Using a lower gear increases torque to the wheels and reduces the load on the engine. With the engine not working as
hard, the driver can apply less throttle.
Why are some vehicles rated to tow 6,000 pounds, some much more and some vehicles rated only at 1,000 pounds capacity? Contrary to what most owners believe, it has very little to do with maximum horsepower.
Motor vehicles have more than enough power to pull a trailer. Even the smallest engines can do it if the driver uses low gear. I have even seen a motorcycle tow a tandem axle U-haul trailer, which is a very foolish thing to attempt.
Some drivers think that the transmission determines how much a vehicle can tow. Manual transmissions are sometimes thought to be better than automatic transmissions, because the torque converter in an automatic transmission produces a lot of heat when it is working. Lock up converters are used in virtually every automatic transmission these days, so as long as the converter is locked, it doesn’t produce any more heat than would be generated in a manual transmission. Therefore either will work for towing.
So what does determine how much a vehicle can tow? I had the opportunity to talk with a couple Dodge truck engineers while road testing the Dodge Dakota pickup. This vehicle, even when equipped with a V6 engine, has a towing capacity that matches many full size pickups. The engineers tell me that they base towing capacity on the ability of the vehicle to tow a load from a stop up an incline that a driver might expect to find on a steep mountain road. In other words, it isn’t horsepower that is needed — it is low-RPM torque.
A seven per cent grade is about the maximum grade you will find on most major highways anywhere in North America. For Dodge, the maximum load a vehicle can get moving up that grade determines the towing capacity. Automatic transmissions are better able to get a heavy load moving due to the torque multiplication provided by the torque converter. Therefore, if a vehicle is only offered with an automatic transmission, it may have a higher towing capacity than one that also has a standard transmission available.
Many factors determine how much low-RPM torque is put to the ground. Drive axle gearing is often different on vehicles that tow. Installing a 4.10 to 1 axle ratio instead of a 3.70 to 1 ratio increases torque to the wheels by about 10 per cent. Changing tire sizes also makes a difference. Larger diameter tires reduce torque, while smaller diameter tires will increase torque. Most drivers don’t change axle gear ratios or tire sizes when towing. Instead, they change the transmission gear they drive in.
Both manual and automatic transmissions are built with overdrive gear ratios in their top gear, and sometimes in the top two gears. An overdrive gear ratio turns the driveshaft faster than the engine crankshaft. With engine RPM lower,
fuel economy is usually better and engine noise and wear are lower. However, an overdrive transmission gear ratio reduces torque output to the wheels. Using a lower gear is usually recommended for towing.
With five or six speed automatics
or manual transmissions, using fourth gear is often recommended for towing heavier trailers. With four speed automatics, towing in third gear works better. Using a lower gear increases torque to the wheels and reduces the load on the engine. With the engine not working as hard, the driver can apply less throttle. Even though engine rpm will be higher, it will typically use less fuel than one that is under constant load.
Many current pickups and SUVs with automatic transmissions have a Tow/Haul mode. This modifies transmission oil pressures and shift speeds so the vehicle uses the lower gears more. On some vehicles, it also modifies transmission shifting so it remains in lower gears on downhill grades allowing engine braking to help the wheel brakes.
Once the engineers have determined what transmission gear ratios, axle ratios, tire sizes and engines they are using, they can determine the towing capacity. Now they match wheel bearing capacity, axle shaft strength, radiator cooling capacity, braking capability and even frame design and strength to the towing capacity. As you can see, there are many parts of a vehicle that contribute to the towing capacity. Just adding a bigger trailer hitch isn’t enough to make it capable of towing a heavier trailer, even though you may have the power to pull it.
Jim Kerr is an automobile writer based in Saskatoon.