I’m not sure I agree that being in high gear saves fuel. Supposing the car is maintaining the same speed on level ground, the engine is fighting rolling resistance, internal friction, and air drag. If engine speed were to drop, the only difference might be internal friction – not sure how much. The other variables are the same. What would essentially happen is the driver would have to give it more gas. If the engine is turning slower, it will require more air/fuel mixture to be put into the cylinders to continue producing the same power output to maintain speed.
We can look at it another way. Since the only factor affected by changing gears is engine’s internal efficiency (provided the engine is not maxed out), theoretically, the best fuel consumption should be achieved at engine’s maximum torque output. This is highly dependent on the engine’s characteristics. We could probably say it wouldn’t be at peak power and wouldn’t be near red line, but we can’t say that it would be at the lower possible rpm (highest possible gear for any vehicle speed).
Why don’t you try some careful experiments. You will find that for the most part, you are wrong. As long as you aren’t overloading the engine, the higher the gear the better the mileage (and engine life).
It will vary from car to car, but in general, unless you are taking it to an extreme, the higher gear will give you better mileage. The only way to really know for your car and your driving style, is to measure it with you driving your usual route.
Don’t expect too much of a difference.
All things are not the same. In high gear, you have the throttle open wider, reducing the vacuum in the intake manifold, thereby reducing what are known as ‘pumping losses’. So higher gears just about always help. Higher (lower numerically) rear axles were quickly put in new cars when the gas prices spiked years ago.
Higher gears will always help fuel economy. Think about it…engine rpm is lower and vehicle speed is higher. That’s why we have overdrive in most vehicles today. My OD lets me cruise all day at 75mph and the engine is barely turning 2400 rpm.
You’re disregarding the fact that you’re opening the throttle more in a higher gear.
Just found an interesting article on efficiency and rpm:
http://www.viragotech.com/fixit/FuelEconomyEngineEfficiencyPower.html
Seems with cars, top gear is indeed usually the best (without lugging the engine).
That’s the whole point. More throttle = less pumping losses. The engine is putting out the same power to the road while operating at lower rpm. Lower rpm = less fuel used to spin the engine. It does not mean more gas consumption. The engine is running at lower rpm, with more air/fuel per cylinder fill. The balance turns out to be better mpgs. Think of the engine as an air pump, you want to minimize the restrictions in the inlet to have it operate at maximum efficiency.
I hear you, but you also have to consider port gas velocity and how gas inertia and turbulence can affect efficiency. I am more used to high-red line motorcycle engines, so it’s possible this is less of a factor with cars, with bigger bores and lower gas velocities and compression.
Look at Texases’ response above. A more open throttle can increase efficiency.
One of the reason diesels are more efficient is that they don’t have a throttle.
They are effectively “wide open” all the time.
I agree that the highest gear is not necessarily the best for MPG. If this was true then every domestic car made would be geared to run 1,000 RPM at 70 MPH if only the engine could pull the car.
Research the documentation of industrial engines, mostly the larger ones suitable for use in cars and trucks. Manufacturers of these engines publish plots of specific fuel consumption when producing HP at various RPMs. This requires a 3-axis plot.
The most efficient (and economical) RPM is that which produces the necessary HP for the least fuel.
Some examples of known better MPG at lower gear: A friend had a 1994 4WD Toyota that he put over 400,000 miles on, often pulling a 3800 pound boat and trailer. 70 MPH in 4th gear was 2 MPG better than 5th when pulling the boat. Without the boat 5th was better MPG.
Riding with a group on similar Honda PC800 motorcycles on the Blue Ridge Parkway where the posted speed limit is often 45 MPH. Some distance later we stopped for fuel and noticed two groups of MPGs, those who were riding in 5th at about 3200 RPM and those who were riding in 4th at about 3800 RPM. 3800 RPM produced better MPG.
I had a 2000 Toyota Avalon which was obvious after 55,000 miles that it got better MPG at 65 to 70 MPH than at 60 to 65. Others have observed the same thing on this generation of Avalon.
