Selecting optional
axle ratios when buying a new truck not as bad on fuel economy as some imagine

H
ang around a car lot for any amount of time and you will hear buyers automatically ask about a potential vehicle’s fuel economy. But after the purchase, the subject seldom arises in conversation as people come to realize that in real world driving, fuel economy is based on how hard a pickup or SUV has to work to pull the load.

That is where axle ratios come into play. This factory option is all too often passed over becaue new truck buyers mistakingly think that the "lower" ratio automatically mean poorer fuel economy.

So the majority of pickup and SUV owners buy the vehicle with the “standard” or base axle ratio, which is typically 3.31:1, 3.42:1 or 3.55:1. Such thinking is more likely to lead to dissatisfaction with the truck’s overall performance than its fuel economy.

Manufacturers offer optional axle ratios for one reason: to improve the vehicle’s pulling performance. The added benfit, however, is fuel economy doesn't take much of a hit--if any at all.

According to Roger Clark, senior manager for General Motors Integration and Fuel Economy Learning Vehicles Program (FELVP), which handles EPA testing of all GM trucks and SUVs, "fuel economy may not change at all by going to an optional axle ratio.

“The typical combined fuel economy impact, based on EPA lab test conditions, is about .4mpg to .6mpg between the base gear ratio and the lowest (4.10) offered. That change is linear. Equipping a truck or SUV with a 3.73 gear ratio, for instance, would affect combined fuel economy by less than a quarter-mile-per-gallon.”

Those fractions of a mile-per-gallon will never be noticed by an owner. Further more, Clark says that in the real world, choosing a lower gear ratio may not even show up in city driving fuel economy. It’s the steady-state, long-distance freeway trips where the lower axle ratios have the most affect on fuel economy.

“The reason we [GM] offers the 3.42 and 3.55 ratios in our pickups and SUVs is those ratios offer the best fuel economy with a four-speed automatic transmission. If you want a truck that responds best to a heavy load or towing a trailer, then 3.73, 3.92, or 4.10 ratios will provide the best wheel torque at the lower engine speeds.”

A lower (numerically higher) gear ratio provides more low-speed wheel torque, which makes it easier to get the vehicle moving when pulling a trailer or carrying a load of passengers and cargo. Lower gears also improve acceleration up to about 60mph. (Passing performance and speeds above 60mph is a matter of horsepower, not axle ratio.)

As for general fuel economy, Clark says, “Sure, most of the axle ratio changes are going to have a cumulative effect in fuel economy over time, but it’s going to be very hard to notice any difference in fuel economy where the vehicle is being driving in a mix of city and highway conditions.”

The reason is aerodynamic drag from the truck’s frontal area, not the gear ratio, is the major contributing factor in fuel economy.

According to the EPA, during the highway test cycle where the average speed is 48 mph, 54% of the engine’s power is used to overcome aerodynamic drag. Drive faster and the engine has to work even harder to push through the air and consumes more fuel in doing so.

“The best fuel economy for the typical truck or SUV is cruising right around 40mph. The EPA test cycle for the highway fuel economy number averages 48mph with a top speed of 60mph,” explains Clark.

“A good example of how drag affects fuel economy is a truck that has a 21mpg highway EPA number. Drive at a 10-percent higher average speed (53mph) and drag causes fuel economy to fall about 1.5mpg. Average 60mph and mileage will drop another 1.5mpg. Run just above70mph and now fuel economy is less than 14mpg instead of 21mpg.”

So, if fuel economy is of a real concern, watch your speed. If performance is your goal, watch the axle ratio you buy.---Bruce W. Smith