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RE: - Mileage
Tony,
I've got a '90 GLi too, and I was getting around
the same mileage as you. However, I just changed the
O2 sensor, and my mileage jumped to about 25-28mpg.
Still not great, but a good improvement considering
my heavy right foot.....
Alex Ting
'90 jetta gli
'86 zender z400 callaway roc
'97 ducati 748 monoposto
Senior Account Manager
Millennium Solutions Group, Inc.
575 Menlo Dr. #5
Rocklin, CA 95765
Phone: 888.801.2001 ext. 228 or 916.630.2001
Fax: 916.630.2000
email: alext@nu2u.net
www.nu2u.net
www.millenniumsolutions.net
icq# 19613213
-----Original Message-----
From: Anthony Pelletier [mailto:vwnuts@yahoo.com]
Sent: Tuesday, January 23, 2001 12:01 PM
To: 16v Jason
Cc: scirocco-l@scirocco.org
Subject: - Mileage
I recently bought a Jetta GlI (2.016v) and its mileage
is complete shit compared to my scirocco 16v. I
averaged 30mpg (with shrick 256 cams and a 3.94 R& P)
I haven't done better than 25 with the jetta. It
hovers around 23-24. Which I feel is complete BS.
There are V8's that can pull that for mileage. My
uncles suburban gets 18-20 for comparison. I put in
new plugs & wires and a bottle of redline injector
cleaner. Any other ideas? I am going to try jason't
theory. Its 90mph on the way home today! Btw jason
your mileage is awesome considering the EPA rated the
scirocco 16v something like 24-28 (cty-hwy). Going to
blast down the highway. I wonder if the extra fuel
mileage will pay for the ticket..
Tony
--- 16v Jason <jason@scirocco.org> wrote:
> At 01:35 PM 1/23/01 , thescirocco@juno.com wrote:
>
> >Only to add...
> >The higher gear, .75, would help to overcome the
> increasing wind
> >resistance...
>
> Right. Gearing is torque multiplication. So having
> a numerically higher
> gear would mean the corresponding torque
> multiplication.
>
> Eg: Assume a constant 100lb/ft at the engine (for
> simplification
> purposes), and a final drive of 1:1.
>
> With a 0.71 gear, you have (torque * final drive *
> gear ratio) = 100*1*0.71
> = 71lb-ft at the wheels.
> With a 0.90 gear, you have 91 lb ft, or roughly 30%
> more torque to
> accelerate the car.
>
> Of course, there is one other big factor: the
> torque curve. Of course,
> with a higher numerical gear ratio, the engine is
> also turning faster. Up
> until a point (4500rpm on a 16V), the engine is
> making more torque to begin
> with at higher revs, so you'r _double_ better off
> having a
> numerically-higher gear.
>
> etc...
>
>
> >Did you get better gas mileage with the .75 5th
> gear?
>
> Kindasorta. Overall, yeah, about 0.5mpg better.
>
> Why so little? Well, in city driving, I'm in 4th
> more than I would have
> been -- so mileage probably dropped slightly... I
> didn't see it, but I run
> very few tanks on city driving alone.
>
> On the highway, I tend to get slightly better
> mileage. Why only
> slightly? Well, here's my Hypothesis:
>
> Like a torque curve that varies with engine speed,
> engines also have a
> specific efficiency curve. Basically, all ya need
> to know is that at some
> RPMs, the engine is more efficient than at others.
> It was my experience
> prior to the 5th gear conversion that my 16V is most
> efficient at around
> 5,000rpm. Why? Well, on the highway, you should
> (if the specific
> efficiency curve was flat), get gas mileage
> indirectly proportional to
> speed. Not directly, but exponentially -- but that
> doesn't
> matter. Meaning, that the faster you go, the worse
> gas mileage you get.
>
> As many of you have no doubt noticed, it doesn't
> always work that way. My
> Scirocco, for example, got the best gas mileage at
> 90mph. In fact, it was
> a _huge_ amount better than 65mph. "Normal" highway
> driving at 65-70mph
> with the stock gear would give me about 31mpg.
> Insane runs where I blasted
> through a whole tank and averaged 90+ miles per
> hour, I would get 36-38 mpg
> -- well above the "normal" figure. Therefore, my
> guess was that the engine
> reaches peak efficiency at like 90mph which was
> around 5000rpm. (There are
> other forces at play, like throttling losses at
> smaller throttle openings,
> but for this illustration, we'll keep them out of
> it).
>
> Now, since my engine hasn't changed, my peak
> efficiency is still at
> 5-Grand, but I'm not at that high revs anymore. So,
> at lower speeds, I
> have a few things working for me:
>
> 1. The lower RPMs help reduce engine friction and
> therefore BOOST fuel
> economy (+).
> 2. Lower RPMs and therefore lower torque output and
> lower torque
> multiplication mean that I have to apply more
> throttle to drive compared to
> the old gearing. This reduces throttling losses,
> thus BOOSTing fuel
> economy (+).
> 3. The lower RPMs move the engine into a band where
> it is not operating at
> its most efficiency, therefore REDUCING fuel
> economy. (-).
> 4. The need to downshift occasionally to pass up
> hills on the highway
> increases the revs beyond that of the old 5th gear,
> and the temptation to
> then "floor it" and fly by someone is enough to
> REDUCE fuel economy (-).
>
> As expected, I saw a small gain of about 1mpg on the
> highway.
>
> However: At higher speeds (90mph or so), I don't
> get as good mileage as I
> did before -- because at 90 before, I was at 5000+
> and at peak
> efficiency. Now, I'm at 3800, i.e. not peak
> efficiency, and get
> 34-36mpg. (Still fucking great, if you ask me).
>
> On the other side of insanity, if I were to average
> 110mph for a tank (I
> wish), I would probably see mileage in the 30-range,
> whereas with the stock
> gearing (6000+rpm) I would expect to see low 20s.
>
> YMMV. :)
> Jason
>
>
>
>
>
> ----------
> 1987 Scirocco 16v
> 1988 Mercedes 190E Sport Euro
>
>
> http://www.scirocco16v.org
>
>
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