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Understanding Axle Ratios

Bruce Smith April 21, 2013

AXLE RATIOS: LOW GEARS, HIGH GAINS

Choosing an optional axle ratio when buying a new pickup/SUV is a benefit, not a liability

 

By Bruce W. Smith / ©2008 Editorial Services 

Whenever I wander through car dealership checking out the new pickups and SUVs I never fail to hear other tire kickers ask about the fuel economy of a tow vehicle they are potentially interested in buying.

Fuel economy is a very important topic, and in some instances is the deal-maker or breaker between one vehicle and another.

Ironically, after the purchase, the subject of fuel economy takes a back seat to overall performance and is seldom mentioned unless it’s to brag that your truck is better than the one whomever you are talking to drives.

One of the most important questions a new truck buyer should be asking is, “What axle ratio is in this truck?”

Fuel economy is an important issue when buying a new tow vehicle. But it doesn’t have the performance impact of having your pickup or SUV equipped with the right axle ratio.

Axle ratios play a huge role in a pickup or SUV’s overall performance—and the optional ratios offered by just about every manufacturer don’t have as much of an adverse effect on fuel economy as one might expect.

All too often a buyer of a new pickup or SUV will not even think about what axle ratio is in the vehicle.

2100 F350 Dually Big Trailer
The larger the trailer or load, the higher (numerically) the optimum axle ratio. Pickups towing big 5th wheel trailers do well equipped with 4.10:1 axle ratios.

The thinking behind the decision to stick with the base axle ratio, which is typically 3.08:1 or 3.42:1, is the ratio comes standard, so that’s what the vehicle manufacturer feels is the best setup.

Or, if they do inquire about a “lower” (numerically higher) axle ratio, they are told by the salesperson the other ratios offered by the manufacturer will drastically cut down the vehicle’s fuel economy numbers.

Both trains of thought are more likely to lead to dissatisfaction with the truck’s overall performance than doing much to hurt its overall fuel economy.

The truth is, vehicle manufacturers offer optional axle ratios for one reason: to improve the vehicle’s acceleration and towing performance.

AXLE RATIO FACTS ON FIGURES

A lower (numerically higher) gear ratio, such as 3.55:1 to 4.10:1, provides more low-speed wheel torque, which means it takes less throttle to get the vehicle and the load it’s carrying or towing moving.

More low-speed torque at the wheels is a welcome help when the fishing boat is heavy, the ramp steep, or there’s a slide-in camper or load in the pickup bed in addition to a trailer of one sort or another in-tow.

Dodge Dually water trailer
(Diesels, because of their massive low-rpm torque, can get by with 3.55:1 or 3.73:1 ratios if trailers weigh less than 8,000 pounds.)

Wth a lower axle ratio you can ease into the throttle and the tow vehicle will respond a lot quicker than it would with the “highway” gears that come as part of the standard equipment.

A lower axle ratio also means your vehicle will accelerate quicker when pulling a trailer or carrying a load of passengers and cargo.

Roll out from a stop light or merge into fast-moving highway traffic and you want a tow vehicle that gets up to speed quickly.

A truck equipped with a lower axle ratio will do just that.

It’s for those very reasons I always buy a new truck with one of the optional axle ratios, typically 3.73s or 3.92s, when the primary boat being towed is less than 21-foot. This is especially true of a vehicle equipped with a V-6 or small V-8.

If I were towing a bigger boat, say a 24-28-foot sport fisherman or center-console, or hauling equipment trailers weighing north of 7,500 pounds,I’d get the optional 4.10s axle ratio if available.

When towed weights get above 5,000 pounds, gasoline V-8s need all the pulling help available, and those 4.10s are just the right ticket. Aren’t I concerned about my tow vehicle’s fuel economy being hammered? No.

AFFECT OF AXLE RATIOS ON MPG

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.

“For typical combined city/highway and city (stop and go) driving), fuel economy is insensitive to final drive ratio,” says Clark.  Optional final drive ratios do have an impact on typical highway driving (steady state driving above 60 mph).”

Axle ratios play an important role in towing performance. Larger tires have a negative impact on fuel economy and efficiency if the axle gears are not changed accordingly.

