greg@isrnix.UUCP (Gregory Travis) (02/14/86)
Can someone explain the relative merits of having a 5 speed transmission
vs. having a 4 speed with an overdrive box tacked on? Is the end
result the same (i.e. 4'th in both schemes is a 1:1 ratio between crankshaft
speed and propeller shaft speed; overdrive/5'th being 1:>1)?
It would seem to me that, this being the case, a 5 speed transmission
would always be superior to a 4 with overdrive - less parts, complexity,
etc. Is overdrive just a marketing ploy? A recent poster mentioned that
Volvo supplies (as an option) an overdrive for its transmissions. Volvo
has addressed the needs of the highway cruisers this way since at least
1960. As far as I know, their transmissions (the M40, M400, M45) have
always been 4 speeds - you can get an overdrive box for any of them
however. Why not simply make a 4 speed and a true 5 speed box? The
cost has got to be lower to slap another gear (and make the box a bit
bigger) into the transmission instead of adding what amounts to a
whole 'nother (2 speed) transmission onto the output of the first.
Note that I know that 5 speed is sometimes called overdrive even on
true 5 speed gearboxes. By overdrive I mean an extra electro-hydraulic
box attached to the output of the main gearbox.
--
Gregory R. Travis
Institute for Social Research - Indiana University - Bloomington, In
ihnp4!inuxc!isrnix!greg
{pur-ee,allegra,qusavx}!isrnix!gregmberns@ut-ngp.UUCP (Mark Bernstein) (02/17/86)
** > From: greg@isrnix.UUCP (Gregory Travis) > A recent poster me > mentioned that > Volvo supplies (as an option) an overdrive for its transmissions. Actually, I don't think it's a true option anymore. Starting with the 1981 models you'd be hard pressed to find one without this feature. > Volvo > has addressed the needs of the highway cruisers this way since at least > 1960. As far as I know, their transmissions (the M40, M400, M45) have > always been 4 speeds - you can get an overdrive box for any of them > however. Why not simply make a 4 speed and a true 5 speed box? The > cost has got to be lower to slap another gear (and make the box a bit > bigger) into the transmission instead of adding what amounts to a > whole 'nother (2 speed) transmission onto the output of the first. Well, that's an interesting question. The Volvo people aren't total ninnies (no flames please ! :-) ) So the question is, why *do* they do it this way? Does anyone know??? > Note that I know that 5 speed is sometimes called overdrive even on > true 5 speed gearboxes. By overdrive I mean an extra electro-hydraulic > box attached to the output of the main gearbox. Right, this is indeed what Volvo has done. -- Mark Bernstein, Univ of Texas at Austin, Speech Communication, Austin 78712 ARPA: mberns@ngp.UTEXAS.EDU UUCP: ihnp4!ut-ngp!mberns allegra!ut-ngp!mberns gatech!ut-ngp!mberns seismo!ut-sally!ut-ngp!mberns harvard!ut-sally!ut-ngp!mberns
davidsen@steinmetz.UUCP (Davidsen) (02/20/86)
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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