Not to sound condescending or anything, but I am glad you
chimed in, because on further reading, I thought it should
be pointed out that a major factor in automatics
traditionally getting worse MPG is the torque converter. The
TC represents a "fluid coupling," whereas the manual
tranny's clutch etc. are a mechanical linkage. Energy
transmission losses are greater with the liquid linkage. As
many of the pros here know. (I am just an amateur who works
on her own car and reads like crazy to understand it.)
But this has changed somewhat with the advent of the "lock
up torque converter."
Optimal gearing is still said to be a factor, though.
Several other factors are said to play significant roles, as
well. So my post did not do justice to why older automatic
trannies were less efficient than manual trannies.
Sure. www.fueleconomy.gov. Just sort of randomly, based on
checking this a few times in the last several years, and
using only the same engine size for a given model:
2007 Civic, same engine size, both five forward speeds:
Manual = 26 MPG city, 34 MPG highway
Auto = 25, 36
2007 Subaru Impreza (an all-wheel drive vehicle)
Manual (5-speed) = 19, 26
Auto (4-speed) = 20, 25
2007 Nissan Sentra
Manual (6-speed) = 24, 31
Auto (variable gear) = 25, 33
2007 Hyundai Elantra
Manual (5-speed) = 24, 33
Auto (4-speed) = 25, 33
2007 Kia Rio
Manual (5-speed) = 27, 32
Auto (4-speed) = 25, 35
From this survey, I think we could argue that newer
automatic trannies seem to do better at highway speeds, even
though it often has fewer gears. The lock up converter (used
only at higher speeds) is the first area I would explore to
explain most of this higher efficiency. I see the lockup
converter started gaining in popularity around the late
1970s but ISTM only recently did all models start having
them. I see the 1995 versions of the cars above never saw
the autos beating the manuals for miles per gallon. Granted
other improvements may have been implemented, like
continuously variable transmissions (CVT).
The Sentra is interesting, since for the two versions I
compared, the big difference is the variable gearing in the
auto. It's the only model that beat the manual version in
both city and highway.
Toyota OTOH seems to consistently have no models where the
auto does better than the manual under city or highway
Again, just an amateur here.
When he wrote "in effect" he was probably right. Unless the RPM at
lockup happens to match between one or more of those combinations, you
get seven different "ratios" of crankshaft to ouput shaft speed, even
though it doesn't happen because of gear ratios changing.
when the converter locks up,the crank RPM equals the converter output
RPM,because they are -locked together-. No slippage.
after that,it's all gear ratios determining output shaft RPMs.
Are you deliberately misunderstanding us? Nobody claimed an
overdrive type shaft speed ratio. What is being said is that when the
lockup engages, you do in fact get the geared ratio - which you
*weren't* effectively getting with the converter unlocked. So for each
gear in which the lockup works, you have two different shaft speed
ratios: one with the lock off and one with the lock on. The *effect* is
the same as having seven gear ratios, with *none* of them being an
overdrive. I don't know how much clear I can make it...
From my reading, the typical auto tranny's lockup does not
fully engage until cruising at speeds upwards of 40 mph. At
40 mph, it is probably in 3rd or 4th gear. At lower speeds,
the lockup is disengaged. But the gear should alway be lower
at lower speeds, too. So I am not sure that I buy Josh's
characterization of doubling the top three gears to yield
effectively seven gears on a 4-speed automatic tranny.
On the other hand, I see that "partial lockup" is possible
and occurs under many conditions, too. This is per the 95-97
Civic's description at
Partial lockup has to be better than no lockup, as far as
overall fuel efficiency is concerned.
So I would say it is not exactly two gears for one that
lockup on/off gives. It's more that lockup is often active
in degrees, being neither fully on nor fully off. When it is
"on" even partly, I expect it usually helps fuel efficiency
compared to no lockup at all.
In this context, where I think Josh was arguing the lockup
feature increases the effective number of gears and so
increases engine efficiency, I would not put it this way.
The slippage is arguably infinite gears but not in a way
that improves efficiency the way direct mechanical linkage
(= lockup) to infinite gears would.
To split hairs.
Just a short anecdote here...
I'm not sure what manufacturer introduced "lock up converters," but
Studebaker began using its self designed automatic featuring a lock up
converter for the 1950 model year.
My 1955 President, a hefty 4,200 lb sedan with 259 V8/DG-250 tranny
achieved 21/28 mpg in real time road tests in that era. Not bad for a 4
bbl carb, auto and pretty good performance. My uncle used to really rub
it in to Chevy/Ford owners...
In a lot of ways, we really haven't advanced much farther.
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