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Car Q&A: Fuel Economy

by Larry Carley copyright 2022

Question: Is there any way to reduce the horsepower of my engine so I can get better fuel economy?

Answer: Yes, ease up on the gas pedal.

Drive your vehicle as if you had a raw egg under the gas pedal. An engine's power output is proportional to the throttle opening, so if you develop the habit of driving with a very light throttle your engine will use less fuel.

The worst thing you can do from a fuel economy standpoint is to drive with a heavy foot. Jack rabbit starts, aggressive driving and speeding all waste gas.

As for actually reducing an engine's normal power output, there is to simple way of derating an engine. Some late model vehicles use a trick called cylinder deactivation that basically turns off up to half of the engine's cylinders when the extra power isn't needed to save gas. This is typically offered on certain GM & Chrysler V8 engines. Hydraulic actuators deactivate the intake and exhaust valves for the cylinders that are being switched off, while the computer cuts fuel and ignition to those cylinders.

Automakers have been downsizing engines in recent years to improve fuel economy. The smaller the displacement of the engine, the less fuel it uses and the less power it produces (unless it is turbocharged). That's why V8s are being replaced by smaller V6 and four cylinder engines.

The nice thing about turbocharged four cylinder engines is that under light load they deliver fuel economy similar to a naturally aspirated four cylinder, but when you step on it the turbo boosts power and delivers performance similar to a larger V6 or V8 engine.

Can an existing engine be detuned to reduce power and fuel consumption?

An engine computer could be reprogrammed to reduce ignition timing and fuel delivery, but the gains would not be that great (maybe 10 to 15%). What's more, reprogramming an engine computer requires special hacking skills and LOTS of experience.

When you start playing around with fuel mixtures and ignition timing, you also change exhaust emissions. A fuel mixture that is too lean (too much air and not enough fuel) may misfire and cause a big increase in hydrocarbon emissions and/or damage to the catalytic converter. A lean fuel mixture may also increase the risk of engine damaging detonation (spark knock). Retarded ignition timing can make an engine run hot.

From a mechanical standpoint, changes that reduce engine displacement (smaller cylinder bore and/or stroke) will reduce power and fuel consumption. Reducing the compression ratio of the engine will also reduce power, as will installing a smaller throttle body or replacing the original camshaft with one that has less valve lift and duration. But these are expensive changes and would not be worth the cost unless an engine was being rebuilt.

Another option would be to replace the original engine with a smaller displacement engine, such as replacing a V8 with a V6 or four cylinder engine, assuming a smaller engine is available that would be compatible with your vehicle's transaxle/transmission and engine compartment. Engine swaps today are much more complex because you also have to replace the engine computer, wiring harness and how the engine control system is integrated with other onboard electronics. This would NOT be a practical option for most late model computer-controlled vehicles.

The better way to save gas is to sell or trade your current gas guzzler and replace it with a newer, more fuel efficient model such as a hybrid or electric vehicle. Many hybrids can deliver fuel economy in the 40 to 50 mpg range, while the typical electric vehicle will get over 100 mpg equivalent!

Question: If I remove the catalytic converter from my truck, will it get better fuel economy?

Answer: Maybe a bit, but not much.

Although removing the converter theoretically reduces exhaust system backpressure, the converter really does not create that much of a restriction, certainly less than the muffler. The only exception would be a converter that is damaged or plugged up with carbon deposits.

The gain in fuel economy and performance by removing the converter would probably be less than 5 percent at best. So if your truck gets 20 mpg on the highway, you might only gain ONE additional mile per gallon by getting rid of the converter.

Be warned that removing the converter is Emissions Tampering, which is ILLEGAL. If you live in an area that requires emissions testing, a missing converter will prevent your truck from passing the emissions test.

The catalytic converter does an excellent job of cleaning up the exhaust by removing unburned hydrocarbons, soot, carbon monoxide and oxides of nitrogen. You would not be doing the environment any favors by getting rid of your converter.