Another example where the jury is still out is the 2008+ Ford Powerstroke Diesel as to whether a 3.55:1 ratio results in better or worse MPG when towing than 3.73:1. At 60 MPH the 3.55:1 with 18" stock wheels the engine runs 1550 RPM. Its not easy to fairly compare one truck to another, or one trailer to another. But if I had to do it over again I’d buy 3.73:1.
Those are all great exceptions to the general rule that (excluding towing) lower ratios (higher gears) typically get better mpgs. Out side of towing and motorcycles that’s going to be true the great majority of the time. Most vehicles specifically recommend against using overdrive when towing.
As long as an engine is not lugging, as long as it’s sufficient into its power curve to keep the car going the speed, it’ll get better mileage on level terrain in a higher gear.
Engines have losses of their own, even not installed in a car. Continually stopping and reversing direction on all the reciprocating parts, and overcoming the internal friction, uses energy. There are also the pumping losses alluded to by Texases. Pumping losses are basically just the energy needed by the engine to pull air in and push air out. The higher the RPM goes, the greater those losses, the more energy is expended just to keep the engine running. The slower the engine is turning, the lower the energy expended to keep the engine running. While a more open throttle presents a lesser restriction to the air inflow, an engine running at higher RPM also needs to be pulling in and pushing out a lot more air.
The car will need whatever energy it needs to maintain the given speed. That’ll remain the same whether you drive in 3rd gear at 3000 rpm or 5th gear at 2000 rpm. It’s the energy needed to keep the engine running that’ll change the MPG, and an engine running at higher speed needs to use more of the fuel’s energy to do so.
You did post an interesting question, however. I like the questions that make me think.
theoretically, the best fuel consumption should be achieved at engine’s maximum torque output. This is highly dependent on the engine’s characteristics.
Yes, and no. The horsepower and torque graphs you see published in various motorcycle and car enthusiast magazines were all measured at full throttle. Closing the throttle changes the engine’s characteristics and tends to make the maximum torque and horsepower peaks to shift to a lower rpm. So even high performance motorcycle engines that make their peak torque at 10,000 rpm at full throttle are better shifted at much lower rpms when the throttle is only 1/10 of the way open.
Below is a dyno test of a typical engine taken at full throttle, half throttle, and quarter throttle along with another graph showing specific fuel consumption. Notice how the horsepower peaks at a lower rpm when the throttle is closed. Torque is not shown but you can bet that it also shifts to a lower rpm as the throttle is closed. Notice also how the specific fuel consumption increases dramatically at high rpms and low throttle openings.
That’s the whole point. More throttle = less pumping losses. The engine is putting out the same power to the road while operating at lower rpm. Lower rpm = less fuel used to spin the engine. It does not mean more gas consumption. The engine is running at lower rpm, with more air/fuel per cylinder fill. The balance turns out to be better mpgs. Think of the engine as an air pump, you want to minimize the restrictions in the inlet to have it operate at maximum efficiency.
That’s oversimplifying the situation in my opinion. From personal experience with changing the final gear ratio on a motorcycle I had, I learned that you can overdo tall gearing and get less fuel economy.
The job of a gasoline engine is to convert heat into energy. A BTU of heat = 778 ft-lb of energy or 1.415 horsepower-seconds of energy. Every BTU of heat supplied by the burning fuel that migrates to the cylinder head, piston crown, and cylinder walls to be removed by the cooling system is a BTU of heat that the engine will not be able to convert into 1.415 horsepower-seconds of energy.
One way to minimize this thermal leakage is to expand the hot gases before the heat has a chance to escape. That is why high rpm is good for efficiency. But at high rpm, another devil jumps up. At high rpm, you have high pumping losses because the throttle restricts air flow more.
This means that the optimum rpm for a given horsepower output is actually a trade off between a rpm high enough to minimize thermal losses and a rpm low enough to minimize pumping losses.
Even with high-red line motorcycle engines, keeping RPMs low will give you better fuel economy. I ride a 2003 Honda Nighthawk 750, and I can get about 50 MPGs by keeping the RPMs down. When I race the engine, the MPGs can be a low as 33. The extra power at mid range and high RPMs comes at a price.