The formula that I use is RPM / Ratio X Cir / 1056 = MPH
RPM is Rev's per minute
Ratio is the overall gear ratio (trans ratio times rear axle ratio)
Cir is the circumference of the driven wheel in inches (Diameter X pi)
pi=3.141592...
1056 is a constant to convert minutes to hours and inches to miles.
Use a little algebra (not my strong suit) the formula can be used to
calculate for RPM needed for desired speed and such.
Allen Brenden
Everett, WA USA
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----- Original Message -----
From: <Herald948@aol.com>
To: <herald1200@comcast.net>; <bill@rarebits4classics.co.uk>;
<triumph_herald@yahoogroups.com>; <nickolarup@hotmail.com>
Cc: <triumphs@autox.team.net>; <ptegler@cablespeed.com>
Sent: Monday, May 19, 2003 11:41 AM
Subject: How fast am I going?
> In a message dated 5/19/2003 9:12:01 AM Eastern Standard Time,
herald1200@comcast.net writes:
>
> > (PS- Math Question- 4,000 RPM in 4th gear(regular 1200 gearbox) into
Spitfire differential(same as 1200 Herald) with 13" tyres(again, standard)
How fast am I going? And how fast if I am at 5000 RPM? I have no idea, and
am looking for a ballpark speed- my speedo is a bit buggy, and jumps
moderately at speeds over 50.)
>
> The original Dunlop Gold Seal 5.20 x 13 tire and 4.11:1 differential would
have given you 15.7 mph per 1000 rpm's. So your "ballpark" is around 16 mph
per 1000 rpm's, or 64 mph @ 4000 and 80 @ 5000, assuming whatever tires you
are currently using are about the same or a bit larger in circumference.
(FYI: for the Vitesse 6/Sports 6 with 5.60 x 13 tires, the magic figure is
16.4 mph per 1000 rpm's.)
>
> --Andy Mace
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