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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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