Just what does a “grade” sign mean?


By Russ and Tiña De Maris

Get out of the flat country and hit the hills, sooner or later you’ll find a sign warning of a “steep grade,” often associated with a percentage number. Road grades seem mysterious at first, but really are simple.

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.

Should you be concerned about steep grades? For RVers, road grades are extremely important. 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.

grade sign

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.

photo: jodastephan on flickr.com

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Greg Illes

Great discussion Russ, but I have some questions:

1. “highway systems grades not allowed over 6%” ? I’ve been on plenty of highways that are way over that number. Utah state 12 comes to mind at 14%. Which highways are you talking about, only Interstates?

2. Many of the grade-ratings I’ve seen are extremely over-stated. Do you think that the number on the signage is perhaps the maximum grade within the distance? For example, “6% grade next 8 miles” would result in a half-mile drop in elevation (2600 ft), which rarely if ever actually occurs.

Phil Atterbery

Most of our travels are in the western states. I’ve come to rely on the altimeter function of my Garmin to indicate the start of a grade. While climbing a grade I monitor the engine temp indicator very closely. With my 300hp engine I find 2250 rpm in 3rd gear usually works well on a 5% grade.
Exhaust braking with the variable vane turbo is a wheel brake saver. The best approach to handling a grade is slow down.

Ken Wolf

I think you miss-spoke. Gas engines have less engine compression braking than diesel engines. Other than that… a fine piece of writing.