Nice summary of mileage stretching

[mf70]

At Fuelly.com:

Fuel Saving Tips | Fuelly

They even mention ecomodder!

MF

[songman]

You often read tips to depress the clutch or drop the car into neutral downhill, on most modern cars with fuel injection this actually uses more fuel than leaving it in gear.

Fallacy, IMO. Yes, no fuel used. But less time spent coasting, because of engine drag.

Just EOC already.

[vskid3]

Originally Posted by songman

Fallacy, IMO. Yes, no fuel used. But less time spent coasting, because of engine drag.

Just EOC already.

I think the intent of that statement is for going downhill, where you would either have to engine brake or use the friction brakes to keep from going too fast. On flat land, yes, neutral is better, but going downhill, you might as well use some of the excess energy to spin the engine.

[serialk11r]

Originally Posted by vskid3

On flat land, yes, neutral is better, but going downhill, you might as well use some of the excess energy to spin the engine.

Most hills aren't steep enough to warrant that. In the SFBA, the only freeway with a lower speed limit than my terminal velocity is Highway 92 off 280 going to San Mateo, where I can go about 75mph in neutral for several miles. Everywhere else the car will lose speed until 40 or lower, and my car is a featherweight aka it has terrible coasting.

[basjoos]

The lower your cd or the greater your weight, the more easily your car will exceed the posted speed limits on a downhill run unless you use braking to limit the speed. The aerocivic is also a featherweight at 2100 lbs, but with its low cd pushing its terminal velocity somewhere above 100mph, it will easily exceed 75 mph on any reasonable downhill run long enough for it to build up speed. It doesn't even have to be a steep downhill.

[Arragonis]

Originally Posted by songman

Just EOC already.

Mention EOC anywhere else and you will be put in the stocks for being a danger to everyone...

[serialk11r]

Originally Posted by basjoos

The lower your cd or the greater your weight, the more easily your car will exceed the posted speed limits on a downhill run unless you use braking to limit the speed. The aerocivic is also a featherweight at 2100 lbs, but with its low cd pushing its terminal velocity somewhere above 100mph, it will easily exceed 75 mph on any reasonable downhill run long enough for it to build up speed. It doesn't even have to be a steep downhill.

Yup, but it's fairly common that a car has 2x the drag of your car. I have around the same drag as a Prius but only 70% the weight. The second steepest long "hill" around here that I've seen is the Bay Bridge, where I top out at 40mph. A Prius would probably top out at 50mph, the speed limit.

Even a Mercedes E class pushing 4000 pounds and with a low Cd would probably have trouble getting to 60mph, but a Lexus LS or Mercedes S class pushing 5000-6000 pounds would probably necessitate engine braking down that moderate downhill, but that's not really the norm.

[Vman455]

Originally Posted by basjoos

The lower your cd or the greater your weight, the more easily your car will exceed the posted speed limits on a downhill run unless you use braking to limit the speed. The aerocivic is also a featherweight at 2100 lbs, but with its low cd pushing its terminal velocity somewhere above 100mph, it will easily exceed 75 mph on any reasonable downhill run long enough for it to build up speed. It doesn't even have to be a steep downhill.

Hold on--I'm remembering my Galileo: bodies of varying mass fall at the same rate. The heavier car will also have more rolling resistance. But, the heavier car will have more momentum. Has anyone proven empirically that in the case of two cars, with identical Cd and frontal area but different masses, the heavier car will roll faster downhill? Does the increased momentum of a heavy car outweigh (ha!) the increased rolling resistance at some point, and where is that point?

[euromodder]

Originally Posted by Vman455

Hold on--I'm remembering my Galileo: bodies of varying mass fall at the same rate.

In vacuum

IRL, the rate of fall is determined by mass, shape & volume (body density and aero drag), the length of the fall and air density.
If you don't believe that, don't go sky jumping

That's why it took until Galileo for people to realise the concept.

Has anyone proven empirically that in the case of two cars, with identical Cd and frontal area but different masses, the heavier car will roll faster downhill?

There's no real need to try it.
Gravity usually exceeds rolling resistance.

The main thing you might need to overcome is inertia, i.e. getting the car rolling - which takes the most force.
Once rolling, the heavier car will accelerate away from the lighter one.

[Vman455]

Originally Posted by euromodder

In vacuum

IRL, the rate of fall is determined by mass, shape & volume (body density and aero drag), the length of the fall and air density.
If you don't believe that, don't go sky jumping

That's why it took until Galileo for people to realise the concept.



There's no real need to try it.
Gravity usually exceeds rolling resistance.

The main thing you might need to overcome is inertia, i.e. getting the car rolling - which takes the most force.
Once rolling, the heavier car will accelerate away from the lighter one.

Originally Posted by Vman455

...in the case of two cars, with identical Cd and frontal area but different masses...

I'll rephrase my question: is there a point at which the inertia of a more massive body overcomes the increased rolling resistance of that body such that it will roll downhill faster than an aerodynamically-identical, less massive body? I see people all the time claiming that a heavy car will roll downhill faster than a lighter, but I've never seen any evidence to suggest this is so.