How is it much more fuel efficient to travel at say 50 mph for longer than 70 mph for a shorter period of time. Like if I was going a set distance, if I drive slower I’m using the car longer so why isn’t it the same fuel being used.
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Air resistance doesn’t scale linearly with speed – twice as fast gives you 4 times the wind resistance. There are other factors of course, but at the speeds we generally drive, air resistance is the biggest factor which varies significantly in a single vehicle.
2 reasons –
1. Air resistance is substantial. It’s harder to push through air going 70 MPH then when going 50. That means the engine needs to work harder. It’s kind of like going 1 mile flat or 1 mile uphill. Same distance, harder to do.
2. Engines are not “universally” efficient. There are tons of variables that determine the efficiency of an engine and you need a multidimension chart to pin-point “optimized” sweet spots. A car manufacturer will target the 50-60 mile per hour range for optimization because that’s where most people drive (at least, it’s around the speed limit). Since 70 MPH is beyond that space, they don’t care as much and the engine might get lousy fuel efficiency at that speed.
EDIT – just because this is gaining some speed (see what I did there) I’ll add that the “multidimensional chart” involves things like car load (weight), aerodynamics, internal friction of the engine, fuel types, even air pressure. All those factors play into the engine’s efficiency so they create these wild “maps” of efficiency. If you imagine the map looking like the surface of the Earth you’ll see occasional mountains, those are the areas where the engine is performing really well. Manufacturers will then try and “move those mountains” so they sit right where the car is going to perform the most. Something like average air pressure, normal fuel, weight = 2 passengers, driving at 65 MPH in 4th gear. Or something like that.
EDIT 2 – This is why Hybrid cars can be so efficient as well. You can use the gas engine not to drive the car, but to recharge the battery, which is a really constant and specific performance on the engine. Drop the efficiency mountain right where engine needs to power the battery and you have a really high performance engine operating at maximum efficiency to reduce emissions and waste.
you have to keep in mind that we usually measure in “miles per gallon”. This unit does not have anything to do with time. It’s about distance (miles) and fuel consumed (gallons). going fast or slow doesn’t have a direct effect on this measurement.
What others have said about the wind resistance and different efficiency ranges is true. It’s a combination of what RPM range your engine is most efficient in, gear ratios on your transmission, and the wind resistance.
Kinetic energy goes as speed squared. Going for longer increases the burned fuel linearly, which is a slower growth.
In addition to wind resistance, most engines and transmissions are more efficient at 50 mph than 70. Picture a bicycle: at a high speed and a high gear, you can cruise along with slow, leisurely pedaling. If you wanted to go 30% faster, you could – but you’d have to pedal like crazy, which would tire you out faster.
Basic physics come into play. The faster you go the more drag (wind hitting your car) your car has to overcome. It goes up exponentially, so the difference between 50 and 60mph’s wind drag isn’t 20%, it’s substantially more (sane with 60 to 70). You can test this by rolling down your window and feeling the drag on your hand at different speeds.
There is also engine efficiency – your car was built to go specific speeds. Some older cars that might be 30 or 40 mph (think a 4 gear VW van). Racing cars may run most efficiently at much higher speeds with great aerodynamics and larger gears.
The reason most cars max out at 55mph for efficiency (or around there) is because there is a complicated formula used to establish MPG advertised on cars. If you hit your max MPG at 55 you can actually maximize the rating you get. This goes back to cars being designed to drive on highways/roads in an era where 55mph was a pretty reasonably fast speed.
However, your car may vary where it does best. Tracking mileage (or using a MPG display) can help you figure out where you car runs best. My friends 1990 volvo shook at 55-65 and for some reason burned gas faster than 50 or 70. So, 70 was ideal (though that seemed really fast in a tin box). My 2001 VW Jetta ran best at 65 exactly. My 2014 impreza does best around 55-60. The 2000 Jeep Cherokee Sport topped at 50ish I think (gas was cheap back then so I didn’t track it as closely).
At its most basic, the engine uses fuel at different rates. The harder you push the accelerator pedal the more quickly the engine burns fuel.
If you are running the engine for less time but burning fuel faster over that time, that could make for more or less fuel burned overall. What tends to happen is that at first the decreasing time “wins” and the car gets more efficient as you drive faster but as you keep going faster, the time savings increase slower and the fuel burn rate increases faster so eventually fuel burn rate wins and it starts to get less efficient.
Car companies engineer their vehicles to have the maximum efficiency at a speed at which they are likely to be driven a lot by most drivers.
What I haven’t seen mentioned is to travel at either 50 or 70mph steady state you will be in the same gear. So while the cars engine does run longer, it actually makes the same number of revolutions to travel from point A to B. So the load on the car is less at 50mph due to air resistance and typically is more fuel efficient at a lower RPM. Couple those two things together and you get better fuel economy.
Almost universally down to the engine speed.
In top gear, with the engine barely rotating, you’ll do 50mph on a flat road. The engine speed will barely be above idle (that speed necessary to keep it rotating), often only around `1000RPM
To get to 70mph, you would have to rotate the engine at a few thousand more RPM.
If you provided another gear, you would be able to do 70mph at engine idle, but at greater engineering cost, and with air resistance, etc. it would actually not be as simple as “yet another gear range”, you would have to do the maths to make it as efficient.
You then risk getting into a situation like trucks have where you have to have LOTS of gears (and gear changes) to make it as efficient at a speed that you won’t necessarily hold for a long time (e.g. every time you want to change lanes or slow slightly, you’d have to change down a gear… generally you can just stay in top gear on a motorway and concentrate purely on driving).
Imagine running a mile in an hour. Not a big deal right?
Now imagine running a mile in a minute. Not so easy right?
It takes a lot more energy to do the same work in less time. A lot more energy per time period.
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