Date:         Sat, 29 Jun 2002 20:14:14 EDT
Reply-To:     FrankGRUN@AOL.COM
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Frank Grunthaner <FrankGRUN@AOL.COM>
Subject:      Re: Speedo Calibration
Comments: To: mikei@ANCORE.COM
Content-Type: text/plain; charset="ISO-8859-1"

In a message dated 6/29/02 3:59:17 PM, mikei@ANCORE.COM writes:

<< Converion:

engine rev/min * 60 min/hr * transaxle ratio =  wheel rev/hour

now,

measure your tires from side to side (to eliminate the little flat spot you

rest on) in inches.


wheel width in inches * 1 ft/12in * 1mi/5280ft = miles / wheel revolution


now,

multiply wheel rev/hr * miles/wheel rev to get miles/hr


simplified :   engine rev (rpm) * transaxle ratio *  wheel dia (inches)  *

946.9697 * 10^-6  = miles per hour. >>

Mike,

Nice arithmetic, bad function, incorrect result. First, C=ÿd=3.1416*d. 
Second, measurement of static tire diameter is irrevelant. The rolling 
diameter (as in 2*rolling radius) establishes the true dimensions of the 
tire. This number is always smaller than the apparent diameter because of 
inflation vs. load, tire flexure  and tread compliance. For new tires, this 
number is given for a specified inflation pressure and load. For example, 805 
revs/mile. Now work backwards through the gear train to get the distance 
traveled of the wheel as a function of engine speed. 

For measurement of the revs/mile for older tires use the chalk mark on the 
sidewall method and measure away. Unfortunately the accuracy will not be 
better than 2 significant figures. 

Frank Grunthaner