Often overlooked, optional gear ratios play a key role in pickup truck performance. If the axle ratio is too tall, the truck may seem to lack power, especially when climbing hills. This may be corrected with a numerically higher axle ratio, often at the expense of unladen highway fuel economy.
Consider that with a 3.31 axle ratio, the driveshaft turns 3 1/3rd times for every revolution of the rear wheel. With a 4.30 ratio, it turns 4 1/3rd times for every revolution of the rear wheel. The numerically higher gear ratio (in this case, 4.30) increases wheel torque and improves acceleration from low speeds.
But selecting the best gear ratio can be tricky. Every engine includes a sweet spot in its power curve between peak power and peak torque. You want the transmission gearing and axle gear ratio to stay in this sweet spot while hauling the payload at the desired speed.
Also consider that the axle ratio is only a portion of the overall pickup gearing. Manufacturers spend a lot of effort on powertrain matching. "Powertrain matching is really a systems approach," says Dennis Slevin, vehicle engineering manager, Ford Commercial Vehicles. "It is the engine, transmission, axles and tires working together. It is the effective ratio of engine power transmitted all of the way through the drive system to the ground."
He adds, "From our perspective there are interconnected reasons to choose optional axle ratios -- if you are hauling heavy loads with large trailers in hilly terrain, and you do a high frequency of towing." Consider a PowerStroke 6.7-liter diesel that offers a choice of a 3.31, 3.55, 3.73 or 4.30 axle ratio. "The 3.31 might be capable of towing the trailer. But if you are towing the majority of the time, you probably want the 3.55 because it will tow better and provide better fuel economy loaded."
Fuel Economy vs. Towing Efficiency
Start with the total combined weight of the vehicle. Then consider the environment.
"If you drive the vehicle on flat ground in Iowa throughout its life, axle ratio is not as critical as pulling the same load up the hills in Denver," comments Greg Martuch, vehicle development engineer, General Motors. "The axle ratio really starts to show when you get into grades."
Note that pickup tow and haul ratings are listed by axle ratio. "The vehicle's tow rating vs. what the customer plans on towing is the best way to determine which gear ratio you should select," says David Williams, truck marketing manager, Toyota.
"The consumer can tailor the vehicle's driveline to meet their particular needs," he notes. "If you will never have more than 1,000 lbs. in the back of the truck, you would want the highest ratio (3.73 instead of 4.30) for improved fuel economy because you will not need the torque. However, if you are towing a 9,000-lb. trailer, you will need that extra torque to get the trailer moving, especially up steep grades (4.30 instead of a 3.73)."
Fuel economy (higher rpms translate to increased fuel use), the weight to be towed or hauled (you want the engine to stay at an efficient speed on hills), roads traveled (steep grades vs. flat highway) and expected speed are all factors to consider when choosing the correct ratio, says Nick Cappa, engineering and technology PR, Chrysler Group. "It really depends on the rpm at which you want to drive," he adds. "You may choose a ratio that keeps the engine at the lowest rpm for fuel economy. But if you are not traveling fast enough on the highway, it will lug the engine and prove to be less efficient. You want to keep the engine at the most efficient rpm for a set speed, no matter what the load."
Cappa advises, "Consider the top speed at which you want to drive, the fuel economy you wish to achieve at that highway speed and the load you intend to tow or haul. For example, if you wish to tow 10,000 lbs. at 55 mph on flat highway, a mid gear ratio will keep the engine rpms up enough to maintain power with efficiency. If you want to drive at a high rate of speed, go with a smaller numerical gear ratio to keep the rpms as low as possible."