In operation this MPG meter has almost instantaneous reaction (bouncy MGA
fuel gauge thread) to changes in throttle modulation, as fast as the fuel
flow will change at the entrance to the carburetor (leaky float valve
thread). The instrument reading during accelleration goes very low, and
during decelleration goes very high, much like the vacuum operated
"power/economy" gauge (current thread sidebar). As such it is very
interresting to watch while you modulate the throttle (how far into the
"power" range thread), and anyone can achieve 59 MPG for a short moment
simply by lifting the foot fully off the throttle at speed (impress the
passenger thread), but that yields very little useful information about
fuel economy, because those conditions are short term and transient (like
some folks interest in this thread). The only practical use for this type
of indication is simply to discourage you from tromping so hard on the
throttle ("loud pedal" thread), which is why the power/economy gauges do
not have numbered calibrations on the face (late model MGB temperature
gauge thread), and why they are so easily supplanted by an idiot light
theory of stupid drivers thread). In fact the idiot light may be better
suited to that cause (some useful idiot lights thread) because it can also
be keyed to engine speed as well as vacuum level (distributor spark advance
thread), and so actually encourage you to use higher throttle settings at
slower engine speeds (tough on the bottom end thread) to keep the engine
torque closer to the power curve where the thermal efficiency is best
(power curve thread).
The real practical use of this MPG instrument comes at constant cruising
speed on the highway (does anybody really do that thread). Here you get a
readout of your exact fuel economy in realtime (appropriate computer terms
thread). As long as you continue at a constant speed in fixed conditions
(depends on the police population thread) the instantaneous MPG indication
is the same as the long distance average MPG result (useful information
thread again, finally). Perhaps the most important information gleaned
here can be the optimum cruising speed for best fuel economy (is this the
real world thread). As different camshafts yeild optimum torque at
different engine speeds (power purchase thread), various cars are also
geared to yield optimum fuel economy (remove two pistons thread) at
different ground speeds. When the national 55 MPH speed limit was imposed
(the not so good old days thread) it was widely advertised that Chrysler
corporation immediately started bulding their "49 MPH cars" (misleading
advertising thread), meaning that they were geared and tuned to optimize
the government mandated Corporate Average Fuel Economy (government
interference thread) in a lower average speed range than was previously
practiced (changing times thread). Unfortunately (depending on your point
of view thread) my 1975 Mustang-II was built to earlier practices (good old
days thread), so it turned out to get the best fuel economy at a constant
62 MPH (try explaining that one in traffic court thread), which is exactly
what I wanted to know.
The next thing you can learn from instantaneous MPG readings is what
happens to fuel consumption (sometimes more is better thread) when you hit
an upgrade and press the throttle a little more to maintain speed going up
hill (not letting the SUV pass you thread). With just a little practice
and a short learning curve you might watch ahead more (defensive driving
thread) and accellerate slightly before getting to the upgrade, so that you
can negotiate the hill at a reasonable but decreasing speed (depending on
who's in the rear view mirror thread) without pressing so hard on the
throttle. This is contrary to what a cruise control unit (lazy driver
thread) will do with the throttle modulation, but can in fact achieve
better fuel economy (prove it to me thread). Another bit of useful
information is what happens when you drive in a strong headwind (I didn't
know we were in Kansas thread) or tailwind (why did the chicken fall down
thread), and the optimum speed for best fuel economy changes, possibly
being a large change in either direction (back in traffic court again thread).
But IMHO the coup de grace on this list (foreign language thread, but only
if you don't speak French) would be to tabulate the real (without prejudice
thread) MPG effect at various speeds with and without overdrive engaged (OD
thread is obvious), and with a few different final drive ratios installed
in your MG (most desireable mod's and models of MG thread). Similarly one
could take the initiative to map differences in MPG related to various high
performance engine components, free flow exhaust, etc (more power = less
throttle = better economy thread), and maybe even investigate the theory of
incremental and cumulative fuel economy improvements through the use of
economy improving gadgets (snake oil ads thread) to the point of less than
zero fuel consumption and having to drain the fual tank occasionally
(useless jokes, endless run-on sentences, and overconsumption of bandwidth
threads). One might even aspire to the grand objective of finally
discerning the optimum cost-vs-benefit combination of components for
lifetime minumal operational cost of your MGB (this is the mgs list isn't
it?). One might also retire and grow old gracefully (old farts thread)
while attempting to achieve some minimal economic benefit (offset by other
important social benefits thread) for a dubious number of disinterrested
owners of useless old junk cars (flame suit thread). Then again, one might
also benefit by getting some sleep (dangerous drivers thread) rather than
sitting up all night corresponding on a mailing list (internet addiction
thread).
Zzzzzzzzzzzz.......,
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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