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- 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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