Clark says on older model pickups with the 4-speed automatic, “The typical combined fuel economy impact, based on EPA lab test conditions, was 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.

“Today with the new 6-speed transmission we offer from a 3.08 to a 3.73 final drive ratio. The 3.73 final drive ratio is part of a heavy-duty towing package and comes with changes to tires and other elements.

“Moving from the 3.42 to the 3.08 final drive ratio the fuel economy in mixed driving can improve [mpg] by up to 1.0 mpg in typical highway driving, but won’t change much in city driving.  So the total impact in combined driving may be in the range of 0.0-0.5 mpg.,” explains Clark.

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.

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.)

Aerodynamics plays a big role in fuel economy at highway speeds. However, it’s engine technology that plays a big part overall.

“The reason final-drive ratio doesn’t have a big impact on our [GM] truck fuel economy is AFM – Active Fuel Management. This system shuts down half the cylinders when less power is needed and greatly improves fuel economy.,” says Clark.

Axle ratios and engine technology do a lot to influence the fuel mileage a pickup gets.

However, the driver has a much greater impact on mpg than anything related to the engine and drivetrain.

“Speed, acceleration, loading and maintenance all impact fuel economy more than final-drive ratio,” says Clark. “For example, driving 10mph faster on the highway can reduce fuel economy by up to 4mpg.”

FUEL ECONOMY KILLERS

  • Rapid Acceleration
  • Driving fast or against the wind
  • Low tire pressures
  • Driving in cold weather
  • Driving in hot, humid conditions
  • Operating the A/C or heater
  • Driving in hills or mountains
  • Stop-and-go driving
  • Towing or carrying a load
  • Dirty air filter
  • Poorly maintained engine
  • Front-end misalignment

The reason fuel economy doesn’t take a big hit when going to those optional axle ratios is there are many other factors involved in mpg.

For example, aerodynamic drag from the truck’s frontal area, weather conditions, and driving habits—not the gear ratio—are all major contributing factors in a vehicle’s fuel economy.

DRIVE SLOWER, BETTER MPG

According to the EPA, during the pre-2008 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.

(EPA fuel numbers in 2008 will drop about 10-15% across the board on new vhicles because they have raised both city and highway test speeds to better duplicate real-world driving.)

“A good example of how drag affects fuel economy,” explains Clark, “is a truck that has a 21mpg highway EPA number. Drive at a 10-percent higher average speed than what the EPA test cycle averages (55 mph) and drag causes fuel economy to fall about 1.5mpg.”

Drive 10mph faster and mileage can easily drop another mile-per-gallon. Run75mph and fuel economy in a bigger pickup or SUV can vary 4-5mpg from the optimum fuel mileage.

Put a trailer in-tow, drive a conservative 65mph, and fuel mileage will be cut by 30- to 40-percent because now your tow vehicle is fighting its own wind drag, that wind resistance of the trailer, and the extra weight of the trailer load.

The saving grace to all this is at least when your truck is running the axle ratio designed for bettering towing performance you don’t have to get as hard or deep into the throttle to get moving and to maintain speed. The lower axle ratio does a lot of the grunt work.

 

_BWS3027
If you plan on adding taller aftermarket tires, get the lowest gear ratio offered from the factory to offset them and keep your truck’s performance.

 

Then there’s the part of upgrading to taller tires.

If you know you are going to put on taller tires, say move up from a 31-inch tie to a 35-inch, order the 4.10 axle ratio from the factory ($50 option).

That way when you put on the 35s your effective axle ratio will be around 3.73:1–ideal for maximum engine performance towing or unladen.

Otherwise, you’ll either have a slug of a pickup after the tire upgrade– or you’ll end up having a gear shop put in 4.10s at the cost of about $500 per axle to get back he “lost” performance.

So, when you buy your next new truck, put a check mark next to the box that lists the optional axle ratio best suited for your driving needs and future upgrades.

I recommend 3.73:1,or 4.10:1 ratios for those who regularly tow heavy trailers–and as I said above, 4.10s for anyone going to larger tires.

Then when your new ride arrives, keep an eye on the speedometer and be aware of you’re your driving habits to maximize fuel economy. You’ll find, as I have, you are a lot more pleased with your new truck’s performance when those lower gears work their magic—trailer in-tow or not. – BWS

 

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