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Fuel economy

There are over 136,000,000 passenger cars registered in the United States, traveling an average of 12,000-18,000 miles (19,000-29,000km) per year. In total, private vehicles consume close to 90 billion gallons of gasoline each year, which is about 2/3 of the oil imported by the United States every year.

The federal government's goal is to reduce gasoline consumption progressively each year. Varieties of methods are either implemented or under serious consideration, all of them affecting your driving and the vehicles you drive. In addition to "down-sizing," the industry is using and investigating alternative engines, alternate fuels, smaller and lighter vehicles, and streamlining, to name a few, in an effort to meet the federally mandated Corporate Average Fuel Economy (CAFE) standards.

CAFE is not the same as mpg. Miles per gallon refers to the miles that any given vehicle will travel on a single gallon of fuel. The CAFE figure is a measure of average fuel consumption of a manufacturer's entire fleet. To determine the CAFE number, the manufacturer uses the EPA miles-per-gallon rating for each model in its line. Each manufacturer is assigned an EPA mileage figure based on a weighted (55/45) average of city and highway fuel economy numbers. This number will be somewhat higher than the official EPA rating because the EPA is only publishing city ratings now. Then a fleet, or corporate, average is computed. The mileage of each model contributes to the corporate average in proportion to the number of units sold of that model.

Consider a hypothetical case. A vehicle getting 20 mpg and a vehicle getting 40 mpg would average 30 mpg, using the mile-per-gallon formula. However, the CAFE formula is different. Let's assume that each vehicle is actually driven 100 miles (160km). The 20-mpg-vehicle would use 5 gallons of fuel to travel 100 miles (160km) and the 40-mpg-vehicle would use 2.5 gallons of fuel to go the same distance. Add the 5 and 2.5 gallons for a total of 7.5 gallons of fuel consumed by the two vehicles to travel a total of 200 miles (320km). Divide the 200 miles by 7.5 gallons and you arrive at a "fleet" average of 26.67 mpg, not 30 mpg, as you would get if you simply averaged the two vehicles' mpg ratings.

Using the CAFE method, not only are the gas mileage figures of each vehicle taken into account, but also the sales mix of each particular model. It is obvious that it takes more than one high-mileage vehicle to offset the sale of one low-mileage vehicle.

This brings us to the diesel. The fuel efficiency of the diesel is another way some manufacturers see to meet the CAFE requirements set down by the federal government. The stakes in the CAFE game are not small, either. If the manufacturer does not achieve the required CAFE figure, there are provisions that they could be fined $5 per vehicle for every one-tenth mile the manufacturer falls short.

Further federal regulation could possibly be avoided if just one gallon of gasoline per week could be saved for every automobile. This would amount to 8% of the government's goal of 10% reduction in fuel consumption.

There are three areas where the motorist can save on fuel-proper maintenance, efficient driving habits, and intelligent purchase of a vehicle.

Care and maintenance
See Figures 1 thru 7

Proper care and maintenance of your vehicle(s) will save you money and conserve gas. tune-ups and a regular maintenance program like the one in this book can save up to 20% in fuel.

Tests by the Champion Spark Plug Company showed a tune-up, on vehicles judged to be in need of one, increased fuel economy by over 11%. The same tests also revealed that of the vehicles checked, ¾ had maintenance deficiencies that adversely affected fuel economy, emissions or performance. A regular maintenance program should at least include:

