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Simply put, road grade is the amount of rise or drop over a given distance. A five-percent grade means over 100 feet, the road will rise or fall five feet. In real life terms, a sign reading, "5% downgrade next 4 miles" indicates that over four miles you'll lose 1,056 feet in altitude over the four miles of run. Here's the math: 5,280 feet (per mile) times four miles = 21,120 feet x .05 (five-percent grade) = 1,056.
In practice, this is something you really should know about. Going up a long, steep grade can lead to overheating your engine and transmission. Heading down a long, steep grade requires preparation: An RV, heavier than most automobiles and trucks, must be kept in control. "Brake fade," resulting from overuse of brakes can lead to an out-of-control situation. Being aware of your rig's handling on a grade is an important part of safe RVing.
So what's a steep grade? Grades are typically marked when they reach 5% or more. On the US Interstate Highway system grades are not allowed to be over 6%; on other roads and highways there is no limit. RVers generally agree that the longer the grade the greater the concern. We've been over short-length double-digit grades that gave us no trouble, but even a five-percent grade can be worrisome if it goes on for miles and you or your vehicle are not prepared for it.
How do get ready for a steep grade? Going uphill keep an eye on your engine comfort. If you're dealing with a long grade you may need to switch off your air conditioner to keep your engine cool. Watch your temperature gauge and--if you have one--your transmission temperature gauge. If things start heating up back off the throttle and down shift. The same is true if your engine begins to lug, drop down a gear.
Going down a steep grade means keeping your rig under control. The old trucker's adage, "You can come down the hill too slow many times, but you can come down the hill too fast only once," applies well to RVing. It's much easier to start out at the top of the grade slower than you "think" you should--once you build up downhill momentum things can get out of hand very fast. The rule of thumb says whatever gear you required to come up the pass is the one (or one gear lower) you'll need to head back down. Beware, diesel engines don't have nearly the compression braking of a gas engine.
Ideally the gear you choose for the downhill run should "hold" your rig at a comfortable speed, not allowing it to gallop away. Some truck drivers advise the use of aggressive braking: Keep the vehicle under control with the proper gear and figure a "safe" speed. When the rig hits the safe speed, bear down hard on the brake pedal and reduce speed by five miles per hour. Get off the brakes and hit them again when the safe speed is reached. NEVER ride your brakes--it's a sure way to overheat them and lose braking power.
Don't let those yellow grade signs throw you into a tizzy. Recognize what they mean, prepare yourself to drive them appropriately and enjoy the scenery!