Is driving slower fuel-efficient?

[MDJH]

I often hear about how driving more slowly is supposed to save on fuel emissions. Are those who say so referring to "less fuel use per unit of time on the road" or do they mean over time as well? If so, why?

[insane_alien]

less fuel burnt per distance rather than time.

 

at high speeds most of the engines power is used overcoming drag. at 70mph it may take 40 horsepower to maintain speed but at 50mph it may only take 30 horse power so you don't need to supply anywhere near the amount of fuel to the engine.

[swansont]

However, engine efficiency depends on speed as well. One of the selling points of hybrids is that you can run the engine at the optimal efficiency point to charge the battery, rather than the varying efficiency of everyday driving.

[Moontanman]

I regularly drive to see my son and the distance is 175 miles, driving 15 mph slower takes significantly less fuel. 3/4 of a tank at 80 mph 1/2 at 65.

[MDJH]

less fuel burnt per distance rather than time.

 

at high speeds most of the engines power is used overcoming drag. at 70mph it may take 40 horsepower to maintain speed but at 50mph it may only take 30 horse power so you don't need to supply anywhere near the amount of fuel to the engine.

So is it safe to say that generally speaking, the higher the speed, the higher the percentage of the engine's power goes into overcoming drag, and the lower the speed, the higher the percentage of the engine's power goes into propelling the car forward?

[insane_alien]

sort of. if you are cruising at a set speed then ALL of the engines power is going into overcoming drag from various sources.

[MDJH]

sort of. if you are cruising at a set speed then ALL of the engines power is going into overcoming drag from various sources.

"Various sources"? From what I recall drag force referred specifically to the force resulting from the vehicle pushing against the air... what other sources are you referring to?

[ewmon]

On a flat smooth road surface at highway speeds, aerodynamic drag is the major force to overcome (followed by rolling friction, etc). Drag is proportional to the square of the speed. Power is drag times speed, and thus is proportional to the cube of the speed.

 

40³ = 64,000

50³ = 125,000

60³ = 216,000

70³ = 343,000

 

So, the power required to overcome aerodynamic drag approximately doubles for every 10mph increase at highway speeds. But there's a lot of waste in ICE's and their drive trains, so only about ¼ of the power (ie, gas) consumed ever reaches the wheels. This means the gas mileage won't have this doubling/halving effect, and is much tamer, say, about 20% decrease in mpg for every 10 mph increase in speed.

 

At low (urban) speeds, the mere running of the engine dominates the gas economy. Consider the extreme situation of a standing idle, which results in 0 mpg. It has been said that maximum mpg occurs at about 40 to 50 mph, and depends on the car, traffic/road conditions, driving habits, etc.

[insane_alien]

"Various sources"? From what I recall drag force referred specifically to the force resulting from the vehicle pushing against the air... what other sources are you referring to?

 

drag is also used to mean friction.

[ninus maximus]

However, engine efficiency depends on speed as well. One of the selling points of hybrids is that you can run the engine at the optimal efficiency point to charge the battery, rather than the varying efficiency of everyday driving.

 

I have often wondered why a 90 % efficient gas turbine is not used to charge the battery set vs using a 10-15 % efficient ice engine.

 

a gas turbine is usually around 90% efficient , and running the turbine at a set rpm would be perfect for a generator head designed to run at that set rpm.

 

good point there swansont

[insane_alien]

gas turbines are nowhere near 90% efficient.

 

they are also far more expensive than reciprocating engines and take longer to start up.

[Newbies_Kid]

A car consumes most gas as it accelerates. It's a simply law of physics (force equals mass times acceleration). A moving car doesn't require much gasoline to keep moving (due to the inherent inertia). In real life this means, in order to improve your mileage you need to keep the ride smooth. I used to ride on my motorbike 50 miles a day, and i just don't think that cruising at low speed shall increase your mileage. It's depend on your engine.. you can feel them.. i save most when i'm cruising at constant speed of 130km/h.

