On Fri, 9 Oct 1998 00:51:11 -0700 Skye Poier <skye@ffwd.bc.ca> writes:
>Until I get a timing light with a tach, I'm not really sure the tach
>or the
>speedo are correct on my B. For kicks, I calculated how fast you
>should
>be going at certain r.p.m.'s...
>
>My tires are 185/70R14s which the online tire calculater
><http://www.mgb.bc.ca/misc/tirecalc.html> gives a diameter of 24.1"
>and using C=2*pi*r = 75.712" circumference. Multiplied by the desired
>
>r.p.m. and by the rear axle ratio (11/43), then convert inches to
>miles and
>minutes to hours. ie:
>
>75.712 in 3000 rev 11 1 mi 60 min
>--------- * -------- * -- * -------- * ------ = ~ 55 m.p.h
> 1 rev 1 min 43 63360 in 1 hour
>
>I came up with the following table (rounded to the nearest number):
>
>rpm mph
>---- ---
>2000 37
>3000 55
>4000 73
>5000 92
>
>This is for 4th gear where the trans ratio is 1:1
>Or, simply multiply r.p.m. by about 0.0184 to get m.p.h.
>
>Is my method sound?
It's so sound that you came up with almost exactly what the factory came
up with 30+ years ago; Ie 17.6 MPH/1000 RPM in 4th gear, no OD
See the chart in, among other publications, the special tuning manuals.
Rick Morrison
72 MGBGT
74 Midget
>Here's some more interesting information. My speedo has the following
>written on it: Jaeger SN 6125/04 1060
>
>Is 1060 the turns/per/whatever? I can only find references to 1280
>and 1240
>speedo's in my material. I compared my odometer to mile markers on
>the I5
>crusing in O/D and found my odometer to be exactly 20% fast (reports
>12 mi
>when I have only gone 10 etc).
>
>If the above table is correct, my tach is way off. I timed myself
>with
>a watch to see what r.p.m. would get me 1 mile in 60 seconds (60 mph)
>and
>the tach reported around 4700 r.p.m. again in O/D so either my
>calculations
>are wrong or I have been cruising at a much lower r.p.m. than I
>thought!
>Which is just as well, because with my tired suspension and not quite
>balanced tires I wouldn't want to go any faster for now. I can dream
>about
>blowing the doors off SUVs though, can't I? ;)
>
>Math is fun.
>
>Skye
>
>--
>1966 MGB GHN3L-70307 __,__\__
>The MGB Experience http://www.mgb.bc.ca/ (_o____o_)
>
or call Juno at (800) 654-JUNO [654-5866]
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