engine RPM and gas consumption
 

Today me and my mom drove about 120 miles overall. During the first
part of the trip, I drove around town doing a few things (going to lunch,
having the tires checked... tires are apparrently brand new), and it seemed
to consume not much fuel at all, I drove about 20 miles total. The diesel
car doesn't seem to suffer much at all in stop and go and short trips, the
fuel tank needle didn't move hardly at all for about the first 40 miles of
the tank.

I went about 55 mph tops within the speed limit, and used cruise control
on a few small highways, going 45 mph. The engine RPM stayed around 1800
RPM most of the time. On the way out to Patrick AFB (my dad is retired
Airforce), my mom drove and we took the toll road/expressway and got up to
about 75 mph for most of the trip. It seemed to suck up alot more diesel
and the RPM's were about 3500 at highest. Still, the combined mileage for
200 miles when we topped up the tank was 39 mpg, but I think the 75 mph
driving ate into the fuel a bit, and with slower driving it would have been
lower. The wind resistance seemed to pick up too, the wind was louder than
the engine.

So I do think 55-60 mph is the fasted this car can go before the fuel
economy starts dropping off. Any thoughts? In the EPA fuel economy tests,
their highway driving is 60 mph tops. If it weren't for so many folks
being ticked off for driving 55 mph, I'd repeat the test at a later time
driving 55 mph on the expressway.

"Magnulus" > wrote in message
...

> Still, the combined mileage for
> 200 miles when we topped up the
> tank was 39 mpg, but I think the 75 mph
> driving ate into the fuel a bit, and
> with slower driving it would have been
> lower. The wind resistance seemed
> to pick up too, the wind was louder than
> the engine.

Considering what most cars get in fuel milage these days, 39MPG in
combined city/highway driving is pretty damn good, IMO.

> So I do think 55-60 mph is the fasted
> this car can go before the fuel
> economy starts dropping off. Any
> thoughts? In the EPA fuel economy tests,
> their highway driving is 60 mph tops.
> If it weren't for so many folks
> being ticked off for driving 55 mph,
> I'd repeat the test at a later time
> driving 55 mph on the expressway.

Is there a secondary road that you could use for the highway speed
portion of the test as opposed to the expressway?

"Paul" > wrote in message
...
> Considering what most cars get in fuel milage these days, 39MPG in
> combined city/highway driving is pretty damn good, IMO.

I suppose it's OK. I'd be curious to know what a Honda Civic Hybrid or
Prius gets doing the same route. I've heard that the trip computers on
these cars can be off, so the numbers I experienced during test driving
might not reflect the "real world" fuel economy. I don't have a fuel
economy computer in my Jetta, so I just use the old fashioned method.

From what I've read of the European Volkswagen diesels, blasting down the
expressway or autobahn 120 kmh (about 75 mph?) or more, seems to burn as
much fuel as congested city traffic. I suspect aero drag is a huge factor
in losing fuel economy at these speeds with a small-engine car.

Popular Mechanics did their own test of the Jetta turbodiesel a few months
ago, and they found their fuel consumption only a few mpg short of the
Prius. I believe they were also driving the automatic version, and other
reviews have found similar results. The Prius, despite it's aerodynamic
front end, actually might have a bit of drag on the back (I know for sure,
the low ceiling on the back sucked so badly I really didn't want the car
anymore). The "beetle" shape is actually not the most aerodynamic, I
believe (I think a tear-drop/wedge would be better).

>
> Is there a secondary road that you could use for the highway speed
> portion of the test as opposed to the expressway?

There are roads with 55 mph speed limits that could get there in a
roundabout way. Some of them have a very dangerous reputation- single-lane
highways where occasionally some idiot tries to pass, despite the signs.
I'm wary in driving them, I'd much rather have a divided median.

