[DeTomaso] Taller 5th gear for cruise

Will Demelo wdemelo at cogeco.ca
Thu Jun 17 17:10:53 EDT 2010


I have the 3.77 and .655 fifth gear. I find it a little sluggish at 75mph 
with a 393 stroker. I don't recommend going any taller on the fifth. The 
3.77 r&p is amazing.
Will

----- Original Message ----- 
From: "Daniel C Jones" <daniel.c.jones2 at gmail.com>
To: "Pantera REALBIG forum" <detomaso at realbig.com>
Sent: Thursday, June 17, 2010 3:41 PM
Subject: [DeTomaso] Taller 5th gear for cruise


> 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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