RE: speed vs RPM-Spiders

["Bennett, George J" <george.j.bennett@xxxxxxxx>]

That matches my '78 Spider  ---  20mph (indicated)/1000 rpm.

But, checking the speedo versus a handheld GPS, my speedo
is indicating a consisent 10% faster than actual. So that 80 mph
is really 72 mph. My tires are 205/60R14 Yoko 509.

Jeff Bennett
Houston, TX

> ----------
> From: 	Alan Lambert
> Reply To: 	Alan Lambert
> Sent: 	Monday, September 21, 1998 1:37 PM
> To: 	alfa-digest@domain.elided
> Subject: 	speed vs RPM-Spiders
> 
> 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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