speed vs RPM-Spiders

[Alan Lambert <gerard@xxxxxxx>]

Below is the formula I came up with to try to use the tach as a speedo 
(my speedo in the '74 Spider being noticeably off)--somehow it seems to 
indicate speeds in fifth that are higher than what I think I'm doing. (or 
else I haven't been by any cops lately)--If you can tell what I did 
wrong, please post the answer! This is based on a 4.56 rear end, .79 
fifth gear, 205/60/14 AVSi's.

speed in mph = (rpm*60)*3.1417*(14/12 + 
(20.5*2*0.6/2.54)/12)/5280)/4.5556/.079

in words, speed in mph equals tach rpm times 60 minutes/hour times PI 
times the rolling diameter ( which is 14inches divided by 12 plus 
(205mmdivided by 10) times the aspect ratio divided by centimeters per 
inch divided by inches per foot divided by feet per mile divided by the rear 
end ratio divided by the transmission ratio.


(try to say it fast without taking a breath.)


Using the formula above, I'm coming out with 19.58 mph per 1000 rpm, or 
78.32 mph at 4000 in fifth.---seems too high, or else everyone else on 280 
(at least the fast ones) is doing over 80mph.--
================================================================================
Regards,
Alan G. Lambert___________________________________<gerard@domain.elided>
           *******Los Altos, California (office)*******

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