[DeTomaso] Taller 5th gear for cruise

Daniel C Jones daniel.c.jones2 at gmail.com
Thu Jun 17 15:41:09 EDT 2010


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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