We covered this in driver training and it is a little foggy (5yrs ago) but i recall that most vehicles reach their optimum fuel economy at about 70kph (45mph???) and before and after that it is less. The drop off after about 125kph (75mph???) is dramatic and will not balance out against the decreased time driven. They also reach their optimum level in terms of least emissions at approx. the same speed. Of course that is cruising speed so i don't know what happens if you are accelerating hard. I have also heard this from environmentalists. They say highways should be limited to about this speed because (a) best economy (b) least wear and tear (c) least emissions (d) safe speed (e) best for good traffic flow.

Thats what i have heard anyways...
Claeren.

 

Claeren is correct. It is true that cars usually maintain thier highest fuel efficiency in 5th gear but the relationship between fuel consumption and speed is not linear.
It is easy to think so because the relationship between RPMs and Speed is linear based on gear ratios.
But the amount of fuel required to maintain a specific RMP range depends on more than the gear you are in. It depends on the efficiency of the transmission, internal friction of moving components, wind resistance, etc.. So typically a car is most fuel efficient at the lowest speed (within normal RPM range) in its highest gear. If there was no friction or wind resistance then your fuel economy would be the same at 45mph in 5th gear as it would at 100mph in 5th gear at a constant speed.

 

I remember reading about meg-lev trains and how they can make them go like 400kph easily in the open but if they put them in pressurized tubes then they would essentially and functionally only be limited by the amount of inertia that the body could handle in acceleration and decceleration because you would have to start slowing down like halfway to the destination. They said that with exsisting technology they could easily hit 800kph. The whole point to this being that controled conditions like a pressurized tube can make a big difference on capabilities.

On a side note i just watched a special on how they are experimenting with launching space shuttles using meg-lev technology...

Claeren.

 

An engine is most efficient near its torque peak. For our cars that's pretty high in the rpm range. However, that's just the engine, not the car.

Air resistance (drag) is a function of the square of the velocity. This makes it an exponential relationship. Air resistance starts to really become a factor around 70miles/hour.

Mike 94PGT had an excellent post about this last week or the week before.

Tyson

 

thanks a bunch guys. that helped a lot. it actually was really bugging me, but now i dont know why i hadnt taken wind resistence into consideration. that seems like what would be the biggest negative factor at highspeeds.

 

Dude, what does your engine do to attain those higher rpms? Uses more fuel. Thats more fuel FOR EVERY REVOLUTION. Not just the fact that it is spinnin fast. I used to think the same thing as he did. :idea:

 

well yes it uses more fuel, but my reasoning behind it was that while you're using more fuel you're also covering more road in a smaller amount of time.

 

Claeren is correct, the most fuel efficient speed to travel at is around 70 km/h, (45 mp/h I think) for most cars.

 

uhh, doesn't depend on the car itself? for instance a z06 in sixth gear is turning like 1900 rpms at 80, and gets better mileage on the highway then cars with half their displacement.

 

MOST vehicles. Very few cars peak would fall outside of a 65-80kph range. After all you are still talking about the same limiting factors like air resistence, drag, weight, inertia, etc. Of course big V8's are often called cruisers and are so for a reason, where-as i4's aften have a smaller range between highway and city numbers. However one must note that although CAR1 at 85kph is more effiecent the CAR2 at 70kph, CAR1 is probably still more effeicent at 70kph then at 85kph, does that make sense...

Claeren.

 

i had a 4 cyl wrangler and now a 6 cyl lifted wrangler... i expected a huge drop in fuel economuy with the bigger engine, and also on paper there is a big difference.. but the fact of the matter is - the smaller engine had to work a lot harder to maintain speed - so even though it was the same rpm, it had to really chug to keep it there...

the reason i used my jeeps as the example is because its so much easier to gauge wind resistance with a jeep (ie: you let off the gas at 75 moph, and it feels like youre slamming on the brakes.) wind resistance REALLY makes a big impact on fuel usage. on a emissions dyno, my jeep would maintain 65 mph at 1/4 throttle, since its not actually moving.. but out on the highway, i have to keep it halfway down or more to keep those same rpms... see the correlation?

 

yeah crapp, thanks for the input. i did get pretty damn good milage on a trip to houston one time though. i averaged about 110 and made it in about 4 1/2 hours(i live in new orleans) i ended up getting a little over 400 to the tank.

 

truth be told - i havent been on a long enough trip in this car to use a full tank in one shot... how is 400 compared to normal? lol mileage for the jeep w/ a 19 gallon tank is good if i hit 280 on a full tank ;p

 

The reason travelling 60 mph is more economical than travelling 120 mph is because of air resistance. Air is matter, and therefore has mass. The quicker you try to displace it, the more force it will resist.

Drag increases exponentially, so you won't have twice as much drag going 120mph than 60 mph, you will probably have 4-5 TIMES more drag, so more fuel is used to maintain the speed. Its kind of a hard concept to understand, but examples can help.


next time you're in a pool, move your hand back and forth through the water, palm flat, to create as much resistance as you can. move it back and forth slowly 10 times. it'll be easy. but if you try to move it back and forth as fast as you can, it will take much more energy. Sure, you got the same amount of work done more quickly, but you also used a LOT more energy to do so.

 

on a normal tank of gas i'll get around 250, but with about 1.5 gallons to spare. i dont know about the math cuz im too lazy. whoever wrote that analogy about the pool, that makes a lot of sense dude. i understood though, as soon as somebody brought it up about air resistence and stuff.

 

well yes it uses more fuel, but my reasoning behind it was that whileyou're using more fuel you're also covering more road in a smaller amount of time.

Covering more road and RPM's ARE directly proportional. You will Get the same number of Revolutions for mile no matter the speed, 20-130 mph. That would be great if your engine used the same Fuel/Revolution at any engine speed, but IT DOESN'T! More fuel is injected into the cylinder to create enough energy to make the engine spin faster. More fuel is also required to maintain the high speed. Thus you don't get as good of mileage at high speeds.

I really hope everybody understands this.

On the other hand, the faster you get to work, the more hours you get, the more green you make! Thus, you can pay for this decrease in fuel mileage. @-