Grex Cars Conference

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Grex Cars Item 36: Driving and car maintnance for the cheapskate
Entered by kaplan on Mon Dec 27 12:57:18 UTC 1993:

If there's no traffic, I'm not in a hurry, there's no speed limit, and
my goal is to minimize gas consumption and maximize the life of the car
while going to a given destination, how fast should I go? If all those 
other conditions are true a couple hundred feet from my parking
space and the temperature gague hasn't started to move yet, is there
a lower optimal speed?

Overdrive seems to kick in some time around 35 or 40 mph.  I assume that
the optimal speed is not too much faster than that mimimum overdrive
speed, right?  Why or why not?

6 responses total.

#1 of 6 by ragnar on Wed Dec 29 18:21:10 1993:

The engine management system will affect things a lot, but basically I
expect best engine efficiency at the lowest possible steady rotation
speed.  While this usually requires a nearly shut throttle which increases
the energy used to induct intake air, more complete buring of the gasoline
should easily compensate - gas being so energy dense.

The transmission fouls this up - an automatic trannie wastes power anytime
a torque is applied, unless it has a lock-up feature and it is locked-up.

The engine shouldn't have much trouble staying in a useful temperature
range.

Air resistance increases dramatically with speed, so slower is better in
that regard.

Mechanical cycling of load bearing members should be reduced at constant speeds
and the loading of drivetrain parts should be reuced at lower speeds.  
Temperature cylces should decrease in amplitude at lower speeds.

So, to skimp on fuel and parts, drive slow and stready.

My personal advice: don't forget what the car is for - you.

#2 of 6 by kaplan on Thu Dec 30 11:52:06 1993:

What is "Steady rotation speed?"  If I'm trying to minimize fuel used to 
get to a given destination, not fuel used per minute while the car is turned 
on, wouldn't going at 15 MPH in second gear waste a lot of fuel because 
the engine isn't going much slower than 35 MPH in 4th gear, but it'll 
take a lot longer to get there?

#3 of 6 by n8nxf on Fri Dec 31 13:33:40 1993:

every engine and engine/car combination has different efficiencies depending on
MANY variables!  Get one of those MPG meeters.  That'll get you in the
ball-park.  Or you can roll your own.  Just monitor engine intake manifold
vacuum.  The more vacuum the better at the highest speed possible.  Also try
inflating your tires to about 40psi. I've done that for decades and have never
had a problem running excessive pressure, though you experiance may be
different.

#4 of 6 by ragnar on Fri Dec 31 20:36:15 1993:

Steady rotation speed - I mean how fast the engine and drivetrain components
are turning.  Usually it takes engine energy to speed these up and that 
energy is not gained back under braking.  It all goes to heat.

Going faster in a higher gear - even cutting engine rpms - will probably
use more gas and will definitely cause more wear.  You get there sooner
so running the engine a shorter time, but the air will push back more than
twice as hard and that work has to be made up by the engine.

#5 of 6 by wjw on Fri May 13 12:14:03 1994:

The simple answer to your question, Jeff, is that the optimal speed varies
from car to car but it's usually around 40 mph.

My one car is a 5 speed and I can put it into 5th gear whenever I want.
The other is an electronically controlled automatic that goes into
overdrive/lockup at exactly 47.28474058374593 mph.  <g>.   Your comment
about the minimum speed at which the car goes into overdrive I 
would also say is correct.

#6 of 6 by volvo on Thu Sep 1 03:27:55 1994:

uuuu! gear heads! misery loves company!

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