A couple weeks ago ,there was a poster who said that they didn't see much
fuel economy loss by going at 75 mph, to which my response was the engine
was designed to run under high load- it's a V-8. I posited that a smaller,
more fuel efficient engine would lose a greater percentage of efficiency at
such speeds. But that doesn't mean the V-8 is more fuel efficient by nature
(I suppose it depends on what you are doing with the engine. If you want to
tow a trailer, the V-8 engine actually might be the more fuel efficient
choice). A large city bus only gets about 3 mpg, but per passenger mile
,it's more energy efficient than even a hybrid car- the same is true of a
highspeed train (even more efficient than a bus). Airplanes, OTOH, are
hundreds of times less efficient than even a huge V-8 car or SUV (fighting
aero drag and the force of gravity).

Magnulus wrote:

>
> There are roads with 55 mph speed limits that could get there in a
> roundabout way. Some of them have a very dangerous reputation- single-lane
> highways where occasionally some idiot tries to pass, despite the signs.

That is most likely your fault. Those roads are grossly undersposted at 55.
One could easily do 70 mph on them--unless, of course, there is a an idiot
slowpoke ahead that cruises at 55 mph. If you're in no hurry to get anywhere
dont assume others arent.

"223rem" > wrote in message
m...
> That is most likely your fault. Those roads are grossly undersposted at
55.
> One could easily do 70 mph on them--unless, of course, there is a an idiot
> slowpoke ahead that cruises at 55 mph. If you're in no hurry to get
anywhere
> dont assume others arent.

The speed limit is 55 on those roads. It's the law and has been for
decades. And frankly I think driving them at 75 would be insane. Some of
the roads had a good deal of curve.

And, the safety issue aside, driving at 75 mph ****es away fuel. If I
choose to conserve some fuel, that's my business. It only loses me a couple
minutes. I think it's worth it. And if the price of gas keeps climbing (it
may well hit 2.50 this summer), I'm sure even you will change your tune
eventually. If I have the choice, I drive in the left lane so the other
speeders can zoom by, I won't stand in their way. But if I have to make a
left turn, of course I'll go in the left lane, and drive the speed limit.
Don't like it, tough. Maybe people like you could try obeying the law and
being a little bit more patient.

"Magnulus" wrote

> I'd be curious to know what a Honda Civic Hybrid or
> Prius gets doing the same route.

About the same.
Hybrids perform in city driving with many stops and accelerations.
The Prius is also good on the highway due to his
very low air drag (cw=0.26).
With the same size and shape, the diesel engine would be
more efficient at constant highway driving.

With fuel efficient driving (in the test cycles the inspectors
have to shift quite inefficient) you can save especially in
city driving. The Prius does it for you automatically.

> From what I've read of the European Volkswagen diesels, blasting down the
> expressway or autobahn 120 kmh (about 75 mph?) or more, seems to burn as
> much fuel as congested city traffic.

Typical consumption according to EU-Norm city is 6 l / 100 km (39mpg),
but this test cycle does not represent really congested city driving.
(But, as mentioned above, inefficient shifting.)

And as you have experienced, 6 litres are also typical for about 120 kmh
or "Richtgeschwindigkeit" (130kmh).

With my previous Opel Astra station wagen (direct injection 75hp diesel)
I had an average of 4.6 litres over 135000km. 51mpg! :-)

Thomas

"Thomas Schäfer" > wrote in message
...
> With my previous Opel Astra station wagen (direct injection 75hp diesel)
> I had an average of 4.6 litres over 135000km. 51mpg! :-)

You can't get new cars with small engines like that anymore in the US. I
think the 90 horsepower of the Volkswagen TDI is about as low as it goes.
Even cars like the Scion xA (a Toyota Echo hatchback) have 110 horsepower
engines. The Honda Civic Hybrid does have about 93 horsepower, total, as
well. It's astounding considering cars in the past had lower horsepower
and seemed to suite people well enough- the early VW Beetles had 25
horsepower, and the early Honda Civic in the US had 45 hp.

Magnulus wrote:
> "Thomas Sch=E4fer" > wrote in message
> ...
> > With my previous Opel Astra station wagen (direct injection 75hp
diesel)
> > I had an average of 4.6 litres over 135000km. 51mpg! :-)
>
> You can't get new cars with small engines like that anymore in the
US. I
> think the 90 horsepower of the Volkswagen TDI is about as low as it
goes.
> Even cars like the Scion xA (a Toyota Echo hatchback) have 110
horsepower
> engines. The Honda Civic Hybrid does have about 93 horsepower,
total, as
> well. It's astounding considering cars in the past had lower
horsepower
> and seemed to suite people well enough- the early VW Beetles had 25
> horsepower, and the early Honda Civic in the US had 45 hp.

