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Re: [Tigers] Gas milage with different tires?

To: drmayf@mayfco.com, tigers@autox.team.net
Subject: Re: [Tigers] Gas milage with different tires?
From: David T Johnson <djoh797014@yahoo.com>
Date: Mon, 5 Jul 2010 08:23:13 -0700 (PDT)
Why not do what they do in Kentucky.
Put 13 in wheels on the front and 15
in on the back.
This way you are always driving downhill.

Then get one of
those secret 200 mpg carbs
that Detroit is hiding.

Dave  90degrees on July
4th..Damn Global warming!

From: drmayf
To: tigers@autox.team.net
Sent: Mon, July 5, 2010 1:10:14
Subject: Re: [Tigers] Gas milage with different tires?

Resent...list owner
said too long
Duke, all....
The issue of better fuel mileage seems to
periodically pop up, which is
not a bad thing.  The Sunbeam's drag coefficient
is published at 0.45,
which amounts to it being a rolling brick.  At a
generally constant
speed, drag, which directly translates to hp and tq needed
to move the
car down the road, you can generally estimate the power needs
based on
the summation of the drag forces. At least to the point where you can
convince yourselves that a change in driving habits is needed for better
mileage, lol.  In this case, we'll run some numbers assuming that
getting to
speed is done in a gentle fashion and not in some screaming
kamakazi banzai
manner.  We'll start here:
frontal area is approximately 17.9 sq ft
Cd is 0.45
We won't use the mechanical or rolling resistance losses since they will
fairly constant is the range of highway speeds we'll look at

Drag =  1/2 *
rho * V * V * A * Cd
  rho = air density, in slugs.. and we will use a STP
value of  0.0023718
  V = car speed in ft/sec  for th etwo cases, we will use
78 mph =
114.4 ft/sec  and 70 mph = 102.7 ft /sec.

Plugging in    Drag (78) =
0.5 * 0.0023718 * 114.4 * 114.4 * 17.9 * 0.45
=  125.02 lbs

Drag (70) = 0.5 * 0.0023718 * 102.7 * 102.7 * 17.9 *
0.45 =  100.8 lbs
HP(78)  = Drag (78) * 114.4 / 550 =  26.004 hp

HP(78)  = Drag (70) * 102.7 /
550 = 18.8 hp

As mentioed, mechanical losses and rolling resistance adds to
the power
required.  As can bee seen, the power difference is significant. And
that power requires fuel in direct proportion. It all points to the old
of  "Speed costs, how fast do you want to go?"  The best fuel
mileage can be
had by simply going slower. In this case a drop from 78
mph to 70 will
increase your mileage quite a bit. But, it also depends
on the other two
facors as well.  If you have happy feet then it is
going to get worse mileage,
happy feet being the ones that are
constantly twiddling with the gas pedal:
everytime you do that, it pumps
raw fuel into the carb through the accelerator

So, my suggestions?
First, slow down

Second, get larger skinnier tires
as fat ones have a lot more rolling
resistance. In fact maybe some old bias
belted tires that can be pumped
up hard. High tire pressures reduce rolling
resistance in those tires.
Also get skinnier tires on the front and keep them
pumped up.

Third, get an AFR meter, not even a fancy wideband. Just one of
that have led lights and show rich or lean. Then tune the car for as
lean or as close to stoic as you can. Then drive it with the AFR meter
side led lit.

Fourth, maybe get a old timey vacuum meter and put it on the
dash as
well. A high vacuum means best economy.

Fifth, keep it tuned. Get an
electronic dizzy or innerds like Pertronic.

I hope this may help. But, as in
all things, your milage may vary..

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