  1. Change the oil and filter as recommended. Dirty oil is thick and causes extra friction between the moving parts; cutting efficiency and increasing wear. Use a "Energy Conserving" type motor oil.
  2. Radial tires have been standard equipment on most vehicles since the middle 80's. However, be sure the tires are properly inflated. Under-inflated tires can cost as much as 1 mpg. Better mileage can be achieved by over-inflating the tires (never exceed the maximum inflation pressure on the side of the tire), but the tires will wear faster and the ride will be rougher.
  3. Replace spark plugs at regularly scheduled intervals. New plugs alone can increase fuel economy by 3%.
  4. Be sure the plugs are the correct type and properly gapped.
  5. Be sure the ignition timing is set to specifications.
  6. If your vehicle does not have electronic ignition, check the points, rotor, and cap in your distributor as specified.
  7. Replace the air filter regularly. A dirty air filter enriches the air/fuel mixture and can increase fuel consumption as much as 10%. Tests show one third of all vehicles have air filters in need of replacement.
  8. Replace the fuel filter at least as often as recommended.
  9. On carbureted vehicles, be sure the idle speed and carburetor fuel mixture is set to specifications.
  10. On carbureted vehicles, check the automatic choke. A sticking or malfunctioning choke wastes gas.
  11. Replace the PCV valve at regular intervals.
  12. Service the cooling system at regular recommended intervals.
  13. Be sure the thermostat is operating properly. A thermostat that is stuck open delays engine warm-up, and a cold engine uses twice as much fuel as a warm engine.
  14. Be sure the drive or serpentine belts (especially the fan belt) are in good condition and properly adjusted.
  15. Be sure the battery is fully charged for fast starts.
  16. Use the recommended viscosity motor oil to reduce friction. Use "Energy Conserving" type motor oil.
  17. Use the recommended viscosity fluids in the drive axle and transmission.
  18. Be sure the wheels are properly balanced.
  19. Be sure the front end is correctly aligned. A misaligned front end actually has wheels going in different directions creating additional drag.
  20. Correctly adjust the wheel bearings. Wheel bearings adjusted too tight increase rolling resistance.
  21. Install a flex-type fan if you don't have a clutch fan. Flex fans push more air at low speeds when more cooling is needed. At high speeds, the blades flatten out for less resistance.
  22. Check the radiator cap for a cracked or worn gasket. If the cap doesn't seal properly, the cooling system will not function properly.
  23. Check the spark plug wires for cracks and burned or broken insulation. Cracked wires decrease fuel efficiency by failing to deliver full voltage to the spark plugs.

Figure 1 Change the engine oil regularly, as dirty oil causes extra friction between moving parts.
Change the engine oil regularly.

Figure 2  Whenever you change the oil, you should also replace the filter. The old filter holds about a quart of dirty oil.
Whenever you change the oil, also replace the filter.

Figure 3  Check your tire pressure often. If the tires are under inflated, they can lower gas mileage substantially.
Check your tire pressure often.

Figure 4  Installing new spark plugs at the proper intervals will help your vehicle run more efficiently. Make sure the plugs are properly gapped.
Installing new spark plugs at the proper intervals will help your vehicle run more efficiently. Make sure the plugs are properly gapped.

Figure 5  This air filter element is dirty and in need of replacement. A new filter element will cut gas consumption.
This air filter element is dirty and in need of replacement. A new filter element will consume less gas.

Figure 6  Replacing a faulty thermostat can increase your fuel economy.
Replacing a faulty thermostat can increase your fuel economy.

Figure 7  On vehicles where ignition timing can be adjusted, it's important that it be set to specifications. Advancing the timing past specifications leads to a rapid rise in plug temperature with little appreciable gain in power output.
Ignition timing chart

Driving habits

Getting the best gas mileage depends not only on how the vehicle is maintained, but also on how it is driven. By planning and driving by intention rather than instinct, gasoline mileage can increase as much as 20%. Here are some fuel-saving driving tips to follow:

  1. Avoid extended warm-ups. As soon as your vehicle is drivable, accelerate gently and slowly until the vehicle is fully warmed.
  2. Avoid unnecessary idling. One minute of idling uses more gas than it takes to restart the engine. Prolonged idling uses gas at the rate of about ½ gallon per hour.
  3. Avoid sudden stops and starts. Hard acceleration uses up to one third more gas. Achieve your desired speed with a steady foot on the accelerator and try coasting to stop.
  4. Drive at a steady pace. Plan your route to avoid stop-and-start conditions and heavy traffic. Be aware of the traffic around you and adjust your driving to avoid constant acceleration and deceleration.
  5. Many traffic light systems are "timed" for a given speed. Try to pace your speed to make the green lights rather than going faster and stopping for red or yellow lights.
  6. Try to anticipate traffic jams and avoid them when possible. Despite stops for traffic signals on other roads, avoiding those expressway traffic jams can lower fuel consumption as much as 50%.
  7. Choose your road surface. The fuel economy penalty for driving on soft or poorly surfaced roads can be 10-30%.
  8. Avoid excessive braking. The need for braking can often be eliminated by downshifting or simply taking your foot off the gas.
  9. Combine several short trips into a single trip. Short trips (fewer than 5 miles) don't let the engine reach its most efficient operating temperature. By combining numerous short trips, you can save on the total miles driven and take advantage of the vehicle's more efficient warmed-up condition.
  10. On long trips, start early in the morning to avoid heavy traffic and to reduce the need for air conditioning in hot weather.
  11. If you own more than one vehicle, use the most economical, especially for commuting or stop-and-go driving.
  12. Use the transmission properly. If your vehicle has a manual transmission, shift gears as soon as the engine can run smoothly in the next gear. Low gear at 20 mph gives only about two-thirds the mileage as high gear at the same speed. In second gear, it's four-fifths the mileage you'd get in high. With an automatic transmission, lifting your foot slightly off the accelerator will make the transmission shift sooner.
  13. When approaching hills, don't wait until the vehicle begins to "lug" before shifting gears. Don't accelerate once you have started up the hill, because speed increase is slight and gas consumption is high. You can minimize the speed loss by gradually increasing speed as you approach a hill.
  14. If you can, take advantage of good weather, and avoid bad weather driving. Rain or snow can reduce gas mileage as much as 2 mpg. A strong headwind can mean a 10% loss in fuel economy.
  15. Summer temperatures above 70°F (21°C) are better for fuel economy than winter temperatures. There is an approximate 85% difference in economy between 70°F (21°C) and 20°F (-7°C).
  16. If equipped, use the cruise control. A cruise control can gain 1-2 mpg by maintaining a steady, preset speed over any kind of terrain.
  17. The best fuel economy is obtained at moderate speeds. More fuel is consumed below 35 mph than at 45 mph, and generally, you'll lose 1 mpg for every 5 mpg over 50.
  18. Use the A/C at highway speeds. Although the weight and operation of the air conditioner reduce economy, tests have shown that wind drag at 55 mph with the windows open can consume more fuel that using the air conditioner with the windows shut. The least efficient time to use the air conditioner is at lower speeds. Turn off the air conditioning, at lower speeds, and use the vents when the outside temperature is in the comfort range.
  19. Don't carry unnecessary equipment in the trunk. Weight is the largest single factor in fuel usage, and every extra hundred pounds in cargo costs about 1% in fuel economy.
  20. Don't load cargo on a roof rack. This just creates frontal area, increases air resistance, and lowers your mpg.
  21. Learn to drive by instruments. Reading the tachometer (if equipped) can keep the engine in the optimum 1000-3000 rpm operating range. A vacuum gauge indicates the highest engine vacuum (best mileage).
  22. Relax while driving. Find a comfortable driving position; fidgeting in the seat leads to constant speed changes and decreases gas mileage.
  23. Avoid buying super-wide tread tires. They only create extra rolling resistance. Stick to the manufacturer's recommendations.
  24. If you drive a manual transmission vehicle, start in second when going downhill.
  25. On a 4-barrel carburetor engine, learn how to move the gas pedal to avoid activating the secondary circuit, except in an emergency.
  26. Avoid using large mud-flaps or oversize rear view mirrors unless necessary. They only create extra drag (air resistance).
  27. Don't drive fast until the engine has fully warmed to normal operating temperature.
  28. In winter, clean accumulated snow and ice from the trunk, hood, and roof before driving. Carrying heavy, wet snow uses fuel.
  29. Keep accurate records. Over a period, you can check your fuel economy; a sudden drop in miles per gallon may mean it's time for a tune-up or other maintenance.

Buying a vehicle

Fuel consumption is the biggest contributor to operating cost, and it should be a primary consideration when buying a vehicle. Some things to remember about options and fuel economy are:

How you will use the vehicle -- Your driving needs may be adequately served by a compact instead of a full-size vehicle.

Engines -- Smaller engines generally require less gas than larger V8s, but an under-powered vehicle will use more gas than one with sufficient power.

Transmissions -- If used properly, a manual transmission can provide up to 8% better mileage than an automatic, in city driving. At highway speeds, the difference is negligible.

Axle ratios -- Numerically higher axle ratios give more power, but numerically lower ratios save gas at highway speeds because the engine doesn't have to rotate as many times.

Weight -- The lighter the vehicle, the less gas it will use. On an average vehicle, every extra hundred pounds will cost about 1% in fuel economy.

Tires -- Radial tires can deliver as much as ½ mpg over bias or bias-belted tires.

Cruise control -- If you do a lot of highway driving, a cruise control can gain 1-2 mpg.

Fuel Injection -- Fuel injection is generally more efficient than a carburetor because it meters fuel more precisely.


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1998 W. G. Nichols - Chilton's Easy Car Care