[insane_alien]

cruising slower WILL increase your mileage as the engine has to do less work against drag which means it requires less force to maintain a constant speed.

[Newbies_Kid]

but if you drive your car at low speed in low gear, the gas should be consumed faster than you drive at higher speed on top gear. Drag is a factor, but acceleration is the main factor i think.

[insane_alien]

yes, if you go into a lower gear where the revs are higher of course.

 

but all other things being equal(same gear, same weight etc) driving slower is more fuel efficient although there is a tipping point as the slower the engine goes the less efficiently it puts power to the wheels so you have to balance minimising drage and keeping the motor running efficiently.

[Newbies_Kid]

another question come out in my mind... why does a car consume a lot of petrol when idling?

[insane_alien]

it has to overcome friction in the engine and keep moving fast enough to remain operational.

[PaulS1950]

The speed for best mleage is around 40 - 45 mph. (flat ground at sea level)

Wind resistance is low and the rolling resistance is also relatively low. Having said that, it is important to remember that the driver has the largest effect on mileage. Keeping the throttle steady and the intake vacuum as high as possible is vitally important. Gradual, smooth acceleration and gradual decelleration using as little braking as possible will do more for fuel economy than most anything else.

If you leave lots of room (4 seconds) between you and the car in front of you will allow you to drive while rarely touching the brakes.

In addition to the above keeping your engine tuned, air filters clean and tires inflated to the maximum will improve results. Using the lowest octane rated gas that prevents pre-ignition or detonation, and keeping excess weight out of your car will also add to your savings.

[Mr Skeptic]

I often hear about how driving more slowly is supposed to save on fuel emissions. Are those who say so referring to "less fuel use per unit of time on the road" or do they mean over time as well? If so, why?

 

Both. Of course, there is a limit. If you go cruising at 5 mph, your engine is going to suck a lot of power just to keep itself going (its overpowered for the job required), and it will be very inefficient. Basically, the faster you go the more you save on the gas used to keep the engine running (like idling), but the more you spend on gas to overcome drag. However, time the engine is running is distance/speed, but the power needed to overcome drag is proportional to speed cubed. At a low enough speed, the idling dominates, but at a higher speed the drag dominates. A more powerful car will be more efficient at a faster speed than at a lower speed, but of course will be more inefficient overall. Check what your car's ideal speed is.

[bluevanghetto]

Has there been any mathematical analysis done on this subject? Would there be a way to use calculus and physics to figure out the instantaneous ideal acceleration for a given road and given car and given situation? I'm doing a calculus course right now. We are doing optimization and it would seem that you could think of finding the most gas-efficient way to drive would be a giant optimization problem...

any thoughts??

[Izzy_Bee]

I was instructed that driving at a constant 30mph maybe 40mph is most efficeint. This is because you need more fuel to get to a higher spped and keep it at that speed. If you rev low, and keep it in 5th gear you will be more efficent. Also chaging your air filters helps the car alot.

[DJBruce]

In this book, Transportation Energy Data Book 22 Ed., it shows the results of three different studies done on the most fuel efficient driving speed. The three studies were conducted in 1973, 1984, and 1997. The 1973 study suggest that the optimal fuel efficiency is achieved around a speed between 30 and 40 mph. The 1984 study seems to confirm the 1973 studies findings showing the optimal speed to be 35-40 mph. The 1997 study, however, suggests optimal fuel efficiency is achieved at 50-55 mph. The study does not discuss this change, however, I would be it was due to improved car technology and design.

 

The 1997 study also analyzes the fuel efficiency of many different mid 1990's cars. This data seems to show that optimal speed varies from model to model.

 

Also here is a graph that shows the fuel efficiency of various cars at different speeds. I found this on Wikipedia, and could not find a supporting study, however, so take it for what its worth.

 

Edited July 9, 2010 by DJBruce
Fixed Chart Link.