Assuming this is a serious comment, the older cars were also a hell of
a lot lighter. This is in part due to the mandatory safety equipment
that's been added (airbags, bumpers, etc.) and in part to the creature
comforts buyers now demand that weren't dreamed of years ago.

To illustrate, my '55 Studebaker coupe probably weighs less than your
new Jetta example - and has a 225HP V-8. By contrast, your early Civic
maybe weighed 2200 lbs. soaking wet (a guess, but I bet I'm not far
off.) So power-to-weight ratios are actually fairly comparable Civic
vs. Jetta, although the Jetta has a slight advantage. Imagine both the
performance and economy that would be possible with a TDI-style engine
in a light body, like say an early Scirocco or Rabbit. But of course
people wouldn't buy such a minimalist vehicle, practical and economical
though it would be. (personally, I'd be tempted to drop in a 1.8T
myself, but I've been accused of not being quite right before.) It
wouldn't even be legal, lacking airbags.

this is progress?

nate

"Magnulus" wrote

>> [75hp car]
> You can't get new cars with small engines like that anymore in the US.

The new model has at least 80hp. 4 diesel engine types up to 150hp.
And 5 petrol from 90 to 200hp. And it is much bigger than the
previous model (452cm instead of 429cm in length, ...).
Why forcing the customers to take a big engine, even the smallest
is good enough for 168kph!?

> the early VW Beetles had 25 horsepower,

Really? We have the modell "Beetle" in Germany too, but it is a quite new
one
with much more hp. Or do you mean its archetype "Käfer" from 1946?

Thomas

"N8N" > wrote in message
oups.com...

"Assuming this is a serious comment, the older cars were also a hell of
a lot lighter. This is in part due to the mandatory safety equipment
that's been added (airbags, bumpers, etc.) and in part to the creature
comforts buyers now demand that weren't dreamed of years ago."

And yet, some folks claim that new cars are lighter than older cars due
to CAFE standards.

"To illustrate, my '55 Studebaker coupe probably weighs less than your
new Jetta example - and has a 225HP V-8. By contrast, your early Civic
maybe weighed 2200 lbs. soaking wet (a guess, but I bet I'm not far
off.) So power-to-weight ratios are actually fairly comparable Civic
vs. Jetta, although the Jetta has a slight advantage. "

My Jetta weighs about 3100 pounds. About the same as a Toyota Camry.
Some of that is no doubt due to the diesel engine having more weight. But
even a gas VW Beetle (the new one) weighs near 3000 pounds.

The original civic weighed 1500 lbs. That's less than a Honda Insight,
but I believe more than a VW Lupo (Euro minicompact, gets 80 mpg).

So the hp to weight ratio would be, with the MPG consumption:

Jetta TDI wagon 0.03 45 mpg
Early Honda Civic 0.03 40+ mpg
1988 Honda Civic CRX-HF 0.03 40+ mpg
2005 Honda Civic: 0.05 36 mpg
2005 Honda Accord: 0.06 29 mpg
2005 Ford Focus 0.05 28.5
2005 Toyota Camry: 0.06 25 mpg
Dodge Viper: 0.14 16.5 mpg

You can see a generalized trend towards more power and acceleration =
lower fuel economy.

"Imagine both the
performance and economy that would be possible with a TDI-style engine
in a light body, like say an early Scirocco or Rabbit. But of course
people wouldn't buy such a minimalist vehicle, practical and economical
though it would be. "

The problem with light cars- how to make them safe enough, and how to
ensure they handle well. That means you have to find materials and designs
that are both light and strong. Suppossedly the all-aluminum Audi A2 and
VW Lupo have good-to-acceptable crash ratings for their respective classes-
by European standards (which in some ways are better than the US's, in some
ways, more forgiving).