[DeTomaso] Taller 5th gear for cruise

Julian Kift julian_kift at hotmail.com
Thu Jun 17 16:00:20 EDT 2010


When I purchased the GT5 the ZF was with Quella being rebuilt and I had him drop in a taller 5th. The only one he could source at the time was a 0.642, but I'm very happy I made the upgrade, 80 mph cruising is a breeze.


For the '74 I'm leaning toward a 3.77 R&P and 0.642 5th for ORR.

 

Julian

 
> Date: Thu, 17 Jun 2010 14:41:09 -0500
> From: daniel.c.jones2 at gmail.com
> To: detomaso at realbig.com
> 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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