Stupid question on 5-speed transmissions

Arde

Arde

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Here is the question, why do 5-speed OD gearboxes exist at all?
Given that one drives most of the miles in 5th gear, and that the most efficient coupling is 1:1 (less losses, less wear, less rings involved) why not make every 5 speed gearbox 1:1 in 5th gear, and get the desired revs by using the desired differential?
By revs I mean that different uses want to be in the optimal power band at different speeds for rallies vs. cruising for example.

Specifically there is the Getrag 5 speed dogleg, which is 1:1 in 5th, why do a 265/6 OD at all instead of changing the differential and reuse the dogleg.
 
depends on the gear ratios i think ? Yes, you can have a dogleg with a long diff to achieve normal rev's at high speed ... but then you have a very big first gear ?
 
4 speeds are 1:1 final drive with the tall Euro rear end, I do not know rear end ratio, I was getting 17.5 mpg. After replacing with a 5 speed with a final 0.8 5th I get 24 to 25 mpg in 5th. I have increased my mpg an average of 64 more miles per tank. The 265 is still 1:1 in 4th.

Now 2500 rpm in 5th is 70ish, 3000 rpm is 85ish. I am also running 16 x 7 BBS RS which have 411 revolutions/mile, while stock wheels are 815/mile. Every little bit helps.
 
I can give the example of my 2002. My ‘71 came with a 4 speed and 3.64 open diff. I swapped in a 3.91 LSD and a 5 speed overdrive transmission. The car feels much more responsive in lower gears with that final drive ratio. It pulls out of corners much better than the 3.64 ever did. The OD 5th gear transmission allows me to reduce some of those revs at highway speed.

Yes, our cars can spend a lot of mind-numbing miles on a highway, but where are you really enjoying “driving” your car? Likely, in the twisty mountain roads and for me, the car responds better with a lower gear ratio final drive.

In a pure “efficiency” equation, yes, a taller final drive and 1:1 transmission may make the most sense, but when did our cars ever enter into an efficiency contest, or “make sense”. The e9 is a sexy beast and makes no apologies for it. :)

My $0.02...

Ed Z
 
Transmissions/engine/vehicle combinations are designed for driving in all conditions including weather, terrain, and gross weight.
My take (relatively level terrain and not fully loaded) after changing to the 265/6 overdrive with the 3:45 open diff and stock 14 inch wheels is that 1st gear seems almost like a creeper gear and 2nd is a little low. However 3, 4, and 5 seem to fit the vehicle and engine very well and are comfortable to drive. It pulls strong even in 4th and 5th gear. In San Francisco, I am sure 1st and 2nd gear would be needed more and 4th / 5th less.
It is definitely an improvement over the ZF 4 speed.
 
Yes, the 265/5 OD benefit is understood and I see the advantage of more gears when I compare my E24 with a 0.81:1 4th gear automatic vs. 0.83:1 5th gear in the E9. It is in the lower gears.

My point is that I could have the same ratios if the 265/6 had a 1:1 5th gear, a taller differential by a factor of 1/0.83
and of course shorter 1-4 gears by the same factor. The manufacturer would have a single transmission model to support. The only difference really would be the dogleg vs. H shift pattern...
 
Given a clean slate for the engineers, not having to leverage any previous designs because of cost or manufacturing (IE: four speed box with fifth gear mounted on the back) I'm guessing they just don't care. If they're not worried about longevity of bearings, etc, they'd design to whatever is easiest. In other words, if an overdrive design had inherent issues and failures they'd be forced to do something else.

New auto transmissions are extremely complicated and expensive only because they're chasing fuel efficiency. If they weren't concerned with that would we be using 30 year old designs in our cars? Maybe they would change manual transmission designs to save manufacturing cost but if they're already reliable enough why change.

Some of the CVT transmissions are garbage nowadays.
 
In order to have the same engine rpm with a 1:1 5th gear 265/5 as opposed to a 0.81 5th gear 265/6 you will have to lower the rear axle ratio by 0.81%. For a 3.64 rear axle you would have to switch to a 2.93. The overall ratios would be, from 1st to 5th:
265/5 with 2.93 rear -- 1st - 10.9:1, 2nd - 7.03:1, 3rd - 5.19:1, 4th - 3.7:1, 5th - 2.93:1
265/6 with 3.64 rear -- 1st - 13.9:1, 2nd - 8.0:1, 3rd - 5.1:1, 4th 3.64:1, 5th - 2.93:1

Basically, once you're in 2nd gear, there's not a lot of difference.

 
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