Some people think removing the converter makes a political statement and is a way of rebelling against tree hugging environmentalists and government regulations. They want their diesel-powered pickup trucks to belch thick black clouds of soot ("rolling coal") when they floor the accelerator. They may be making a statement by this kind of behavior, but all they are really accomplishing is wasting fuel, polluting the air we ALL breathe and showing others how obnoxious and inconsiderate they really are.

Question: If I fill my tank with Premium gas, will my car get better fuel economy?

Answer: It depends on the type of engine, but the difference in mileage between regular and premium unleaded is negligible.

The main difference between regular unleaded gas and so-called premium gas is its octane rating. In most of the U.S., regular unleaded gasoline is rated 87 octane, while premium can range from 90 to 91 or higher.

The octane rating tells you how resistant the fuel is to engine-damaging detonation (spark knock) when the engine is working hard under load. The higher the octane rating, the SLOWER the fuel burns. This reduces the risk of detonation when the engine is lugging under a heavy load or accelerating hard. It does not affect how much heat energy is produced by combustion.

Premium fuel is generally required or recommended in high compression performance engines, as well as supercharged and turbocharged engines. Although most late model engines that are turbocharged can also run on 87 octane unleaded fuel, the engine computer will retard ignition timing when it senses detonation to compensate for the lower octane rating of the fuel. This, in turn, will reduce power, performance and fuel economy.

The energy content of a gallon of gasoline depends on the "type" of gasoline (regular unleaded or "reformulated" unleaded), how much ethanol alcohol it contains (E10 contains 10 percent ethanol, E15 contains 15 percent ethanol) and the blend of hydrocarbons and other additives in the fuel itself.

According to the U.S. Energy Information Administration, one gallon of gasoline that contains 10 percent ethanol contains 120,286 BTUs (British Thermal Units, a measure of energy content).

One gallon of straight unleaded gasoline (no alcohol) is rated at 116,090 BTUs

So how can unleaded gas that contains 10 percent ethanol have a higher energy rating when pure ethanol only contains 76,330 BTUs per gallon? The answer is that the refiner can use more lower octane (but higher energy content) hydrocarbons when they blend the fuel. The resulting blend ends up having MORE energy than gas that contains no alcohol.

One thing to keep in mind about gasoline and its energy and octane ratings is that the numbers will actually vary somewhat from one area of the country to another, with the season (winter vs summer fuel), and with the brand or supplier of the fuel.

The bottom line is you can fill your tank with PREMIUM if you want, but if your engine does not require the higher octane you really won't gain anything and are just wasting money.

Question: If I increase the air pressure in my tires, will my vehicle get more miles per gallon?

Answer: Yes, but don't overdo it!

The rolling resistance created by a vehicle's tires can have a significant and noticeable effect on fuel economy. The lower the rolling resistance, the less fuel the engine has to use to push the vehicle down the road. That's why some hybrid and electric vehicles come factory equipped with special low rolling resistance tires.

Adding more air to a tire makes it stiffer and reduces its rolling resistance. But it also places more physical stress on the tire itself and increases ride harshness.

DO NOT OVERINFLATE YOUR TIRES! The recommended inflation pressure for your tires can be found on a sticker in the drivers door pillar or your owners manual. On most vehicles, the recommended pressure is 32 to 35 PSI (Pounds per Square Inch). This will vary somewhat depending on the size and weight of the vehicle and the type of tires. You can usually add an additional 4 or 5 PSI to reduce rolling resistance, but DO NOT EXCEED the MAXIMUM INFLATION PRESSURE indicated on the sidewall of the tires (typically 44 to 51 PSI depending on the tire). Overinflating the tires too much increases the risk of tire damage, tire failure or a sudden blowout. It may also make the ride unacceptably harsh.

If your tires are underinflated, add air to bring them up to the recommended pressure on the door decal or owners manual, NOT the maximum pressure indicated on the tire itself.

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