tdoh

Ah, if it were that simple (I know you said your example was hypothetical using BS numbers)...however--and if there are any physics majors out there, don't hesitate to correct me:

If you try to determine how much energy an object has or requires to maintain a certain velocity (when I say velocity, I mean change in position), I think the formula to use is: KE=(1/2)(mv²), where KE is kinetic energy, m=mass, and v is velocity. Now, this formula won't tell you directly how much gas you'd be burning per hour at a given speed (velocity), but as you can tell from the formula, kinetic energy increases with velocity squared. IOW--the amount of energy required as velocity increases isn't linear. So, although I see where you're coming from in your example, the increase in the amount of fuel burned to make a car move at 90 mph as opposed to 75 would be much more than what your example would indicate, again in part due to the non-linear increase of energy required to move a mass at a given speed. IOW--unless I'm mistaken in my physics application here, the amount of energy required to move a mass is not directly proportional to velocity, i.e., not some simple numerical ratio.

Yeah, I know that most of you don't give a rat's a** about any of this, but my point was to show that (all other things being the same) higher velocity does not necessarily equate into better fuel economy. Need anecdotal proof? Just do a search to see how many people here have commented about lower fuel economy after some spirited driving.

Tony

daverman

tdoh: you're right about kinetic energy increasing to the square of velocity, but that makes a difference only during acceleration (or deceleration), not at steady state. The only work you do to maintain cruising speed is to overcome the losses due to friction. This is why you get such improved mileage when you cruise: you're not doing work to regain the kinetic energy lost during braking in stop-and-go traffic.

thephantom

I love this physics talk, you never know who is going to come up with what. Let me put in my $0.02, may not make sense but let me give it a shot. Here is my theory...

If your car is revving at let's say... 2500 RPM and steadily, your speed may vary but the RPM is consistent, wouldn't that save you more gas than let's say... 70 MPH but the driver adjusts the speed with variables such as hills, wind... The RPM changes because the driver has to accelerate or decelerate to keep the speed constant. Any thoughts?

jbrown

Thanks to both tdoh and daverman. What most of these people are missing is that maintaining 75 mph is easy, but how fast you get there time and again has the biggest impact on your fuel economy. My hypothetical situation was a theoretically possible occurrence, depending on rolling resistance, driveline friction, and aerodynamics, but absoulutely NOT based on tests involving our cars, just used for illustration and explanation.

But I don't really give a sh!t about milage, this car is bad @ss!

tdoh

See? I knew there would be at least one braniac out there!!

So basically, much of the fuel is burnt getting the car up to speed, right? That it takes more fuel/energy to get a car up to speed than it would to maintain that speed? If so, then that in itself would validify why going really fast (like 90-100 mph) wouldn't necessarily get you better gas mileage--you still have to burn a lot of fuel to get up to such speeds. I sorta figured that KE applied to rate of change of velocity (AKA acceleration) but wasn't sure if it could partially be applied to constant velocity.

In any case, I think it's safe to say that in the real world, better gas mileage can be attained at legal freeway speeds than at triple-digit speeds.

Tony