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From: davidsen@steinmetz.UUCP (Davidsen)
Newsgroups: net.auto.tech
Subject: Re: 5 speeds vs. Overdrive
Message-ID: <658@steinmetz.UUCP>
Date: Thu, 20-Feb-86 14:10:11 EST
Article-I.D.: steinmet.658
Posted: Thu Feb 20 14:10:11 1986
Date-Received: Mon, 24-Feb-86 07:42:06 EST
References: <693@isrnix.UUCP>
Reply-To: davidsen@kbsvax.UUCP (Davidsen)
Organization: GE CRD, Schenectady, NY
Lines: 62


There are a number of effects which prevent any one best solution to
transmission and gearing. A few of these conditions:

An engine gets its best mileage at the peak of the effective torque curve.
That is, the effective torque curve is diferent for every throttle
setting. It is also effected by tune, plug gap, gas quality, temperature,
... you get the idea.

A transmission with more speeds gives you the chance to operate the engine
nearer the most efficient point (for a given throttle setting). Therefore,
if you operate your car at all speeds equally, the more speeds the better.

In the real world... city driving 10-40mph, highway driving 55-80mph. The
purpose of an overdrive is to adjust the engine speed for best efficiency
(not always lowest rpm, since lugging an engine hurts mileage). The term
"overdrive" indicates that the driveshaft turns faster than the engine,
and I am not going to get into internal vs. external, since I have never
seen a good technical reason to think that there's a notable diference.

A transmission uses power. Every gear to gear engagement take some
percentage of the torque applied and converts it to heat. In *most*
transmissions the 4th gear is direct, implemented with some form of dog
clutch or coupler, thus eliminating one gear to gear loss. I know there
are exceptions, please don't feel the need to enumerate them.

At any rate, given a transmission with direct drive, one with overdrive,
and one operating in a reducing gear such as 3rd, if the overall ratio
(engine rpm at a given speed) is the same, the direct drive system will
have better mileage (given everything else the same).

As a car buyer or changer, you should select the transmission and
diferential as a group, to suit what you want for performance vs.
efficiency. For a car driven mostly in the city, a slightly lower ratio
overall may give better city mileage. Given an overdrive to keep the engine
at the right rpm, and the total mileage will *probably* be better.

Finally, keep in mind that the tires are part of the gear train. The magic
number is "miles per hour per thousand rpm", and that is the number you
can adjust to get the most out of a car. Larger tires may lower rpm but
have more rolling resistance, while wider tires are frequently the
reverse. Radials are usually 1-2mpg better than bias tires if the sizes
are close.

Parsonal experience: when I was driving my 64 Plymouth on the street, over
the course of several years I ran rear gears from 3.23 to 4.56. The best
city mileage was with 3.56 or 3.90's, and there was only a slight loss at
4.11. Highway mileage dropped with any ratio higher than 3.90, although
that was back in the days before 55. The point is that I was able to enjoy
the performance gains of stronger gears without a mileage penalty (other
than that caused by too much right foot), and that the 3.30 gears which
kept my engine rpm down actually hurt mileage in town.
-- 
	-bill davidsen

	seismo!rochester!steinmetz!--\
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