that the tire was made during the 35th week of 2006.


ê The Rubber Manufacturers’ Association’s website offers information for all aspects of RV tire care and RV weights/weighing: (Or, use “TinyURL”:

ê Individual manufacturers’ web sites offer application, size and load rating information for their tires; examples:

ê and list many manufacturers’ tires according to size, load rating, application, costs, etc.
ê Look for the “certification” sticker on the door jamb of your LD; it contains information about tire size/type, recommended tire pressures, and tire weight information.

ê Use the recommended tire application, size and load rating for your RV.

Light Truck” is the somewhat confusing tire designation used for tires that are the appropriate size and application, i.e., designed for “commercial” or “RV” use, for your E-350/450 chassis. These tires are usually made of better grades of rubber, contain more UV and ozone protectants, and may have additional plies of polyester (or another material) in the sidewalls, and/or extra belts under the tread; these are usually steel “mesh”.

Use the appropriately-sized tire for your wheel. Using even a slightly wider tire than the 225/75R/16, i.e., a 2/5” larger 235/75R/16, reduces the amount of space between the duals. Less space means excess heat build-up and possible “blistering” (and weakening) of the tires’ sidewalls. Also, “non-spec” space between duals leads to possible contact between the duals when the tires flex during normal use; contact can cause the tires to “scrub” and weaken the sidewall. (And, the wider tire may not form a thorough seal when “seated”.) Never install a 16.5” (bead width) tire on a 16” rim, or vice versa.

Load rating “E” is the appropriate choice for the E-450 Ford chassis. All-steel, i.e., steel mesh belting and sidewall reinforcement is the best, but the choices are limited. These currently-available all-steel, i.e., steel belts under the tread and steel-reinforced sidewalls, are the choices for tire replacement for 225/75R/16 E tires:

Michelin XPS “rib”
Goodyear G947RSS (M+S tread), G949RSA (“highway” tread)

Currently, Michelin, Goodyear, Bridgestone, Kumho, Toyo, and B.F. Goodrich are among manufacturers who offer at least one LT/225/75R/16 E-load range tires that are appropriate for use on the E-450 chassis. Check each manufacturer’s website for tire choices of the appropriate size, load rating, and application; availability of tire models and applications changes frequently!

ê The most common causes of tire failures are overloading and underinflation.

Weigh your coach, loaded and ready for the road, on all four “axle corners”; the weight (of each corner of) your coach determines the inflation pressure(s) of your tires.

All tires on the same axle should be inflated to the same pressure; if there is a weight discrepancy between the right and left side of the RV, air the tires to accommodate the higher weight.

Don’t exceed the maximum cold tire pressure, stamped on the tire sidewall, or the GVWR of your coach.

ê Tire manufacturers’ web sites offer load and inflation tables for their tires. For example, see: (PDFs available for download; load/inflation tables and more)

Note: The “M+S” tire tread designation means that the tread grooves are wide and begin close to the outside edge of the tire; the wide tread grooves allow mud and snow to be “thrown off” more easily than from a narrow-grooved “highway” tire. M+S tires are not true “snow tires”; they are classified as “all-season” tires, suitable for a variety of road applications.


ê Inspect your tires regularly; don’t forget the inside duals! Check the sidewalls for cracking, “checking”, bulges, tears or (excessively) scuffed areas. (Also check the tire bead area for any gouges or shredded or missing rubber.) Check the tread for tread depth, uneven wear, rocks or other “road hazard debris”, and any cuts or areas of missing rubber.

ê Tires deteriorate much more quickly when exposed to UV light and ozone in the air; keep tires covered while the RV “sits” in storage or at a campsite.

ê Do not use “tire dressings” or vinyl/rubber treatments containing petroleum distillates, e.g., Armor All, on your tires or any vinyl or rubber surface; using these products can actually increase the rate of deterioration. (“303 Aerospace Protectant” is the good stuff!

ê RV tires should be replaced when the tread depth reaches 1/8” or the tires are over 5-6 years old, even if the mileage is low and the tread still appears “good”. (Time and exposure can deteriorate tires faster than use.)

