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BACKYARD MECHANIC: Choose enough vehicle to safely tow trailer

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Manufacturers set the towing capacity considering everything from frame strength to axle size.


2015 Dodge Journey CVP/SE Plus for $19,990

2015 Dodge Journey CVP/SE Plus

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QUESTION: I purchased a small travel trailer and require 4,000-pounds towing capacity. Do you think the Nissan Frontier rated at 6,000 lb. would be adequate to tow my trailer through the mountains of British Columbia? Would a rule of thumb be to use 70 to 80 per cent of any vehicles rated towing capacity? I don't want my motor screaming all the time while towing. Thanks for any advise you can give me.

ANSWER: Towing capacity is based on several design factors of the vehicle and there appears to be no standard for measuring it. Braking capacity may be used to set the limit of towing capacity of some vehicles. The ability to start a load up a steep incline from a stop may be used to determine the towing capacity of other vehicles. Manufacturers set the towing capacity considering everything from frame strength to axle size.

When looking for a towing vehicle, look at the vehicle towing capacity. The bigger the capacity, the easier it will pull the same load. For example, a truck rated at 9,000 lb. capacity will have more reserve to pull your 4,000-lb. trailer than the Frontier.

Engine torque is another spec to look at. Two engines may have the same torque output, but if one develops maximum torque at 4,000 r.p.m. and the other at 3,000 r.p.m., the engine that develops torque at the lower r.p.m. will pull better because most of the time is spent driving at lower r.p.m. Turbocharged and diesel engines produce peak torque starting at low r.p.m. so that is why they are usually good for towing. There is no general rule of thumb to use 70 to 80 per cent of rated towing capacity.

Another factor to look at is the profile of the trailer. A tall, boxy house trailer creates more wind resistance than a low fold-up tent trailer and requires more pulling torque.

Would the Nissan pull a 4,000-lb. trailer? Yes, it's rated to pull it. You may have to use lower gears, though, to pull through the mountains. Big highway trucks may have 13- or 15-speed transmissions so they can use the right gear to keep the engine r.p.m. running at peak torque. Depending on the steepness of the hill, you will likely have to use lower gears in the transmission and engine r.p.m. will rise. If r.p.m. is too high, then slowing the vehicle speed is the best way to drive. Operating the engine at a higher r.p.m. such as at 3,000-4,000 r.p.m. for an engine with a 6,000-rpm redline is easier on the engine than lugging it too slow below 2,000 r.p.m.

Finally, you say you need a 4,000-lb. towing capacity. Does that include the weight of items you put in the trailer, such as drinking water, waste water, clothing, food, etc? These items can add up quickly, so your 4,000-lb. trailer can suddenly be a 5,500-lb. trailer!

QUESTION: I purchased a Dodge Journey. It had 19-inch tires and when winter came there were no 19-inch snow tires available for sale. My dealer sold me 17-inch snow tires and it meant also buying 17-inch rims. My question is: when I am travelling at 100 kph as shown on my speedometer, am I actually travelling less than 100 kph or more than 100 kph as shown on my speedometer?

ANSWER: The original tire size on your Journey should be 225/55R19. These tires would rotate approximately 701.6 revolutions per mile. If the correct 17-inch tire was selected, such as a 225/65R17 that rotates at 707.3 revolutions per mile, the speedometer would read virtually correct.

Selecting a replacement tire with the same outer circumference regardless of wheel size is the correct way, and this is probably what your dealer has done. If you had tires with a smaller circumference, such as 225/55R17 size that rotates at 754 revolutions per mile, the speedometer would read higher that the vehicle is actually going by about four miles per hour (about seven kph).

Look up the revolutions per mile for your 17-inch tires on the Internet and compare it to the rotation of the original tires to determine your speedometer reading change.

Jim Kerr is an experienced mechanic, instructor of automotive technology, freelance journalist and member of the Automobile Journalists' Association of Canada.