Custom Tranny Gears

[Huw Powell]

> By my calculations, if I use the 0.640 5th gear of the early CGTs in
> place of the stock 0.537 5th, it should lower my RPMs in 5th by about
> 16%. Right? So @ 70 mph when I now turn around 3,000 RPM (just a
> guess, I can't remember)I would be turning only 2,520 RPMs, right?

I'm not gonna follow the math this early in the day, but in my
experience the 85-87 trannies generally run about 3k rpm at 60 mph or so.

In my QF trannied '82 coupe, I get about 80 mph at 3k in fifth - bu
tthat is partly the higher gear, and partly the higher final drive.

I'm sure your math is good, just multiply or divide rpms &speed or speed
@ rpm by the ratio of the two gearsets.

> Any clue what this would do to my highway mileage? I know it will at
> least make for a quieter highway ride... Thoughts on this mod?

It should improve slightly, I would imagine.  It will certainly cut
noise a bit, especially if you have "free flowed" your exhaust system at
all.  Highway mileage may be more dependent on drag/weight/rolling
friction than gearing, though.  It should take the same amoutn of fuel
to overcome those factors, its just a matter of whether there is less
energy wasted spinning the engine faster, and whether the engine is
actually more efficient (at producing power from fuel) at one of those rpms.

>>>> On this same line, are there any compatable 5th gears I could
>>>> swap into my CGT that would lower my RPM while cruising on the
>>>> highway? I'm not worried about loosing acceleration in 5th as I
>>>> only use 5th for real highway crusing and would just downshift
>>>> to 4th...
>>>
>>> The gears are higher in the early (pre-1984) coupe trannies, but
>>> it's partly a function of final drive ratio, too.
>>>
>>> final drive, 4.45 vs. 4.90
>>>
>>> fifth 0.537 vs. 0.640
>>>
>>> (later tranny numbers last)

--
Huw Powell

http://www.humanspeakers.com/audi

http://www.humanthoughts.org/