Re: Could be simple question on gear ratios..

[16V Jason <jason@scirocco.org>]

At 11:35 AM 10/31/2001, you wrote:
>--- Cory Langford <cory.langford@icbc.com> wrote:
> >
> > I have been looking at the chart for gear ratios on the Org site and
> > I
> > am now completely confused :)
> >
> > So someone want to explain all the basics to me..
> >
> > 5th gear ranges from 1.26 to .71 <- what is this ratio to?  Is this a
> > percentage of travel or something (ie: one turn on the fly wheel =
> > .71
> > turns on the dif flange?)  Simple question: Is bigger better in this
> > ratio or is smaller better?

Hey Cory,
         Cheapass gave you a correct explaination, but I'll say it again in 
different terms.

The 5th gear ratio of 0.71 means that the output of the transmission to the 
differential (essentially) will turn 0.71 times for each 1 turn that comes 
in.  A 0.91 gear ratio would make the output turn faster.

The final drive ratio (or ring and pinion as it's sometimes called) 
multiplies this again.  So with a final drive of 3.94, the input turns 3.94 
faster than the output.

To calculate the overall drive ratio, you multiply the final drive and the 
gear ratio.  So with a 0.91 5th gear and a 3.94 final drive:

         3.94 x 0.91 = 3.5854.

This means that your engine turns 3.854 times for each time your wheels turn.

When you're looking at gear ratios and trying to pick one that's best for 
you (there is no "better" gear ratios, it all depends on what you're 
looking for) you don't have to worry about the final drive because it's a 
constant.

Essentially, the choices you have for 5th gears are listed below, and I 
wrote in a few benefits of each.

0.71:  The lowest gear, meaning the lowest revs at highway speeds.  You 
will almost definitely lose top speed vs. the other gears at the expense of 
quiet cruising, reduced engine temperature and wear, and marginally better 
fuel economy.

0.75:  The second lowest.  Your top speed will be higher than the 0.71, and 
elasticity will be better (top gear passing power).  Revs will be slightly 
higher than with the 0.71, and likely so will noise. Fuel economy will 
probably be the same since any benefit the 0.71 would have is reduced by 
forcing you to downshift into 4th more often.

0.80:  I call this the top speed gear.  For most 16V Volkswagens, this gear 
will give you the highest top speed of any of the 5th gears; but that 
depends on your engine's torque curve.  Revs are again higher than the 
0.75, so 5th gear passing will be quicker, but fuel economy will be ever so 
slightly worse.  This gear is usually the best compromise.

0.91:  The stock 16V 5th gear.  This gear is unnecessarily short, I will 
never understand why VW used it.  Top speed is not as high as with the 0.80 
(stock 16V motor), but passing power is considerably better.  This gear 
will give you the most noise, the most engine wear, and the worst fuel economy.

You need to pick a gear based on what is important to you -- top 
speed?  elasticity?  economy/quietness?  Once you decide what's important, 
use a torque curve to decide which one meets your needs the best.  If you 
need help with that, don't hesitate to ask.

Jason



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