Posted on 05/01/2005 6:19:00 AM PDT by MississippiMasterpiece
To push a vehicle at low speeds where rolling resistance predominates, the force required is proportional to speed. More speed -> more force for the given distance --> more work/mile. Have you ever pushed a vehicle any distance? Tried to push it faster? I believe that you will find it takes more force to push it faster.
The required power to meet the rolling force needs is a constant * V^2. Aero force is a constant*A*Cd*density*V^2. Aero power is proportional to V^3. So total required road load horsepower is rolling constant * V^2 + Aero constant * V^3.
Work done/mi = power * Time. That divides the velocity out for a given mile getting back to work done = ~ (rolling resistance force + Aero force)* distance.
So one measures the coast down time. The vehicle mass (ignoring the energy in rotation of wheels/tires/brakes/driveline (you do do the coastdown in neutral? right?) times V^2 gives the energy at the starting speed and the ending speed. The difference gives you the energy expended. Divide by the time and you get average horsepower over that time. Multiply the required horsepower times the expected engine BSFC (at the appropriate engine speed and torque) * gal/lb (~1/6.15 for gasoline) * driveline efficiency gives you gallons/hr. Take mph and divide by the above and you should have an estimate of mi/gal.
I've owned a diesel in one form or another since 1985. I love 'em.
Recently purchased a 2005 VW Passat Wagon diesel. Rates 28 city and 38 hwy. It's a cruiser!
Unfortunately, it only came with automatic. First time I've owned an auto in almost 30 years. Still trying to get used to it.
We just bought a Jetta TDI 5-speed manual station wagon. The fat torque curve from ~1500 to 2500 gives the equivalent horsepower of a ~3.2L gas engine at those engine sppeds. So in the cut and thrust of every day driving, one gets decent performance without having to downshift and respectable FE. Diesels don't typically wind out so the 0-60 times are a little slow, but to accelerate into the space when changing lanes is a piece of cake. Now I only have to buy fuel (14 gal vs 22) every other week instead of once a week.
From your lips to God's ears. I have a 2001 Corvette that gets 32 mpg @60 mph in 6th gear. It also does 0-60 in 4.7 seconds when I'm feeling frisky.
Manual Trans C4? 'Vette's are the poster child for performance and economy. There's no reason a Suburban shouldn't be able to do at least 25 mpg at 60mph. Looks like I have to (re)build my own.
I thought about buying one of those, but I'm leaning towards the Jeep Liberty CRD, as I live in the mountains, and could use the 4WD. Too bad VW doesn't offer the Passat TDI with 4MOTION.
C5, 6 speed manual tran. @ 60 mph in 6th gear it does a rumbling 1200 rpm.
Yep.
For one, you'd have to pay truckers that extra day it takes to get a shipment in.
I was going to get a 6-spd Wrangler unlimited, and then stumbled across the TDIs. The Jettas don't have much ground clearance, so for the deep stuff I'll have to get the '90 'burb ready for this winter.
I was looking at getting a CRD but the mileage just wasn't there for me. Couldn't justify it otherwise. Nice vehicle though!
I had a 2004 Jetta Wagon TDI. 5-spd too. My wife flipped it.
She's OK. Not a scratch. Car was totaled though. I loved that car. And I averaged 40mpg in it; 36mpg going thru the VA mountains going 80mph loaded with bikes and gear :-)
Think about it. If inflation was the only factor to the equation that formula would work, but its not. I personally had my wages frozen for 3 years and then cut by $2.00 per hour in the forth. Non of this is factored in.
I'm comparing to my own wages.
I have a feeling that the EPA estimates run low on diesels. What is your Passat giving you in real-world driving?
Or in cities like Boston where the local hacks have no turn on red signs up at just about every intersection.
that's always been the east coast solution and contempt for the western states that drive longer distances.
I drive an SUV and pay 30-50 dollars a week on gas depending on how much i drive. I dont think what I'm paying right now is all that egregious even on what I'm making on a college student's wage. I have noticed that the primary poeple b*tching about gas prices drive little hondas and pay 20 bucks a week if that on gas. While spending well over 100 dollars a week on booze, smokes, and drugs.
I was trying to simplify the problem to make it easy for people to understand. Technically speaking the force of rolling resistance has two components: one is constant with velocity, the other part tends to be proportional to velocity; I ignored the second part to make the analysis easier for people to understand the effect of aero drag.
Have you ever pushed a vehicle any distance? Tried to push it faster? I believe that you will find it takes more force to push it faster.
Why, yes I have; 180 yards at a time in the low 20 second range, to be very specific about it. And I've done it enough to understand that the increase in force is required not because the rolling resistance increases by any significant amount, but because Newton says you have to add force to accelerate the mass of the vehicle to a higher velocity. And once at the higher speed. the power required remains higher at a higher speed because you are doing the same amount of work (force over distance) in a shorter period of time. P=Fv.
The fact remains that, as I said originally, to a first order approximation, the force you exert on the pushbar with your hand is essentially the same whether you're going 10 mph or 20 mph, as long as the speed is constant and the pitch of the hill is neglible. That's a fact I have empirically verified countless times. That's why the rolling resistance equation, as it applies to a particular wheel/tire setup, is usually stated in the form
You'll notice the conspicuous absence of a velocity term in the equation.
"'We need to look at what it will take to get manufacturers to offer technologies that people want.' One obvious step, which politicians are loath to even mention, would be to increase taxes on gasoline."
What a grand idea, let's increase taxes so people will 'want' to change their behavior.
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