LIQUID WEIGHTS (pounds per...
LIQUID WEIGHTS
(pounds per gallon)
WATER: 8.3
GASOLINE: 5.6
DIESEL FUEL: 6.8
PROPANE: 4.2
True off-roaders sometimes get so caught up in their rigs' shoes that they back-burner the rubber that meets the road. Trailer tires are often an afterthought-until they fail. In other words, appropriate funds go toward Boggers, Krawlers, Claws... Whatever's left gets spent on tow-rig tires. One blowout easily undermines this thought process. Add a trailer and tow vehicle behind a blown-out tire, and things can get ugly.
The goal with this article is to determine proper tire pressures for tow vehicles, specifically motorhomes, and look at other aspects of safe load-carrying. (Many of the tips also apply to dualies and even to medium-duty tow rigs.)
Weighing In
To find out what you need, you first need to find out what you have. This involves weighing the RV. Just as being overweight taxes the human body's organs and joints, an overloaded tow vehicle takes its toll on tires and mechanical parts.
To find a scale, look in the Yellow Pages. Businesses that have certified scales include moving/storage companies, gravel pits, and truck stops. Call ahead to explain what you need and make an appointment. Load the RV with your normal rations of people, supplies, water, gas, propane, and gear, then attach your trailered load before going to the scales.
Plan on spending at least 30 minutes on the scales. You'll need to weigh the overall vehicle as well as its front and rear halves, each side, each corner, and the loaded trailer. Compare the scale's numbers to those on the vehicle's Federal Dataplate placard. Any corner that weighs more than half of its rated Gross Axle Weight Rating (GAWR) is overloaded-even if the sum of both sides is under the GAWR. Ideally, you should redistribute the load and reweigh until it's under the GVWR and allocated as even as possible. A balanced load makes steering and braking more predictable and also extends component life.
 First, know your vehicle's...  First, know your vehicle's weight ratings. According to the Federal Dataplate, this '80 Apollo motorhome has a 14,500-pound GVWR, a 5,000-pound front GAWR, and a 10,000-pound rear GAWR. (RVIA member manufacturers include a more-detailed placard.) |  To some extent, tires tell...  To some extent, tires tell you what they want. The sum of their individual load capacities should at least equal the vehicle's GVWR. If this isn't the case, upgrade to higher-load-rated tires. | |
Pressure
The double-whammy: loads that exceed the manufacturers' ratings combined with tires that don't have a high enough load rating. Assuming that total weight is under the GVWR, look at the tires. If their maximum load ratings (usually stamped into their sidewalls) don't exceed their respective corner's loaded weights, the tires should be replaced with higher-capacity ones.
Most wheelers know the importance of proper tire pressure. Still, a Bridgestone/Firestone sampling found that 80 percent of RVs have at least one underinflated tire-often an inner dual. As most people now know from extensive media coverage, underinflation increases friction, which generates more heat, which can eventually cause a blowout. Normal pressure loss is 1-2 psi a month.
Since the vehicle literally rides on air, pressure is critical. The higher the pressure, the heavier the load that can be supported. Proper pressure begins with the manufacturer's recommendation as listed on the placard in the vehicle. Never run less than this. Next, refer to the tire's sidewall for the maximum inflation pressure. Never exceed this-overinflated tires are more likely to be damaged by sudden impacts.
 Know your tires' age, especially...  Know your tires' age, especially if you bought your tow rig used and are still running the rubber it came with. The manufacturer's DOT code showed that our new Bridgestone R187s were built the 25th week of 2006. We bought them to replace tires that were born in 1997. |  Inboard dual tires don't get...  Inboard dual tires don't get the same visual attention as their outboard partners. This inner dual had severe cracking-not readily visible, but potentially deadly. | |
To narrow down the proper pressure, refer to the manufacturer's load and inflation table for your tires. This lists the minimum pressure needed for various load weights, for both single and dual tire applications. Verify that you can safely run high enough pressure to support your loaded vehicle-and that said pressure doesn't exceed your wheels' maximum-inflation recommendation. Also, every tire on an axle should have the same pressure. When in doubt about any of this, consult a good tire store or RV shop or refer to the sources listed at this article's end.
Now for the most important part: Check pressure at least weekly with the tires cold-three to four hours after driving or before driving a mile. Never rely on visual sidewall-bulge inspection. Always use a quality pressure gauge: double-angle models are easiest on RVs.