[DeTomaso] Taller 5th gear for cruise

Kirby Schrader kirby.schrader at gmail.com
Thu Jun 17 16:08:11 EDT 2010


Dan,

It sounds like you already know what you want.

:-)

The wimpy cast iron 470HP 377 Cleveland in my Pantera struggles at the
high end. I have a .655 5th in it with the standard 4.22.
On the back straight at Pocono is just wouldn't go any faster with 15"
wheels and tires.
Now, with the 18" wheels and tires which are an inch taller, it is
worse. I guess I need more HP.

I have a .642 5th in the GT40 and find it to be very nice with the
4.22. First gear is a tire smoking demo gear, but it was heading north
of 175mph real fast at Road America in 5th and according to the
spreadsheet, should still do 205mph at 7000rpm.

The .620 is probably just what you need for cruising.

FWIW,
Kirby

On Thu, Jun 17, 2010 at 14:41, Daniel C Jones <daniel.c.jones2 at gmail.com> wrote:
> I've decide to have a taller 5th gear installed in the red Pantera's
> transaxle.  The standard 5th gear is 0.705.  The optional 5th gear
> ratios are:
>
>  0.655
>  0.642
>  0.620
>
> The tallest 5th gear with the standard ring and pinion is very similar
> to the standard 5th gear with the 3.77:1 ring and pinion:
>
>  0.620 * 4.22 = 2.61640
>  0.705 * 3.77 = 2.65785
>
> I ran some numbers assuming:
>
>  335/35/17 tires
>  4.22:1 final drive ratio
>  current gear ratios
>  1st 2.23
>  2nd 1.47
>  3rd 1.04
>  4th .846
>  5th .705, 0.655, 0.642, 0.620
>
>  Speed   Ratio  RPM
>  80 MPH  .705   3050
>  80 MPH  .655   2834
>  80 MPH  .642   2777
>  80 MPH  .620   2682
>  70 MPH  .620   2347
>  60 MPH  .620   2011
>  60 MPH  .642   2089
>  60 MPH  .655   2125
>  60 MPH  .846   2745
>
> Typical highway cruise speed is 75 to 80 MPH.  The engine is a 407
> Fontana with C302B heads and IR EFI.  Looks like I should be able
> pull the 0.620 gear at 80 MPH.  It might be sluggish at 60 MPH but
> there's not much difference in RPM between the three gears at that
> speed so I'm leaning towards the 0.620 gear.  I can always drop
> back to 4th at 60 MPH which would be 2745 RPM.  Any thoughts?
>
> Dan Jones
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