The "J" Type is also approx. 20%. I know because that is about how far my
speedo is off on the '72 after adding a single rail tranny with "J" type
O/D.
Ralph Jannelli
Matthews, NC
'65 Spitfire MKII
'72 Spitfire MKIV
'80 TR8 DHC
----- Original Message -----
From: Andrew Mace <amace@unix2.nysed.gov>
To: Cometman <cometman_98@yahoo.com>
Cc: <spitfires@autox.team.net>
Sent: Friday, October 08, 1999 11:02 PM
Subject: Re: Poor man's speedomete
>
> On Fri, 8 Oct 1999, Cometman wrote:
>
> > It's not long till my speedometer gear gets here from England, but I
> > was told that in top gear, you double the rpms/100 to get the
> > approximate speed.
>
> It's not quite that simple with a Spitfire, although it works pretty well
> with most GT6s and the big TRs, where each 1000rpm in top gear is worth
> approximately 20 mph. On the earlier Spitfires with the 4.11:1 diff, 1000
> rpm in top gear is worth 15.7 mph with original size tires; figure 16 mph
> as an easier number to work with. Similarly, later Spitfires with the
> 3.89:1 diff will give around 16.7 (figure 17) mph per 1000 rpm in top
gear.
>
> > If you invoke the overdrive how does that affect
> > the calculation?
>
> It's a reduction of about 20% (although the later "J" type OD units might
> be different? Help me out here).
>
> --Andy
>
> * * * * * * * * * * * * * * * * * * * * * * *
> * Andrew Mace, President and *
> * 10/Herald/Vitesse (Sports 6) Consultant *
> * Vintage Triumph Register <www.vtr.org> *
> * amace@unix2.nysed.gov *
> * * * * * * * * * * * * * * * * * * * * * * *
>
|