ê Consider installing long, specially configured brass valves; these valves make checking tire pressure and airing the tires much easier! Two sources of “extended” valves are: and

ê Carry a reliable, accurate ‘truck/RV’ tire pressure gauge that registers tire pressures up to about 100 pounds. The ‘pencil’ types are generally less accurate than ‘numbered’ or digital read-out gauges. Make sure that the gauge head is deep enough to allow a complete “seal” around the tire valve when you check the tire pressure. Don’t rely on the gas station or even tire shop gauges that are attached to the air hose; these are notoriously inaccurate. and are two sources of gauges. Make sure that the gauge selected is for truck/RV use, i.e.; the gauge needs to measure high enough pressures and be of a configuration that allows you to reach the valves to check the air pressure. Don’t skimp on quality.

Besides the gauge, other handy “tire tools” are: a valve core tool, extra valve caps, an old (dull) screwdriver to pick rocks out of the tread, and a depth gauge measure.

ê Check your tires’ air pressure before each day’s driving; the tires have to be ‘cold’ for an accurate reading. (“Eyeballing” doesn’t substitute for using the gauge!)

Always use valve caps to keep dirt, debris, and moisture out of the tire valves; these can damage the valve core and cause a slow leak.

ê Depending on speed, ambient temperature, and length of driving time, a tire can inflate to several pounds over its cold inflation (even exceeding the maximum) pressure. This is normal; never ‘bleed’ air from a hot tire.

ê Carrying a small 120V air compressor makes airing the tires so much easier!

ê If you use wooden or plastic “blocks” for leveling your coach, make sure that the blocks are wide enough and long enough to support the entire footprint of the tire!

Always block both rear duals when leveling the coach; never block just the outside dual.

ê Air the spare to the maximum pressure listed on the sidewall.

(Illustrations and text from

Note: The diagrams and accompanying information below are no longer available on the Justtires website!

Wheel Alignment Terms

Wheel alignment refers to how flat your tires sit on the ground, and how straight they are pointed down the road. In technical terms, your vehicle’s wheel alignment can be measured in terms of (1) camber, (2) toe, and (3) caster.

Camber refers to how flat your tires sit on the ground. Over time your vehicle’s suspension gets weaker and your vehicle actually begins to slowly sag. As this happens your tires can start to ride on the inside or outside edge. This is referred to as positive and negative camber. The result of positive and negative camber is usually very fast tire wear on the edge of the tire. It can also result in poor steering and handling.

Toe refers to how straight your wheels are pointed down the road. Wheels that point slightly in towards each other have "toe in." Wheels that point slightly out away from each other have "toe out." When your wheels have significant positive or negative toe, your tires are literally dragged down the road. The result is often fast tire wear, or uneven tire wear which results in tire noise.

Caster is the orientation of the wheel on the axle. Imagine standing to the side of your vehicle and looking at the wheel and its position within the fender or wheel-well. If your wheel was pushed towards the front of the wheel-well it would have positive caster. If your wheel was pushed towards the back of the wheel-well it would have negative caster. Your vehicle was designed with a specific caster. As the caster changes your vehicle’s steering and handling can be affected.

Tire Wear Guide
The following Tire Wear Guide will help you identify the causes and solutions of most common tire wear patterns.
Wear Pattern
Center Wear
Over Inflation
Adjust pressure to particular load per tire catalog
Edge Wear
Under Inflation
Adjust pressure to particular load per tire catalog
Side Wear
Loss of camber or overloading
Make sure load doesn't exceed axle rating. Correction is 3/4 - 1 degree positive camber (top of wheel rim 3/16" further out than bottom.
Toe Wear
Incorrect toe-in
Correct toe-in is 0 - 1/2 degree.
Check bearing adjustment and balance tires.
Flat Spots
Wheel lockup & tire skidding
Avoid sudden stops when possible and adjust brakes.

ê It may be advisable to rotate front tires every 5000 miles or so, but if the tires on your vehicle are wearing evenly, i.e., if all wheels are balanced and in correct alignment, rotating rear duals may not be necessary. If you choose to rotate all the tires, follow the 6-wheel rotation pattern pictured on the Rubber Manufacturers’ web site or the recommendation from your tires’ manufacturer.

Informative video by Michelin on tires, leveling and weighing.

Contributor: JC Taylor