Re: Speedo error & differential ratio

[Brian Kemp <bk13@earthlink.net>]

> I am interested in info that I can use to calculate my overall ratio and
> supposing normal transmission ratios, the differential ratio.

Tony, when you get a speedo or tach, you can check them off each other using the
formula below.  I used this to see the speedo error I would have by changing
tire sizes.  The theory tested out when I had my car tested at a AAA speedometer
calibration station.

     Speed = (dia)(rpm) / (ratio)(336)
     or rpm = (speed)(ratio)(336)/dia

     dia is of the tire
     336 is 63360 in/mi / 60 min/hr / 3.1416
     ratio = overall gear ratio (taken from Bentley)
         sample ratios for my TR6 with a 3.7 differential
         1     11.62
         2     7.44
         3     4.92
         4     3.70
         4od 3.03 (a-type)

See more info about tires or download my Excel spreadsheet with tire size
comparisons at http://home.earthlink.net/~bk13/tech.htm

Brian Kemp
72 TR6