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A Look at Fuel Economy

With no gas price relief in sight, motorists are looking for ways to stretch their fuel dollars.

As many motorists have noticed, the fuel economy numbers posted on new vehicle window stickers rarely reflect what is received after purchase.

Unleaded gasoline prices are currently averaging $2.20 a gallon, and more than half of Americans say the high prices are starting to cause them problems. According to a recent Associated Press-AOL poll, 58 percent of motorists have reduced their driving, 57 percent have cut back on other expenses, 41 percent have changed vacation plans and 41 percent are seriously considering purchasing a more fuel-efficient vehicle.

Vehicles used for towing are notorious for receiving poor fuel economy, and as many motorists have noticed, the fuel economy numbers posted on new vehicle window stickers rarely reflect what is received after purchase. According to Greg Dana, vice president of Environmental Affairs for the Alliance of Auto Manufacturers, rather than being derived from road tests, fuel economy numbers are based on testing performed on a dynamometer in a laboratory environment. Because pump gasoline from different suppliers and batches varies considerably, a special 96-octane gasoline called Indolene is used to ensure consistent test results. According to Dana, the high octane rating does not affect fuel economy.

Fuel economy ratings are determined by special computer sensors attached to the tailpipe. By measuring the weight of carbon exiting the tailpipe, the sensors are able to determine how much fuel was consumed during the test period.

During the test drive, the dyno is calibrated to flat conditions and the driver modulates the throttle according to specific driving conditions determined by the Environmental Protection Agency (EPA). For the 11-mile city driving test, the vehicle is driven for 31 minutes at an average speed of 21 MPH. Ambient temperatures are between 68 and 86 degrees F, and the test includes 23 stops and a 10-minute “heat soak” period where the vehicle is stopped and the engine is shut off. Six minutes are spent idling, and there are two one-minute segments where the vehicle speeds up to 50 MPH.

The highway driving test is a 13-minute, 10.4 mile trip at an average speed of 48 MPH and a top speed of 59.9 MPH. Ambient temperatures are the same as the city test and there are no stops. Road conditions imitate gently
rolling hills and a combination of rural and interstate roads.

The EPA determined in 1985 that drivers in real world conditions receive about 90 percent of the city fuel economy figure and 78 percent of the highway figure. Thus, the final city value is determined by multiplying by 0.90 and the final highway value is determined by multiplying by 0.78.

“Once the computer delivers the raw EPA MPG numbers they are ‘adjusted’ by those multipliers to closer emulate realworld driving,” says Dana. “Those adjusted numbers are what appear on the window sticker.”

Small changes in driving habits, road conditions and weather all have noticeable effects on fuel economy. “A driver may not notice it, but a vehicle’s fuel economy drops on cold days because the air is denser and it takes more energy to push the vehicle through the air than on a warm, dry day,” explains Dana.

Other factors that affect fuel economy are use of the air conditioner, a heavy load on the vehicle’s electrical system, engine condition, tire pressure, size and shape of the vehicle and the load being carried or towed.

Lower axle-gear ratios improve pulling performance by providing increased low speed torque and improving acceleration. Though it is a common belief that lower axle gear ratios have a negative effect on fuel economy, the effect is minimal at worst.

According to Roger Clark, senior manager for General Motors’ Energy Integration and Fuel Economy Learning Vehicles Program (ELVP), fuel economy doesn’t change much with an optional gear ratio. “The typical combined fuel-economy impact, based on EPA lab test conditions, is about 0.4 MPG to 0.6 MPG between the base gear ratio (e.g. 3.42:1) and the lowest (e.g. 4.10:1) offered,” says Clark. “Thatchange is linear. Equipping a truck or SUV with a 3.73 gear ratio, for instance, would affect combined fuel economy by less than 1/4-mile-per-gallon.”

According to Clark, these fuel economy changes may not even be detectible during city driving. Lower axle ratios have more of an effect during long distance freeway driving. “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 driven in a mix of city and highway conditions,” says Clark.

“The reason we (truck manufacturers) offer the 3.42:1 and 3.55:1 ratios is those ratios offer the best fuel economy with a four-speed automatic transmission,” says Clark. “If you want a truck that responds best to hauling a heavy load or towing a trailer, then 3.73:1, 3.92:1 or 4.10:1 ratios will provide the best wheel torque at the lower engine speeds.”

EPA research conducted during the highway fuel economy test indicates that 54 percent of a pickup or SUV’s engine power is used to overcome aerodynamic drag. “The best fuel economy for the typical truck or SUV is cruising right around 40 MPH,” says Clark. “The EPA test cycle for the highway fuel economy number averages 48 MPH with a top speed of 60 MPH.”

In a pickup rated at 21 MPG on the highway, a 10 percent increase in speed over 48 MPH (53 MPH) will decrease fuel economy by about 1.5 MPG. An average speed of 60 MPH causes fuel economy to drop another 1.5 MPG. Increasing speed to 70 MPH brings fuel economy to less than 14 MPG, a 33 percent drop from the previous 21 MPG.

Aerodynamic and fuel economy research performed at the Lawrence Livermore National Laboratory and Michigan Technological University indicates that the added weight and wind drag associated with towing a trailer forces an engine to work harder and consume more fuel.

“If you increase driving speed from 50 MPH to 70 MPH, you double the drag force and increase the engine power required to overcome drag by almost three times,” says Scott L. Post, Assistant Professor of fluid mechanics at Michigan Tech University.

“Of course, not all of the tow vehicle’s engine power goes to overcome wind resistance,” says Post. “A rough estimate is that for cruising on the highway with no hills, about half of the engine power goes to overcoming drag and the remainder goes to overcoming rolling resistance.

“When you add a trailer, you add more wind resistance and more weight, which increases the rolling resistance. But the overall ratio of drag/rolling resistance may not change much. So if you are doubling the drag and ground friction when hauling a trailer, you should expect the mileage to be cut nearly in half as a result.

“The total drag for the truck-and-trailer combination, however, will not equal the sum of these two components because the wake from the pick-up will reduce the drag on the front of the trailer,” explains Post.

The coefficient of drag (CD) for pickups is between 0.4 and 0.5 CD, while a trailer is around 0.9 CD. Post estimates total drag for the pickup/trailer combination to be 1.0 CD, but warns that driving in a strong crosswind cancels out much of the reduced drag resulting from the wake of the pickup.

“The exact amount of drag reduction is hard to estimate because it will depend on several factors,” says Post. “For instance, the higher the trailer, the greater the drag, and the longer the gap between the tow vehicle and the trailer, the less the wake effect and more drag.”

Post’s studies conclude that every 10 percent increase in drag results in 1 MPG decrease in fuel economy.

The use of AMSOIL synthetic motor oils is a proven way to improve fuel economy. Independent tests show the use of synthetic lubricants can increase fuel efficiency by two to five percent, while many AMSOIL customers report even larger gains.

Fuel Efficiency Tips:

The following tips from the EPA help motorists receive maximum fuel efficiency from tow vehicles:

1) Avoiding aggressive driving, including speeding and rapid acceleration and braking, can improve fuel economy by 5 to 33 percent.

2) Replacing a dirty air filter can improve fuel economy by up to 10 percent.

3) Repairing engines that are out of tune or have a defective oxygen sensor can improve fuel economy by 4 to 40 percent.

4) Properly inflated tires can improve fuel economy by up to 3.3 percent.